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The use of microgravity to detect small distributed voids and low-density ground


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Microgravity is established as a technique for the detection of natural and man-made cavities. However, past published examples have concentrated on substantial natural cavities or mine workings. Two cases are described for which the voids present are relatively small and discontinuous, and where much of the affected ground is characterized by low-density ground rather than large open voids. Here we use the term 'low-density ground' to encompass ground that has been disturbed by the collapse or partial collapse of material into a void such that the affected ground shows an anomalously low density in comparison with the surrounding unaffected ground. Case study 1 was undertaken with Mouchel Parkman on behalf of Hertfordshire County Council. A doline had opened up within a school playground. The collapse was expected to be related to natural voids within the chalk at 7-10 m depth. The low-density areas identified in the microgravity survey, in addition to some control locations, were subsequently proved by dynamic probing. The investigation accurately identified voided and poorly consolidated ground, and provided the basis for an assessment of the risks of imminent and potential future collapse. Case study 2 was undertaken with Laing O'Rourke, who were principal contractor for the construction of the A590 bypass, Cumbria. Exploratory boreholes confirmed the presence of loose material in the glacial till overburden associated with suspected voids within the limestone bedrock. The microgravity survey identified areas of low subsurface density, which were subsequently targeted by boreholes. A detailed ground model constructed from borehole data allowed a synthetic gravity map to be calculated for comparison with the measured gravity. Poorly consolidated ground, depth to bedrock and surface topographic effects could be isolated and a clear interpretation of subsurface produced. The microgravity technique is shown to be an effective tool for the investigation of poorly compacted ground and small shallow voids in complex and disturbed ground conditions. When followed by targeted intrusive investigation such surveys can yield a great deal of information that would not otherwise be available to the engineer.
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2008; v. 41; p. 371-380 Quarterly Journal of Engineering Geology and Hydrogeology
G. Tuckwell, T. Grossey, S. Owen and P. Stearns
The use of microgravity to detect small distributed voids and low-density ground
Quarterly Journal of Engineering Geology and Hydrogeology
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George William Tuckwell on 18 August 2008
© 2008 Geological Society of London
The use of microgravity to detect small distributed voids
and low-density ground
G. Tuckwell, T. Grossey, S. Owen & P. Stearns
STATS Limited, Porterswood House, Porters Wood, St. Albans AL3 6PQ, UK
icrogravity is established as a technique for
the detection of natural and man-made
cavities. However, past published examples
have concentrated on substantial natural
cavities or mine workings. Two cases are described for
which the voids present are relatively small and discon-
tinuous, and where much of the affected ground is
characterized by low-density ground rather than large
open voids. Here we use the term ‘low-density ground’ to
encompass ground that has been disturbed by the col-
lapse or partial collapse of material into a void such that
the affected ground shows an anomalously low density in
comparison with the surrounding unaffected ground.
Case study 1 was undertaken with Mouchel Parkman on
behalf of Hertfordshire County Council. A doline had
opened up within a school playground. The collapse was
expected to be related to natural voids within the chalk at
7–10 m depth. The low-density areas identified in the
microgravity survey, in addition to some control locations,
were subsequently proved by dynamic probing. The
investigation accurately identified voided and poorly
consolidated ground, and provided the basis for an
assessment of the risks of imminent and potential future
collapse. Case study 2 was undertaken with Laing
O’Rourke, who were principal contractor for the construc-
tion of the A590 bypass, Cumbria. Exploratory boreholes
confirmed the presence of loose material in the glacial till
overburden associated with suspected voids within the
limestone bedrock. The microgravity survey identified
areas of low subsurface density, which were subse-
quently targeted by boreholes. A detailed ground model
constructed from borehole data allowed a synthetic
gravity map to be calculated for comparison with the
measured gravity. Poorly consolidated ground, depth to
bedrock and surface topographic effects could be iso-
lated and a clear interpretation of subsurface produced.
The microgravity technique is shown to be an effective
tool for the investigation of poorly compacted ground and
small shallow voids in complex and disturbed ground
conditions. When followed by targeted intrusive investi-
gation such surveys can yield a great deal of information
that would not otherwise be available to the engineer.
Dierent subsurface materials have dierent bulk densi-
ties. Microgravity surveys seek to detect areas of con-
trasting or anomalous density by collecting surface
measurements of the Earth’s gravitational field. There
are three common origins of local lows in microgravity
data: subsurface voids, variations (usually deepening) in
the depth to bedrock, and low-density ground in the
shallow subsurface. Here we use the term ‘low-density
ground’ to encompass ground that has been disturbed
by the collapse or partial collapse of material into a void
such that the aected ground shows an anomalously low
density in comparison with the surrounding unaected
A gravity meter is a highly sensitive instrument that
measures the acceleration due to gravity. When posi-
tioned above a dense material it records the acceleration
(g) as a relative high (a positive gravity anomaly). When
positioned above a low-density feature (e.g. an air-filled
cavity) a relative gravity low (or negative gravity
anomaly) is recorded. Gravity anomalies arising from
natural or man-made subsurface features such as voids
and cavities are superimposed on much larger variations
related to height, latitude and regional geological varia-
tions. To isolate the subtle signal of interest, careful data
acquisition and processing are required.
A number of examples of successful microgravity
surveys have been described in the past over large
features including natural cavities (e.g Patterson et al.
1995; Cooper 1998; Styles et al. 2005) and mine work-
ings (e.g. Bishop et al. 1997; Emsley & Bishop 1997).
The technique has also been used to monitor the pro-
gressive collapse of abandoned salt mines through time
(Branston & Styles 2003). In addition to the discrete
void itself, it is common for the cavity to be associated
with a ‘halo’ of disturbed ground, typically characterized
by enhanced porosity or fracturing. This low-density
ground may act to eectively enlarge the target visible to
a geophysical technique.
The interpretation of microgravity data begins with
the identification of local negative anomalies from the
processed residual Bouguer anomaly map. More sophis-
ticated analysis can be performed to calculate the depth
to causative bodies by Euler deconvolution (e.g.
Durrheim & Cooper 1998; Cuss & Styles 1999) or by
iterative matching of predicted gravity signals from 3D
models to the measured data (Styles et al. 2005).
Gravity has been proven eective for large voids at
depth. Other geophysical techniques such as ground
penetrating radar, EM ground conductivity, resistivity
and seismic methods are capable of locating voids in the
shallow subsurface. These shallow techniques are not
appropriate in all site conditions. For example, if surface
Quarterly Journal of Engineering Geology and Hydrogeology, 41, 371–380 1470-9236/08 $15.00 2008 Geological Society of London
types vary considerably (e.g. from concrete to tarmac
to landscaped grass) this may manifest in the EM
conductivity data as variations caused by changing
thickness of made ground and dierent drainage con-
ditions associated with dierent cover materials, which
may mask more subtle features associated with low-
density ground. Where the ground materials are electri-
cally conductive the depth from which reliable ground
penetrating radar reflections can be obtained may be
severely reduced. Where there are areas of hardstanding
it may not be possible to drill holes for electrodes to
undertake resistivity surveys. Of the seismic methods,
the reflection technique is best suited to the detection
of voids; however, these surveys can be slow and expen-
sive to undertake. Noisy site conditions and dicult
access to place geophones can make such surveys
Therefore there is a need for a geophysical technique
that can detect small voids in the relatively shallow
subsurface, to bridge the gap between reliable near-
surface investigations using other geophysical tech-
niques and the proven ability of microgravity as a
technique for the detection of large deep features. Fur-
thermore, it is desirable for the technique to be able to
support other geophysical methods or to operate in
situations where the other geophysical techniques may
give data that are unreliable or dicult to interpret.
Here we present data and interpretations from two
projects in which microgravity measurements were used
to direct subsequent intrusive investigations. In both
cases the voids present are relatively small and discon-
tinuous, and much of the aected area is characterized
by low-density ground rather than large open voids. The
combination of geophysical and intrusive data allows a
detailed assessment of the interpreted gravity datasets,
and an assessment of their ecacy in detecting relatively
small near-surface voids.
Case study 1
This project was undertaken with Mouchel Parkman on
behalf of Hertfordshire County Council. The trigger
event for the investigation was the appearance of a
doline in one corner of a school playground. The hole
had the characteristics of a small collapsed solution
feature, and was c. 0.6 m in diameter and 1.3 m deep
(Fig. 1).
The site is within an area known to be aected by
natural voids within the chalk. The published 1:50 000
scale geological map of the area (Sheet 256, ‘North
London’) indicates that the site is underlain by the
Lambeth Group of the Palaeogene overlying White
Chalk of the Cretaceous. In this area the chalk is
expected to lie at a depth of 7–8 m. Previous investiga-
tions at the site established that the ground profile
comprises metastable granular (sandy) material overly-
ing chalk that lies at a depth of 7–8 m below ground
surface. A layer of cohesive material has been observed
between the sandy material and the chalk. As a result of
erosion and dissolution, the interface between the chalk
and overlying material is likely to be irregular. Based on
the published geological map and the results of unpub-
lished investigations near the site, the water table is
expected to be at a depth of 20 m below surface. No
direct measurements of water table were made at the site
as part of this project.
There is a considerable range of natural solution
feature geometries possible in the shallow subsurface. A
detailed discussion of formation mechanisms is beyond
the scope of this study. An overview of possible solution
feature geometries is shown in Figure 2. Of particular
relevance to this study is that it is possible that no void
is present and the solution feature manifests as a volume
of low-density ground. It is also possible that a void may
lie within the chalk oset laterally relative to the associ-
ated near-surface low-density ground so as not to lie
directly beneath it.
The school grounds are relatively flat, and are pre-
dominantly tarmac hardstanding, with some areas of
landscaping, artificial soft play surfaces and grass.
Access was restricted in some areas by trees, and by play
A geophysical survey was commissioned to determine
the possible presence of voids within the chalk bedrock
elsewhere on the site that may have the potential to
cause future subsidence.
Microgravity survey
The survey was designed to allow for near-complete
coverage of the whole site with the microgravity tech-
nique. The gravity survey was conducted on a 4 m by
4 m grid with three independent readings collected
Fig. 1. Photograph of the doline feature that opened up in one
corner of the grounds of the school.
at each station to reduce error. Because of access
restrictions the original collapse feature lay outside the
survey area.
Data recorded on site include accurate position and
height measurements for each recorded station. A
theodolite total station was used to survey each station
location with a repeatability of <5 mm.
To account for time-variant eects on gravity read-
ings such as Earth tides and instrument drift, a base
station reading was recorded every hour during the
survey so that the appropriate corrections could be
made during the data processing stage. A Scintrex CG5
instrument was used to collect the data. The instrument
has a quoted accuracy of 1 µGal. The survey was
undertaken in normal working hours, but in the school
holidays, so there was little environmental noise from
human activity. Weather conditions were calm and dry,
so there was also little disturbance from wind or tree
roots. Repeat readings taken as part of the survey
provided repeatability of less than 5 µGal.
The raw data were corrected for Earth tides and
instrument drift on the basis of linear variations between
base station readings, resulting in an expression of the
raw gravity data with temporal variations removed. The
data were corrected for the free air and Bouguer eects
to remove the gravitational eects of elevation and
ground material in order to produce a Bouguer residual
anomaly map (Fig. 3). There was no significant topo-
graphic gradient across the site, and therefore no correc-
tive calculation was required beyond that accounted for
by the Bouguer correction. Gravity observations were
made adjacent to the school building. Analysis of these
data points in the context of the complete dataset
indicated that the mass of the single-storey school build-
ing did not have an eect on the data of sucient
magnitude to obfuscate an interpretation of the ground
Six clear negative anomalies can be identified in the
data based on their relative low values compared with
their immediate surroundings. Anomaly 1 is the largest
in plan view extent. Anomalies 2 and 3 are greatest in
magnitude. Anomalies 1–3 appear to be elongated in the
same orientation, and as such may reflect natural align-
ments, perhaps weaknesses, within the chalk. Anomaly 4
is relatively small in plan view extent in comparison
with others highlighted. Anomaly 5 is relatively low
amplitude. Anomaly 6 is relatively high amplitude in
comparison with the ‘background’ values immediately
around it.
The depth and size of the features causing each
anomaly can be estimated by assuming that they take
the form of simple geometric shapes. As a first approxi-
mation it is useful to assume spherical features. This
assumption allows a quick and simple calculation of the
depth of the feature using the standard technique of
measuring the half-width of the anomaly profile. In this
case calculations suggest that each of the six highlighted
anomalies is likely to be located in the upper 5 m below
ground level. This places each anomaly above the ex-
pected depth to bedrock of c. 7–8 m below ground level.
In turn, this suggests that the highlighted anomalies
represent areas of loose fill or unconsolidated material in
the shallow subsurface above the chalk. It is expected
that these features have been caused by voids within the
chalk bedrock causing low-density ground above.
Dynamic probe results
The dynamic probing test is used to determine the
resistance of soils in situ to the intermittent penetration
of a cone, driven dynamically in a standard manner and
in accordance with BS 1377 (British Standards Institu-
tion 1990). The cone was driven by a weight of 50 kg
falling 500 mm, each fall being termed a blow. The data
Fig. 2. Schematic diagram to illustrate the range of natural solution features that may be present in Chalk. (1) Swallow hole where
stream water permeates down through pre-existing solution pipe below surface hollow. (2) Swallow hole where stream water
permeates down through solution-widened joints and bedding. (3) Solution cavity formed within chalk by solution widening of
joints and bedding as ground water permeates downwards to reach the water table. (4) Solution cavity formed by vadose or phreatic
dissolution, whose roof has become destabilized causing collapse of cover into cavity, forming a collapse doline at the ground
surface. (5) Solution pipe remnants, downslope from cover deposit margin, not infilled with disturbed cover deposits. (6) Solution
pipe formed below thin cover deposits, shallow zone above pipe mouth being disturbed by solution subsidence. (7) Solution pipe
present beneath cover deposit, where upward migration of metastable ground conditions has resulted in ground subsidence forming
a subsidence doline at the ground surface. (8) Solution pipe beneath cover deposit, where downward solution subsidence movement
of the cover deposits has ceased because of the presence of a resistant layer, whereas material below this has continued to subside,
creating a metastable cavity. Material above the cavity is undisturbed. (9) Solution pipe formed below thick cover deposits, zone
above pipe mouth being disturbed by solution subsidence. After Edmonds (1987).
recorded are the number of blows required for the cone
to penetrate 100 mm into the ground. At each 100 mm
interval this blow count is recorded as the N
The locations of the dynamic probe investigations
were chosen so that a comparison could be made
between areas expected to contain low-density ground
and those areas that are expected to be unaected.
Within Anomaly 3 access with the probing rig was
restricted because of the presence of climbing frames in
the playground. There was no access for dynamic prob-
ing within Anomaly 4 because of a climbing frame and a
A summary of the dynamic probe results is presented
in Table 1. A blow count of two or less for the
penetration of 100 mm of the dynamic probe into the
ground indicates low-density ground. Interpretation of
dynamic probe results is subjective, there being no
widely recognized published correlation between blow
counts per 100 mm and material properties. Dynamic
probing in the undisturbed granular material generally
reveals relatively low blow counts in the upper metre or
so, typically less than N
= 5, rising rapidly to N
values of 5–10 through competent material. At the
interface with the chalk, blow counts drop to typically
= 2–3. Low-density ground is generally recognized
by its weak resistance to the probe, with blow counts of
Control locations The control probes away from areas of
negative microgravity anomaly showed a mixed pattern.
DP6 refused at shallow depth on an obstruction, DP7
and DP8 showed the expected ‘normal’ profile of weak
Fig. 3. Microgravity residual Bouguer anomaly plot for the survey area in case study 1. Identified negative anomalies are highlighted
with dashed lines. Dynamic probe locations are indicated and logs are associated with each location. All dynamic probe logs are
plotted at the same scale.
soils to around 1.5 m depth with dense material beneath.
DP7 penetrated to the presumed top of the chalk at
around 8 m below ground level (mbgl), whereas DP8
refused at around 6 m in dense soil. At the DP11
location initial blow counts were low, typically
= 0–1, to a depth of 1.5 mbgl, after which blow
counts remained suppressed at N
= 3–6 to a depth
of around 5 mbgl, where more competent soils were
encountered. The top of the chalk is inferred to be at
about 7.2 mbgl.
Anomaly 1 Three dynamic probes (DP1, DP2 and DP3)
were carried out at the Anomaly 1 location. DP1 shows
a ‘normal’ ground profile with no evidence of weak
ground and is probably located just outside the
anomaly. The top of the chalk is inferred to occur at
around 8 mbgl. DP2, however, shows very weak to weak
ground to a depth of around 4 mbgl, and below this
depth N
values remain depressed in relation to typical
values, with blow counts of typically three or four blows
per 100 mm. The boundary between the chalk and the
overlying soil is not clear. The results from DP3 are
similar, although blow counts are generally a little
higher and the interface with the chalk is evident at
around 8 mbgl.
The dynamic probing has confirmed that Anomaly 1
is an area of weak ground. However, N
values are for
the most part above two and there is no evidence of
large voids at depth. It was concluded that there was no
immediate hazard to site users at the Anomaly 1 loca-
tion; however, as with all such features in the area, the
soils within the anomaly have already been weakened
and further loss of strength and associated settlement
may occur in the future, particularly if excessive water
ingress occurs as a result of poor site drainage, leaking
services, or a period of unusually wet weather. The soils
may also be susceptible to collapse under the influence of
additional load from building foundations, etc.
Anomaly 2 Anomaly 2 was investigated by DP4. The
results from DP4 indicate fairly competent ground,
although a little weaker than normal, to a depth of
c. 3.5 m, where blow counts drop to N
= 0 or 1. Only
two blows were required to drive the probe from 3.6 m
to 5.0 m, indicating very weak or voided ground at this
location. N
values remain at zero or one to a depth of
around 8 m, where the top of the chalk is inferred. N
values then rise to N = 6 or 7 to the full depth of the
The ground beneath Anomaly 2 appears to be much
weaker than that beneath Anomaly 1, with a probable
1.4 m void at a depth of 3.6 mbgl and very weak ground
beneath to a depth of around 8 m.
Anomaly 3 Anomaly 3 was investigated with dynamic
probe DP5. The N
values were initially low at N =0
to 2, with a small void or very weak zone at 1.5 mbgl.
Below this depth, N
values increase gradually to
attain a ‘normal’ profile by about 2.8 mbgl. The top of
the chalk is inferred to be at about 7.6 mbgl.
Based on the results of this single dynamic probe,
Anomaly 3 would appear to represent an area where the
low-density ground is present to a slightly greater than
normal depth below ground level, with no evidence of
very weak ground or voids beneath. However, past
experience has shown that within the areas of low-
density ground, soil strength can vary markedly over
very small horizontal and vertical distances, and weaker
ground may exist within the Anomaly 3 location that
has not be revealed by the single probe. It should also be
noted that access restrictions prevented the investigation
of the centre of the anomaly. It is possible that more
extensive volumes of poor ground are present but have
not been encountered by the single dynamic probe
Anomaly 5 Dynamic probes DP9 and DP10 investigated
Anomaly 5. Neither probe show any unusual areas of
low-density ground, therefore the Anomaly 5 location is
not considered to be a cause for concern. DP10 suggests
that the top of the chalk is c. 8.2 mbgl.
Anomaly 6 Dynamic probe DP12 was used to investigate
Anomaly 6. Initially, DP12 N
blow counts were
slightly low, but roughly consistent with the nearby
DP11, formed away from any anomalous microgravity
data. However, at a depth of c. 4.6 m, N
values drop
dramatically with N
= 0 or 1 to the full depth of the
probe at 9 mbgl. From 6.9 to 9 mbgl, only four blows
were required to drive the cone 2.1 m. No increase in
resistance was evident at 9 mbgl and it is likely that the
low-density or voided ground extends to greater depth.
It is also noted that this is the only anomaly investigated
where poor ground would appear to extend into the
Table 1. Summary of dynamic probe results, case study 1
Dynamic probe
Zone of low-density ground
= 0 to <2) (mbgl)
DP1 1.0–1.7 and 8.1–9.5
DP2 0.7–3.0
DP3 0.5–1.5 and 7.9–8.9
DP4 1.3–1.9 and 3.6–8.0
DP5 0.0–2.0
DP6 n.a.
DP7 0.0–1.4
DP8 0.4–1.7
DP9 0.6–1.3
DP10 0.0–1.1
DP11 0.0–2.2 and 7.0–7.6
DP12 0.0–2.0 and 4.6–9.0
n.a., not applicable.
With the possible exception of Anomaly 5, the intrusive
investigation confirmed that the low gravity anomalies
identified by the microgravity survey do represent areas
of low-density ground and/or voided ground. The
dynamic probing has also shown that away from the
anomalies, the ground strength profile is ‘normal’ for
the site.
Of the anomalies investigated, none would appear to
present an immediate hazard to site users. However, past
experience has shown that all such features in this area
are metastable and further loss of strength and associ-
ated settlement or collapse may occur in the future,
particularly where changes occur in groundwater flows
or load conditions.
Case study 2
Case study 2 is an investigation of a section of the A590
bypass road constructed at High Newton, Cumbria, and
was undertaken on behalf of Laing O’Rourke, who were
principal contractors for the project. The geology of the
site comprises c. 14 m of glacial till over the Dinantian
Limestone. The water table was proved to lie between
5 and 10 mbgl in the area surveyed. High Newton is
located to the north of Grange-over-Sands, and lies on
the southern edge of the Lake District fells (Fig. 4). The
area surveyed is on the western slope of a valley,
immediately to the west of the present A590 road.
Low-density ground thought to be associated with the
presence of solution features within the limestone was
discovered during initial excavation and ground investi-
gation works at the site. A preliminary geophysical
investigation was undertaken over a small section of the
route. The survey highlighted three main areas for
further investigation, one of which resulted in the identi-
fication of low-density ground in the near-surface.
A second phase of geophysical investigation was
commissioned to extend to the north and south of the
area surveyed in the first phase of investigation. The
principal aim of the geophysical investigation was to
identify variations in subsurface density associated with
voids or low-density ground within the limestone or
overlying glacial till.
Microgravity survey
The gravity survey was conducted on a 5 m by 5 m grid
with three independent readings collected at each station
to reduce error. Data recorded on site include accurate
position and height measurements for each recorded
station. An RTK dierential global positioning system
(GPS) was used to survey each station location with a
repeatability of <12 mm.
To account for time-variant eects on gravity read-
ings such as Earth tides and instrument drift, a base
station reading was recorded every hour during the
survey so that the appropriate corrections could be
made during the data processing stage. A Scintrex CG5
instrument was used to collect the data. The survey was
undertaken in normal working hours. The site was
cleared of surface vegetation. Although construction
works were continuing on site, these were restricted to
areas away from the survey location. Site trac occa-
sionally generated problematic ground vibrations, but
data quality checks undertaken in real time allowed
for aected data to be rejected and the observation
repeated. Repeat readings taken as part of the survey
provided repeatability of less than 8 µGal.
Over the two phases of the project four separate
survey grids were defined to allow for access restrictions
from continuing construction works. Data from Phase 1
were combined with the Phase 2 data to provide a map
with continuous coverage. Continuing site activity made
it impossible to reoccupy the base station location or
any other observation points from the Phase 1 survey,
therefore the relative gravity residual values between the
two phases have been matched by equating the average
readings from each survey grid where they overlap. The
overlap between the survey grids is shown in Figure 4.
Errors between adjacent observation points from the
two phases of the survey are estimated to be less than
14 µGal.
A regional trend was clearly identifiable in the data,
with values increasing or decreasing over a length scale
of 100 m or more. This ‘regional’ trend was calculated
and removed from the data to provide a ‘flatter’ residual
Fig. 4. Location map of survey area located on a section of the
proposed route of the A590 High to Low Newton bypass,
Cumbria, UK.
value map from which local anomalous lows could more
easily be identified.
The data collected from the area to the south of the
access road show consistently higher values than those
for the rest of the survey areas. The data as presented for
this area have been processed separately from those for
the rest of the areas to remove the eect of the regional
dierence, and therefore allowing local lows in the data
to be identified. The resulting map (Fig. 5a) represents
the gravitational variations associated with subsurface
density variations, and is the map from which interpre-
tations can be made.
A number of negative anomalies can be identified in
the data based on their relatively low values compared
with their immediate surroundings, and these are high-
lighted in Figure 5a. The anomalies themselves are
irregular in plan view, suggesting complex ground con-
ditions. This is not unexpected in areas of glacial till,
where the heterogeneous nature of the material can
aect its response to subsidence. The anomalies do,
however, align along a postulated lineament running
approximately north–south through the survey area.
Geological faults strike approximately north–south or
NW–SE through this area, with fault blocks down-
thrown to the west. It is possible that a structural feature
associated with the regional tectonics has in the past
localized the natural processes of void formation.
Borehole data and synthetic gravity model
Further analysis has been made possible by an extensive
programme of drilling undertaken around the central
anomaly (highlighted in Fig. 5). A significant volume of
‘soft’ or ‘broken’ ground has been identified in the
borehole logs. In these logs ‘soft’ ground refers to
low-density soils and glacial till material, and ‘broken’
refers to extensively fractured (low-density) limestone
bedrock. A summary of the borehole logs is provided in
Table 2. The measured microgravity data show two
main lows in the area (Fig. 5a and b). Figure 5c shows
the depth to bedrock determined from the borehole logs,
with simplified stick logs superimposed at each borehole
Fig. 5. (a) Microgravity residual Bouguer anomaly plot for the survey area in case study 2. Inset indicates area detailed in (be). (b)
Detailed view of measured residual Bouguer anomaly data as shown in (a). (c) Contours of depth to bedrock as determined from
borehole data. Local borehole data annotated as stick diagrams at each hole location as marked. Green indicates ‘good’ ground
conditions; red indicates ‘soft’ or ‘broken’ ground. (d) Gravity anomaly predicted by ground model defined by borehole data. (e)
Measured residual Bouguer anomaly plot adjusted to remove the eects of variations in the depth to bedrock.
location indicating good ground (green) and low-density
ground (red).
If a density contrast between the low-density ground
and the surrounding ‘intact’ ground is assumed then it is
possible to calculate a theoretical Bouguer residual
anomaly pattern that would be predicted to be measured
over the area. The calculations used here are based on a
simple Bouguer slab model (Robinson & Coruh 1988);
they assume a simplified geometry of the subsurface
defined by the borehole logs, and a constant density
contrast, and therefore do not accurately capture the
likely complexity of the ground in the area. They do,
however, provide a useful basis on which to compare
the predicted with the measured gravity values so as
to better understand the accuracy of the measured
gravity map in representing the presence of problem
ground conditions. The density contrast assumed is
, and is the same regardless of whether the
low-density ground is within the soils, glacial till or
Table 2. Summary of borehole data, case study 2
Top of
Base of
Feature type Exploratory
Top of
Base of
Feature type
3 0 15 Soil 3N 0 13.5 Soil
15 15.1 Bedrock 13.5 14.5 Broken ground
3A 0 14 Soft ground 14.5 15.4 Soil
14 14.1 Bedrock 15.4 15.5 Bedrock
3B 0 10.5 Soil 3O 0 9 Soil
10.5 11 Broken ground 9 9.6 Broken ground
11 11.1 Bedrock 9.6 14.3 Soil
3C 0 3 Soil 14.3 14.4 Bedrock
3 11 Soft ground 3P 0 12.9 Soft ground
11 11.2 Soil 12.9 15.4 Soil
11.2 11.3 Bedrock 15.4 17.5 Bedrock
3D 0 12.9 Soil 3Q 0 10.8 Soil
12.9 13 Bedrock 10.8 10.9 Bedrock
3E 0 7 Soil 3R 0 5.9 Soil
7 9.9 Soft ground 5.9 7.2 Soft ground
9.9 10.5 Broken ground 7.2 10.6 Soil
10.5 13.1 Soil 10.6 11.1 Broken ground
13.1 21.9 Broken ground 11.1 11.2 Bedrock
21.9 22 Bedrock 3S 0 7 Soil
3F 0 11.4 Soil 7 8.3 Soft ground
11.4 11.5 Bedrock 8.3 11 Soil
3G 0 8 Soil 11 11.8 Broken ground
8 10 Soft ground 11.8 13.1 Soil
10 10.1 Soil 13.1 13.2 Bedrock
10.1 10.2 Bedrock 3T 0 7 Soil
3H 0 5.1 Soil 7 22.5 Soft ground
5.1 10 Soft ground 22.5 22.6 Bedrock
10 12.1 Soil 3U 0 8.2 Soil
12.1 21.5 Broken ground 8.2 13.3 Soft ground
21.5 21.6 Bedrock 13.3 13.4 Bedrock
3I 0 10.2 Soil 3V 0 11.6 Soil
10.2 11.9 Soft ground 11.6 19.5 Broken ground
11.9 16 Broken ground 19.5 19.6 Bedrock
16 16.1 Bedrock 3W 0 13.1 Soil
3J 0 6 Soil 13.1 13.2 Bedrock
6 9.5 Soft ground 3X 0 9.3 Soil
9.5 14 Soil 9.3 10.2 Soft ground
14 14.1 Bedrock 10.2 16.7 Soil
3K 0 14.4 Soil 16.7 19.7 Broken ground
14.4 14.5 Bedrock 19.7 19.8 Bedrock
3L 0 11.5 Soil 3Y 0 7.6 Soil
11.5 11.6 Bedrock 7.6 12 Soft ground
3M 0 15.1 Soil 12 14 Soil
15.1 15.2 Bedrock 14 14.1 Bedrock
The predicted anomaly map is shown in Figure 5d.
Lows are located in approximately the same positions as
those identified in the measured data; however, the
predicted amplitude of the low centred on observation
point 18.1 is significantly greater than that observed.
Most notable is that whereas borehole 3T located a
significant thickness (15.5 m) of low-density ground ex-
tending from a depth of 7 mbgl the predicted gravity
anomaly is lower in amplitude than that calculated for
borehole locations 3A and 3P, with a thickness of
low-density ground of 14 m and 13 m, respectively,
extending from the surface. This highlights the eect of
the depth of the feature on the gravitational signature.
Features at or near surface will have a greater signature
in the gravity anomaly map than equivalent features at
greater depths. Although this depth eect is well known,
these results emphasize the need to provide a careful
analysis of the data as an interpretation of the gravity
signal to the engineer.
A further influence on the measured gravity values is
the depth to bedrock. Assuming that the overlying
glacial till has a lower density than the underlying
bedrock, then a deepening of the depth to bedrock will
manifest in the data as a relative low. The measured
depths to bedrock are shown in Figure 5c. The depth
varies considerably within the area defined by the group
of boreholes, from 10 to 22 m.
It is possible to estimate the eect of these variations
on the measured gravity data using equivalent calcula-
tions to those undertaken above to predict the signature
of low-density ground. Figure 5e shows the measured
residual gravity anomaly map adjusted to correct for the
calculated eect of variations in the depth to bedrock. A
density contrast of 0.2 g cm
was used in the calcula-
tions. The locations of the low anomalies have not
changed significantly from the uncorrected data (Fig. 5a
and b) but relative values have changed. Most sig-
nificantly, the magnitude of the anomaly centred on
observation point 18.1 has increased. As a result, the
corrected anomaly map shows a closer agreement with
the predicted anomaly distribution (Fig. 5d).
In the context of this location, the principal factor
aecting the ability to detect poor ground is the depth at
which the aected ground lies beneath the surface. In
this example relatively large volumes of soft or broken
ground present at depth have been shown to manifest as
relatively low-amplitude negative anomalies in the
measured microgravity data. Although still detectable
by the microgravity technique, these features are super-
imposed on larger signals attributable to shallow ground
conditions. This produces a complex picture that re-
quires careful consideration by the geophysicist, and
also clear communication to the engineer. The detailed
borehole investigation of one of the major anomalies
identified in this dataset allowed, through some simple
and quick calculations, a link to be demonstrated be-
tween the proven ground conditions and the micro-
gravity survey results.
We note that more sophisticated data analysis tech-
niques are available, and may be appropriate in other
circumstances; for example, the removal of the gravita-
tional eects of above-ground structures and buildings.
However, these would be inappropriate in cases such as
those discussed here, where the major limitation on the
accuracy of a ground model is the spacing of reliable
borehole data.
Complex ground conditions comprising heterogeneous
soils such as made ground or glacial till in addition to
low-density and voided areas may give rise to irregular
and complex anomaly profiles that may subsequently be
verified by targeted intrusive investigations. Should it be
necessary or useful for the engineering requirements of
the project to further analyse complex gravity anomalies
it is possible to unravel these signals by calculating
and summing anomaly profiles from simple geometric
Based on the analysis of the two microgravity datasets
described here, under the scrutiny of extensive follow-up
intrusive investigations, it has been possible to demon-
strate that the microgravity technique is capable of
locating low-density ground associated with natural
voiding processes. The method is eective even where
the causative void is either no longer present or lies at
some greater depth and is not itself detected by the
microgravity observations.
The technique has an established record of the detec-
tion of large natural and man-made voids at depth. It
has been demonstrated here that, provided the feature is
suciently close to the surface, it should be possible to
use microgravity techniques. It is therefore a valid and
useful technique for the detection of near-surface voids
and low-density ground.
The use of microgravity has, on some occasions, been
hampered in commercial settings by its relative expense
per unit area compared with other techniques; however,
it should be given proper consideration by the project
engineer. Other geophysical techniques (e.g. ground
penetrating radar, EM ground conductivity, resistivity
imaging, magnetic gradiometry, and seismic refraction
and stiness measurements, to name a few) may provide
a more cost-eective method of covering large areas. An
informed consideration of all available methods is im-
portant; however, we emphasize that microgravity is, in
essence, the only non-intrusive technique that directly
measures the principal defining property of a void: the
absence of mass. For this reason, it should be considered
at the earliest stages of a project. Decisions on the
investigative programme should be based on a careful
analysis of the relative risk (and economic cost) of using
a less reliable method of non-intrusive investigation.
Where the preferred course is an extensive programme
of boreholes, consideration should be given to undertak-
ing a microgravity survey to identify areas of ground to
target and a comparison made with the cost of obtaining
equivalent coverage and detail of information using
boreholes alone. A ‘geophysics first, boreholes later’
approach is likely in many cases to provide the most
detailed and cost-eective method to investigate the
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... Microgravity surveying (detection of signals from subsurface features of a few microGals) is widely used commercially for mineral exploration, environmental studies and the detection of subsurface voids, mine workings, sinkholes and other subsurface hazards (Hinze 1990, Nabighian et al. 2005, Tuckwell et al. 2008. It is, especially useful for deeper void targets where ground conditions can make the use of other active geophysical techniques difficult as the depth of investigation is limited by signal attenuation, for example when using Ground Penetrating Radar (GPR) in conductive ground. ...
... Microgravity survey involves taking a series of discrete points over the survey site using a precision gravimeter instrument to collect a measurement of the local gravitational field (e.g. Debeglia and Dupont 2002, Tuckwell et al. 2008, Kaufmann 2014, Martínez-Moreno et al. 2016. After applying corrections to remove the influence of terrain and other unwanted signals such as the tides, the resulting map is proportional to the underlying mass distribution of the ground. ...
... However, the instruments are also affected by a broadband range of frequencies from other seismic modes and anthropogenic vibrational sources (Goncharenko et al. 2018). In surveying practice, the higher frequency acceleration noises (> 0.1 Hz) are removed by using a long measurement window (30-60 s) for each measurement cycle to average the effects out by measuring several cycles of the noise, whereas longer period diurnal noise sources as well as the drift of the instrument (caused by the slow relaxation of the quartz spring) are removed by making repeated measurements at a single point (base station) over the course of the survey day and removing the underlying trends (Gabalda et al. 2003, Tuckwell et al. 2008. One common estimation of the error for a single cycle of microgravity measurement is the root mean square (RMS) error, which can be estimated using equation 1 (Scintrex Ltd. 2006). ...
Microgravity surveying is a widely used geophysical method, especially for detection of deeper features which are difficult to detect with other techniques. However, the accuracy of the readings is strongly affected by the amount of microseism noise, which is typically rejected by using long integration times for each measurement cycle. Large seismic events such as earthquakes create large ground acceleration waves which have been known to affect gravity readings. While previous data which have been collected serendipitously, tend to only record the final readings, unique long-duration datasets during and after three earthquakes including the raw data (sampled at 6–10 Hz) from field gravimeters (Scintrex CG-5 and CG-6) in static locations recorded during surveys are presented. The aim of the current study is to characterise both long and short-term effects of the generated seismic accelerations on measurement accuracy and repeatability, and identify changes to commercial practice to mitigate these effects. During earthquakes, the nature of the microseism noise was fundamentally altered by each of the different associated seismic waves. Minor effects were found for body P- and S-waves, but much larger effects were found for surface waves, especially the Rayleigh waves which gave errors many times those which would occur in normal conditions. These accelerations persisted in the data for several hours or even days after the earthquake affecting instrument performance. The main finding is that the optimum course of action is to identify the earthquake early by analysing the data in the frequency domain through FFT, switch to 60 s cycles and return to the base station until the strongest waves have passed. Surveying on subsequent days was affected by lower frequency free earth oscillations requiring removal of the unwanted signals using linear trends between at least hourly base stations. Using these techniques will both facilitate data collection as well as improve data confidence in these challenging conditions. Instruments operating in a gradiometer configuration were shown to be comparatively unaffected by the increased noise for typical commercial integration times paving the way for the next generation of instruments to operate successfully, even with this challenging environmental noise.
... Whilst work has been carried out to quantify the scale of the soil noise for magnetic and electromagnetic measurements (Hendrickx et al., 2001;Van Dam et al., 2004), no work has been conducted to quantify variations in near surface density which affect measurements with gravity instruments. Current spring based gravimeters (such as the Scintrex CG-5 and CG-6) have a nominal resolution of 1 μGal and 0.1 μGal respectively, but in practical applications are typically only capable of a practical resolution of ±5 μGal with realistic integration times in field conditions (Boddice et al., 2018;Jiang et al., 2012;Tuckwell et al., 2008) due to the presence of environmental and instrumental noise. This means the contribution of soil density variations to the recorded gravity map is too small to be concerned with in current microgravity surveys, and separating these effects from the instrument and other environmental sources of noise which limit the practical resolution for feature detection is difficult. ...
Natural density variations in the near surface soil (i.e. the top 5 m) cause variations in the values recorded by geophysical surveys undertaken with gravity instruments. Whilst this ‘soil noise’ is too small to be noticeable with current instruments (e.g. Scintrex CG-5 and CG-6), the future use of more accurate instruments such as quantum technology gravity sensors, especially if used in a gradiometer configuration makes this noise source more significant and in need of characterisation and quantification. This paper reviews the magnitude and distribution of density variations in the near surface using data from the British Geological Survey (BGS) national geotechnical properties database which is then used to quantify the effect on practical gravity measurements in computer simulations. The desk study identified that the scale of density variation in the near surface was typically within a range of 600–900 kg/m³, and showed no obvious relationship with underlying geology, superficial deposits or depth below the surface. The distribution of density varied, from normally distributed to between normal and uniform or bimodal distributions. The forward modelled computer simulations showed a significant impact on the measurements of gravity if new instruments can reach greater levels of accuracy, especially for gravity gradient instruments. Analysing possible methods of suppressing this noise source through the design of gravity gradient instruments showed that, although increasing the height of the instrument above the ground is almost twice as effective at decreasing the scale of the soil noise, increasing the sensor vertical spacing may be the preferred option. This is due to relaxed sensitivity requirements on the new sensors and the preservation of the noise in shorter signal wavelength bands than the targets of interest, which not only reduces the cases of mistaken features of interest but also provides the possibility of spatial filtering to be used in order to enhance the signals from targets of interest.
... The Gal is an older unit for gravity acceleration in the CGS unit system, still used in gravimetry; 1 Gal = 10 −2 m s −2 . In different branches of geological prospection there exist many successful applications of microgravimetry for the detection of shallow density inhomogeneities (Arzi, 1975;Beres, Luetscher, & Olivier, 2001;Butler, 1984;Castiello, Florio, Grimaldi, & Fedi, 2010;Colley, 1963;Fajklewicz, 1976Fajklewicz, , 1983Mrlina, 1994;Pivetta & Braitenberg, 2015;Styles, McGrath, Thomas, & Cassidy, 2005;Styles, Toon, Thomas, & Skittrall, 2006;Tuckwell, Grossey, Owen, & Stearns, 2008;etc.). In non-destructive archaeological prospection, for subsurface voids/cavities (crypts, larger tombs, cellars, tunnels, etc.), a large number of important achievements have been published (Abad et al., 2007;Barton, Pašteka, Zahorec, Papčo, & Brady, 2011;Cuss & Styles, 1999;Eppelbaum, 2011;Fais, Radogna, Romoli, Matta, & Klingele, 2015;Issawy, Mrlina, Radwan, Hassan, & Sakr, 2002;Lakshmanan & Montlucon, 1987;Mrlina, Křivánek, & Majer, 2005;Sarlak & Aghajani, 2017;Slepak, 1997; and many others). ...
Detailed and precise measurement of the Earth's gravity field (microgravity method) can be effectively used for the detection and quantification of subsurface voids and/or cavities. There exist a variety of successful applications of the microgravity method in near surface geophysics, namely in geotechnical, environmental and archaeological prospection. Using state‐of‐the‐art ‘microgal’ relative gravity meters, cavities of several metres in each dimension (positioned at a similar depth) can be detected and interpreted. Such objects produce negative anomalies with amplitudes of several tens of microGals (1 microGal = 10−8 m s−2). This contribution is focused on a methodological overview of the most important acquisition and processing steps in archaeological microgravimetry. In the processing of acquired gravimetrical data into a Bouguer anomaly, the so‐called building correction plays an important role, because the gravitational effect of building masses can produce false, usually negative anomalies. Several selected methods for quantitative interpretation are presented, these are based on depth estimation and density modelling. These interpretation methods give satisfactory results in the case of these type of negative anomalies that are caused by subsurface cavities. Microgravimetry can obtain good support from electromagnetic and electrical methods, mainly from ground penetrating radar and electrical resistivity tomography, respectively. Finally, we present successful case‐studies of microgravimetrical detection of crypts in various churches from the Middle Ages and period of Modern History, surveyed during recent decades in Slovakia and Czechia.
... Near-surface geophysical surveys have been shown to be highly useful to characterize ground conditions for geotechnical site investigations; for example, to characterize sedimentary layer thicknesses (Mellett 1995;Williams et al. 2005;Mitrofan et al. 2008;Mohamed et al. 2012) or low-density ground (Tuckwell et al. 2008), and to locate cracks and joints (Cardimona 2002), fractures and faults (Mohamed et al. 2012;Baek et al. 2017), landfills (Wang et al. 2014), cavities (Orlando 2013), and mines and mineshafts (Banham and Pringle 2011;Pringle et al. 2012). ...
There has been significant structural damage of newly-built residential buildings in Quarter-27 District in the South of Cairo, Egypt. The proximity of an active limestone quarry may also be affecting ground stability. This paper shows how a near-surface geophysics survey could characterize the site, unusually after the initial housing construction had already been undertaken. Geophysical surveys included seismic refraction (acquired between phases of quarry blasting), electrical resistivity and ground penetrating radar 1D and 2D datasets. Geophysical results produced maps of a 3D ground model that also included water table depth, known major faults and a saturated layer that may have caused the building damage. ERT and GPR data was deemed optimal of the geophysical techniques trialled. This study shows that it is possible to undertake geophysical surveys to characterize a restricted urban site development.
... The contour or line data combined with other data like a polygon in the form of coloring.Microgravity method solves problems targeted by the present work is the presence of voids or cavities underneath the sites under the survey area. According to the previous research(Tuckwell, Grossey, Owen, & Stearns, 2008) that micrografity establihed as technique for detection natural or man-made cavilities. In the case Mounchel Parkman on behalf of Herfordshire Country Council, a doline had opened up within a school playground. ...
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A microgravity investigation on bedrock topography was conducted at Maluri park reference level in Kuala Lumpur, Malaysia. The study aim to mapping the near-surface structure and soil and land cover distribution for geography and geophysics surveys. Two types of cross-section modeling of the residual anomaly generated the MaluriBouguer Anomaly model for site-1 and site-2 at Maluri Park. The 2D microgravity models produced the contour map, displaying the characterization due to density contrast in rock types while mapping the subsurface geological structure at different depths. Moreover, a synthetic model was initiated with the assumption of lateral distance on the left and right sides taken at 50 m and a depth of 60 m. The results of modeling confirmed that the soil and rock type composition on both models site tests are topsoil (1.1 to 1.92 g/cm3), soil (1.8 g/cm3), clay (1.63 g/cm3), gravel (1.7 g/cm3), sand (2.0 g/cm3), shale (2.4 g/cm3), sandstone (2.76 g/cm3), and limestone (2.9 g/cm3). The 2D gravity modeling using two model site tests obtained a correspondence with the observed microgravity data. Keywords: Bouguer anomaly, limestone, microgravity, soil structure, topography. References Amaluddin, L. O., Rahmat, R., Surdin, S., Ramadhan, M. I., Hidayat, D. N., Purwana, I. G., & Fayanto, S. (2019). The Effectiveness of Outdoor Learning in Improving Spatial Intelligence. Journal for the Education of Gifted Young Scientists, 7(3), 667–680. Arisona,A., Mohd N., Amin E.K., &Abdullahi, A.(2018).Assessment of microgravity anomalies of soil structure for geotechnical 2d models.Journal of Geoscience, Engineering, Environment, and Technology (JGEET)3(3), 151-154. Georgsson, L.S. (2009). Geophysical Methotds Used in Geothermal Exploration. Presented at Exploration for Geothermal Resources, 1-22 November 2009, 1-16. Grandjean, G. (2009). From Geophysical Parameters to Soil Characteristics.Florida: Report N°BRGM/FP7-DIGISOIL Project Deliverable 2.1, Final ReportDepartment of Civil and Coastal EngineeringUniversity of Florida. Hiltunen, D.R., Hudyma,N.,Tran,K.T.,&Sarno,A.I. (2012).Geophysical Testing of Rock and Its Relationthipsto Physical Properties.Florida:Final ReportDepartment ofCivil and Coastal EngineeringUniversity ofFlorida. Kirsch,R. (2006).GroundwaterGeophysics, ATool for Hydrogeology.New York: Springer. Kamal,H.,Taha,M.,&Al-Sanad,S. (2010). Geoenvironmental Engineering and Geotechnics, GeoShanghai 2010 International Conference. (accessed 02.03.17) Lilie, R.J. (1999).Whole Earth Geophysics: An Introductory Textbook for Geologists and Geophysicists. New Jersey:Prentice-HallInc. Pringle, J.K., Styles, P., Howell, C.P.,Branston, M.W., Furner, R., &Toon,S.M. (2012). Long-term time-lapse microgravity and geotechnical monitoring of relict salt mines, marston, cheshire, uk. Geophysic77(6), 165-171. Samsudin, H.T.(2003).A microgravity survey over deep limestone bedrock.Bulletin of Geological Society of Malaysia4(6), 201-208. Tan, S.M. (2005). Karsticfeatures of kualalumpur limestone. Bulletin of the Institution of EnginnerMalaysia 4(7), 6-11. Tajuddin, A.&Lat, C.N. (2004).Detecting subsurfacevoids using the microgravity method, a case study from kualalipis, pahang.Bulletin of Geological Society of Malaysia 3(48), 31-35. Tuckwell, G., Grossey, T., Owen, S., & Stearns, P. (2008). The use of microgravity to detect small distributed voids and low-density ground. Quarterly Journal of Engineering Geology and Hydrogeology, 41(3), 371–380. Wanjohi, A.W. (2014). Geophysical Field Mapping. Presented at Exploration for Geothermal Resources, 2-23 November 2014, 1-9. Yusoff , Z.M., Raju,G. &Nahazanan, H.(2016).Static and dynamic behaviour of kualalumpur limestone. Malaysian Journal of Civil Engineering Special Issue Vol.28 (1), p.:18-25. Zabidi, H. & De Freitas, M.H. (2011).Re-evaluation of rock core logging for the prediction of preferred orientations of karst in the kualalumpur limestone formation. Engineering Geology, 117(3-4), p.: 159–169. Copyright (c) 2019 Geosfera Indonesia Journal and Department of Geography Education, University of Jember This work is licensed under a Creative Commons Attribution-Share A like 4.0 International License
... Depends of the size, density contrast and depth. Compactness Tuckwell et al. (2008) use this method to characterize low-density ground. ...
Fluvial levees are assets of major importance for protecting human lives and activities. To prevent the risk of failures and to assess their stability, survey campaigns involving geophysical and geotechnical methods are needed. These two types of methods play highly complementary roles and provide information of different natures, uncertainties and spatial distributions. In this work, we first present the function of fluvial levees and describe the associated physical properties that may become pathological and lead to failure mechanisms. Then for each presented investigation method, we introduce : its basic principles, its ability to characterize specific physical properties (or characteristics) of fluvial levees, and then its advantages and drawbacks under specific environmental conditions. It should be emphasized that geophysical and geotechnical methods replaced on an equal footing in order to assist agencies and levee managers in selecting the most relevant ones to assess the condition of their levees and characterize potential weak zones, according to a specific type of pathology and to the environmental constraints. Throughout this review, we introduce a certain amount of literature that is believed to be of potential interest to the research community as well. We also draw attention to some lack of information on the application of specific methods to levee investigation. Finally, we discuss some complementarities of the presented methods and we propose a dedicated comparative table, leading us to suggest new method combinations for levee investigation.
... Some multidisciplinary field studies include gravimetry (e.g. Patterson et al., 1995;Tuckwell et al., 2008;Dahm et al., 2010;Ezersky et al., 2013;Kaufmann, 2014;Pazzi et al., 2018) but focus structural interpretations of the Bouguer anomaly above and around assumed subrosion features. Hence, and with sparse exceptions (Lambrecht et al., 2005;Benito-Calvo et al., 2018), the majority of the mentioned ground-based methods are applied to localize sinkholes, image the actual state of sinkhole development, and concentrate on their spatial extent or physical parameters at a certain point of time. ...
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We present results of sophisticated, high-precision time-lapse gravity monitoring that was conducted over 4 years in Bad Frankenhausen (Germany). To our knowledge, this is the first successful attempt to monitor subrosion-induced mass changes in urban areas with repeated gravimetry. The method provides an approach to estimate the mass of dissolved rocks in the subsurface. Subrosion, i.e. leaching and transfer of soluble rocks, occurs worldwide. Mainly in urban areas, any resulting ground subsidence can cause severe damage, especially if catastrophic events, i.e. collapse sinkholes, occur. Monitoring strategies typically make use of established geodetic methods, such as levelling, and therefore focus on the associated deformation processes. In this study, we combine levelling and highly precise time-lapse gravity observations. Our investigation area is the urban area of Bad Frankenhausen in central Germany, which is prone to subrosion, as many subsidence and sinkhole features on the surface reveal. The city and the surrounding areas are underlain by soluble Permian deposits, which are continuously dissolved by meteoric water and groundwater in a strongly fractured environment. Between 2014 and 2018, a total of 17 high-precision time-lapse gravimetry and 18 levelling campaigns were carried out in quarterly intervals within a local monitoring network. This network covers historical sinkhole areas but also areas that are considered to be stable. Our results reveal ongoing subsidence of up to 30.4 mm a⁻¹ locally, with distinct spatiotemporal variations. Furthermore, we observe a significant time-variable gravity decrease on the order of 8 µGal over 4 years at several measurement points. In the processing workflow, after the application of all required corrections and least squares adjustment to our gravity observations, a significant effect of varying soil water content on the adjusted gravity differences was figured out. Therefore, we place special focus on the correlation of these observations and the correction of the adjusted gravity differences for soil water variations using the Global Land Data Assimilation System (GLDAS) Noah model to separate these effects from subrosion-induced gravity changes. Our investigations demonstrate the feasibility of high-precision time-lapse gravity monitoring in urban areas for sinkhole investigations. Although the observed rates of gravity decrease of 1–2 µGal a⁻¹ are small, we suggest that it is significantly associated with subterranean mass loss due to subrosion processes. We discuss limitations and implications of our approach, as well as give a first quantitative estimation of mass transfer at different depths and for different densities of dissolved rocks.
Geologically, the Marlagalla area represents the southern part of NNE-SSW trending Nagamangala schist belt, which is a member of ancient supracrustals of Sargur Group and situated in the southern part of Western Dharwar Craton. The area is mostly soil cover with limited exposure of amphibolite/hornblende schist and intrusive pegmatite. Pegmatites occur as lensoidal bodies within the amphibolite. Microgravity surveys are attempted for the first time to detect pegmatites within the amphibolite. The density between pegmatite (Density= 2.58–2.61 gm/cc) and host rock amphibolite (Density=3.09–3.14 gm/cc) gives detectable contrast to identify pegmatite within amphibolite. Gravity data are acquired over a grid of 5m × 30m using CG-5 Autograv gravimeter which has 1 µGal sensitivity. Elevation data are acquired using SP80 post processing DGPS instrument which has 5 mm elevation accuracy. Analysis of residual gravity and first vertical derivative (FVD) filters facilitated in delineating the pegmatite bodies. The pegmatite bodies show low gravity anomalies with an amplitude range of −5 to −200 µGal. FVD map of the residual gravity indicated four major N100E trending linear anomalies of −36 to 3.0 µGal/m corresponding to pegmatite bodies of varying strike (100–200m) and width (10–15m). 2D-Forward modeling of microgravity data helped in deriving the subsurface geometry of the pegmatites. Borehole drilled subsequently based on the results of microgravity surveys intercepted pegmatites within the amphibolite. Thus, the technique has proved for exploration of pegmatite in contrasting environment of amphibolite. Pegmatite are known for rare metals and lithium minerals. Indirectly the model proposed here will help in exploration of rare metals and Li bearing pegmatite.
The characterization of shallow subsurface formations is essential for geological mapping and interpretation, reservoir characterization, and prospecting related to mining/quarrying. To analyze elastic and electromagnetic properties, we characterize near-surface chalk formations deposited on a shallow seabed during the Late Cretaceous–Early Paleogene (Maastrichtian-Danian). Electromagnetic and elastic properties, both of which are related to mineralogy, porosity, and water saturation, are combined to characterize the physical properties of chalk formations. We also perform rock physics modeling of elastic velocities and permittivity and analyze their relationships. We then use measured ground penetrating radar and P-wave velocity field data to determine the key model parameters, which are essential for the validity of the models and can be used to evaluate the consolidation degree of the rocks. Based on the models, a scheme is developed to estimate the porosity and water saturation by combining the two rock physics templates. The predictions are consistent with previous findings. Our templates facilitate fast mapping of near-surface porosity and saturation distributions and represent an efficient and cost-effective method for near-surface hydrological, environmental, and petrophysical studies. In the current formulation, the method is only applicable to rock type (chalk) comprising a single mineral (pure calcite). It is possible to tailor the formulation to include more than one mineral; however, this will increase the uncertainty of the results.
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Peter Street is an area of terraced houses in Northwich suffering from subsidence, thought to be related to salt extraction in the 19th century. Microgravity and resistivity profiling have been used as non-invasive techniques to investigate the cause of this subsidence. Repeat (or time-lapse) microgravity has been used to assess the stability and evolution of the low-density areas. Time-lapse microgravity uses the characteristics of anomaly size and gradient to track the development of cavities as they propagate to the surface. It is possible to monitor the change in gravity with time and to model the increase in cavity volume and/or depth. A gravity low was found to be coincident with the area experiencing subsidence. Integrated modelling techniques including Euler deconvolution, Cordell and Henderson inversion and GRAVMAG modelling have been used to investigate the depth and size of the body responsible for this anomaly. Resistivity imaging has been used to investigate the conductivity of the near surface and constrain the gravity models. Results from both techniques suggest that low density ground is now present at a depth of 3-4 m below the surface in the subsidence affected area. The use of time-lapse microgravity has shown that there has been an upward migration of a low-density zone at gravity anomaly C over the monitoring period.
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Measurements of the geomagnetic field can be used to help determine the structure of the Earth, since rocks often contain magnetic minerals. Measurements of the strength of the Earth's gravitational field can also be used, since the more dense the rocks, the stronger the field. The interpretation of large amounts of data can be aided by "automatic" interpretation techniques such as Euler deconvolution. This considers the anomalies to be caused by many relatively simple sources (such as dipoles or lines of dipoles) and produces the positions and depths of these sources. This data can then be used as a basis for a more detailed interpretation.
The presence of mining-related cavities (workings, shafts and tunnels) or karstic (solution cavities and sinkholes in limestone) within the top 100 m in the rock mass restricts land utilisation, and their migration to the surface may damage property or services or cause loss of life. Confirmation of features marked on existing plans prior to design and construction may be sufficient but it is often necessary to determine the detailed sub-surface structure. The standard method of site investigation is to drill a pattern of boreholes to locate the spatial extent of any cavities. However, unless the spacing is less than the cavity dimensions it is possible to miss it completely. A cavity may be filled with air, water, or collapse material resulting in a contrast in physical properties which may be detected using appropriate geophysical methods. One powerful technique is microgravity which locates areas of contrasting sub-surface density from surface measurements of the earth's gravity. Although the method is fundamentally simple, measurement of the minute variations in gravity (1 in 108) requires sensitive instruments, careful data acquisition, and data reduction and digital data analysis. Final interpretation must be performed in conjunction with independent information about the site's history and geology. This paper presents three examples in both mining and karstic environments demonstrating that microgravity is a very effective technique for detecting and delineating cavities in the sub-surface.
Introduces geophysical methods used to explore for natural resources and to survey earth structure for purposes of geological and engineering knowledge. These methods include seismic refraction and reflection surveying, gravity and magnetic field surveying, electrical resistivity and electromagnetic field surveying, and geophysical well logging. Modern field procedures and instruments are covered, as well as data processing and interpretation methods, including graphical methods. All basic surveying methods are described step-by-step, and illustrated by practical examples. -from Publisher
This paper is part of the special publication No.165, Geoarchaeology: exploration, environments, resources. (eds A.M. Pollard) This article presents the results of a high-resolution microgravity survey which was successful in delineating the 150-year old Williamson tunnels beneath inner-city Liverpool, England. The tunnels, which date from the Napoleonic Wars, lie at depths of c. 5 to 15 metres, and are poorly mapped because of several phases of later development and subsequent dereliction. The 'brown-field' nature of this site created substantial noise signals which required the application of careful terrain corrections and second derivative and Euler deconvolution methods to isolate and identify the tunnel signatures. Succcessful delineation was only possible because of a comprehensive initial desk study and prior modelling, which assessed the likely depths and conditions of the impassable tunnels and allowed appropriate techniques and survey parameters to be determined.
The detection of subsurface cavities, including mine workings, mine shafts and solution features, is an essential component of any site investigation for major civil engineering works and often relies on drilling investigations to identify the presence of any cavities. However, there is no standard, cost-effective site investigation technique which can be readily used for the physical investigation of such features. Whilst a desk study may yield documentary information on the presence of recorded mine workings and shafts, the location of solution features is generally even more problematical. Two complementary approaches have been developed for the location of subsurface cavities. Firstly, closely spaced boreholes are drilled in a specific pattern to locate cavities. This method can prove prohibitively expensive, with no guarantee of intersecting all voids or cavities. Secondly, remote sensing geophysical techniques have been used. Such techniques rely on the existence of contrasts in physical properties between the rock mass and the cavities, which can be detected using suitable geophysical methods. This paper describes the application of the microgravity technique to the detection of solution cavities and mine workings with reference to three case histories. In the first and second examples the microgravity technique was used as a reconnaissance method for defining targets for subsequent physical investigation; in the third, the technique was used to define the extent of solution features, having been initially and unexpectedly encountered by a drilling programme. These examples demonstrate the applicability of the microgravity method in detecting and delineating both solution cavities and mine workings within differing geological settings.
Microgravity is the interpretation of changes in the subsurface density distribution from the measurement of minute variations in the gravitational attraction of the Earth. As a technique, it is particularly suited to the investigation of subsurface structures, mapping of geological boundaries and, most importantly in this case, the location and characterization of voids or cavities. Gravity variations due to the geological/petrophysical changes associated with fracturing and changes in pore composition are superimposed upon much larger variations due to elevation, latitude, topography, Earth tides and regional geological variations. However, these external changes can be modelled or monitored with sufficient accuracy to be removed from the data. With the recent development of high-resolution instruments, careful field acquisition techniques and sophisticated reduction, processing and analysis routines, anomalies as small as 10 microgal can be detected and interpreted effectively. This paper describes the 'state-of-the-art' application of the microgravity technique for the detection and characterization of karstic cavities in a variety of limestone terrains, including the Carboniferous Limestone of the United Kingdom and Eire and the coral limestones of the Bahamas. The case study examples show how the recorded gravity anomalies have revealed the location of density variations associated with underground cave systems and, ultimately, provided information on their depths, shapes and morphology from a combined analysis of their spectral content, characteristic gradient signatures and modelling responses. In addition, mass deficiencies have been estimated, directly from the anomaly map, by the use of Gauss's theorem without any prior knowledge of the exact location, or nature, of the causative bodies.
An introduction to geophysical methods used to explore for natural resources and to survey earth's geology is presented in this volume. It is suitable for second-and third-year undergraduate students majoring in geology or engineering and for professional engineering and for professional engineers and earth scientists without formal instruction in geophysics. The author assumes the reader is familiar with geometry, algebra, and trigonometry. Geophysical exploration includes seismic refraction and reflection surveying, electrical resistivity and electromagnetic field surveying, and geophysical well logging. Surveying operations are described in step-by-step procedures and are illustrated by practical examples. Computer-based methods of processing and interpreting data as well as geographical methods are introduced.
Dissolution subsidence affords some of the most difficult ground conditions with which engineering geologists have to deal. Within the UK, areas underlain by gypsiferous Permo-Triassic strata, most notably around Ripon in Yorkshire, are prone to dissolution structures and resultant building failures are well documented. Conventional drilling of such unstable sites is often a 'hit and miss' affair and most geophysical techniques do not provide sufficient resolution to offer adequate confidence in the results. Proposals for the redevelopment of a site within the urban area at Ripon could not rely on such frequently inconclusive methods and it was necessary to implement a phased approach to site investigation. Following a desk study, high-resolution microgravity geophysics was carried out both inside and outside the existing building. This indicated a major negative anomaly of peak amplitude - 74 muGal. Subsequent static core probing, rotary drilling and trial trenching confirmed the existence of a potentially unstable breccia pipe which could therefore be taken into account in the engineering design.
About every three years natural catastrophic subsidence, caused by gypsum dissolution, occurs in the vicinity of Ripon, North Yorkshire, England. Holes up to 35 m across and 20 m deep have appeared without warning. In the past 150 years, 30 major collapses have occurred, and in the last ten years the resulting damage to property is estimated at about £1000000. Subsidence, associated with the collapse of caves resulting from gypsum dissolution in the Permian rocks of eastern England, occurs in a belt about 3 km wide and over 100 km long. Gypsum (CaS0 4 .2H 2 0) dissolves rapidly in flowing water and the cave systems responsible for the subsidence are constantly enlarging, causing a continuing subsidence problem. Difficult ground conditions are associated with caves, subsidence breccia pipes (collapsed areas of brecciated and foundered material), crown holes and post-subsidence fill deposits. Site investigation methods that have been used to define and examine the subsidence features include microgravity and resistivity geophysical techniques, plus more conventional investigation by drilling and probing. Remedial measures are difficult, and both grouting and deep piling are not generally practical. In more recent times careful attention has been paid to the location for development and the construction of low-weight structures with spread foundations designed to span any subsidence features that may potentially develop.