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GEOSCIENCE CANADA
Remote Predictive Mapping 7.
The Use of Topographic−Bathymetric Lidar to
Enhance Geological Structural Mapping in
Maritime Canada
Tim Webster, Kevin McGuigan, Nathan Crowell,
Kate Collins and Candace MacDonald
Applied Geomatics Research Group
Nova Scotia Community College
Middleton, Nova Scotia, B0S 1M0, Canada
Email: tim.webster@nscc.ca
SUMMARY
An airborne topo-bathymetric lidar survey was conducted at
Cape John, on the north shore of Nova Scotia, Canada, using
the shallow water Leica AHAB Chiroptera II sensor. The sur-
vey revealed new bedrock features that were not discovered
using previous mapping methods. A thick blanket of glacial till
covers the bedrock on land, and outcrops are exposed only
along the coastal cliffs and offshore reefs. The seamless land-
seabed digital elevation model produced from the lidar survey
revealed significant bedrock outcrop offshore where ocean
currents have removed the glacial till, a significant finding that
was hitherto hidden under the sea surface. Several reefs were
identified offshore as well as a major fold structure where
block faulting occurs along the limbs of the fold. The exten-
sion of the Malagash Mine Fault located ~10 km west of Cape
John is proposed to explain the local folding and faulting visi-
ble in the submerged outcrops. The extension of this fault is
partially visible on land, where it is obscured by glacial till, and
its presence is supported by the orientation of submerged bed-
ding and lineaments on both the south and north sides of
Cape John. This paper demonstrates how near-shore high-res-
olution topography from bathymetric lidar can be used to
enhance and refine geological mapping.
RÉSUMÉ
Un levé lidar topo-bathymétrique été réalisé à Cape John, sur
la rive nord de la Nouvelle-Écosse, Canada, en utilisant un cap-
teur Leci AHAB Chiroptera II. Ce levé a permis de repérer des
affleurements que les méthodes de cartographie plus anci-
ennes n’avaient pu détecter. Une épaisse couche de till glaciaire
recouvre la roche en place sur le continent, et la roche affleure
seulement le long des falaises côtières et des récifs côtiers. Le
modèle numérique de dénivelé en continu terres et fonds
marins obtenu par le levé lidar a révélé l’existence d’affleure-
ment rocheux considérables au large des côtes, là où les
courants océaniques ont emporté le till glaciaire, une décou-
verte importante demeurée cachée sous la surface de la mer
jusqu’alors. Plusieurs récifs ont été identifiés au large des côtes,
ainsi qu’une structure de pli majeure, à l’endroit où se produit
un morcellement en blocs le long des flancs du pli. Une exten-
sion de la faille de la mine Malagash situé ~ 10 km à l’ouest de
Cape John est proposé pour expliquer les plis et les failles
locaux visibles dans les affleurements submergés. L’extension
de cette faille est partiellement visible sur la terre, voilée par le
till, et sa présence est étayée par l’orientation de la stratification
et des linéaments submergés tant du côté sud que nord de
Cape John. Cet article montre comment la topographie haute
résolution du lidar bathymétrique peut être utilisée pour
améliorer et affiner la cartographie géologique.
Traduit par le Traducteur
INTRODUCTION
In this paper we present the results of offshore coastal map-
ping using airborne topo-bathymetric lidar at Cape John, Nova
Scotia along the Northumberland Strait in the Gulf of St.
Lawrence (Fig. 1). Traditional remote sensing mapping meth-
ods such as aerial photography and boat-based echo sounding
used in the mapping of geological structures on the seabed can
be difficult, time-consuming and expensive to locate. It is gen-
erally assumed that terrestrial outcrops extend underwater;
Cape John is known to have outcrops along the coast but there
Volume 43 2016 199
Geoscience Canada, v. 43, http://www.dx.doi.org/10.12789/geocanj.2016.43.099 pages 199–210 © 2016 GAC/AGC®
SERIES
is little known about the distribution of geologic formations
underwater, nor the fine details of the bathymetry, aside from
a paper chart based on soundings from 1945 (Canadian
Hydrographic Service 1945). At Cape John, the lack of out-
crop on land, except for the coastal cliffs is a result of the dep-
osition of glacial till during the last glacial period; however, the
ability of an airborne sensor to accurately survey the nearshore
bathymetry offers an opportunity to overcome the challenges
of locating offshore exposures using traditional methods by
providing detailed information on geologic structures that
extend across the land-sea boundary.
Bathymetric data are traditionally collected using echo-
sounding techniques in water depths greater than 10 m
(Moustier and Matsumoto 1993; Clarke et al. 1996). However,
these boat-based techniques are expensive and potentially haz-
ardous in shallow water because of a limited survey field of
view and the prospect of running aground. To overcome these
issues, passive and active remote sensing techniques have been
developed for shallow water bathymetry (Hedley and Mumby
2003; Jay and Guillaume 2014). Decker et al. (2011) have com-
pared various methods for mapping bathymetry and water col-
umn properties from passive remote sensing. Hedley and
Mumby (2003) presented a passive remote sensing technique
whereby they modified a spectral unmixing method to calcu-
late depth and substrate type from passive imagery. The tech-
nique requires pure spectral information of the substrate types
very shallow and the water diffuse attenuation coefficients
(Kd) for the site in the same spectral regions. Collecting Kd
values during remote sensing data acquisi-
tion can be challenging, especially as these
values change with conditions, such as
increased turbidity or other water quality
factors. Hedley and Mumby (2003) use
realistic Kd values (0.15−0.25) along with
random and actual spectra to test their
spectral unmixing approach to mapping
benthic cover, and found it to be insensi-
tive to inaccuracies in depth estimation. Jay
and Guillaume (2014) used a maximum
likelihood estimation method for depth
and water quality. Their method assumes
that water column properties are similar
for a group of at least 400 pixels having
similar water clarity conditions, which is
easier with high-resolution hyperspectral
data.
Active remote sensing techniques for
surveying depths utilize airborne topo-
bathymetric lidar systems. Topo-bathymet-
ric lidar works by emitting near-infrared
(NIR) and green laser pulses from an air-
craft and measuring the travel time of the
pulses to and from the land, water surface,
and seabed. The NIR laser pulse reflects
off the land and sea surface, whereas the
green laser pulse is refracted at the air-
water interface, attenuated through the
water column, and reflected from the seabed. Although topo-
bathymetric lidar is a relatively new technology, it has been
proven to be effective for mapping the fine detail of underwa-
ter bathymetry and geologic structures (Kennedy et al. 2008,
2014; Arifin and Kennedy 2011; Collin et al. 2011a, b, 2012;
Coveney and Monteys 2011; Le Gall et al. 2014). The emerging
uses of airborne laser bathymetry (ALB) for coastal research
and coastal management are summarized in Brock and Purkis
(2009). For example, Kennedy et al. (2014) used ALB (with the
Laser Airborne Depth Sounder (LADS) Mk II sensor) and
multibeam echo sounding to examine the erosion of granitic
domes along the coast of southern Australia. Kennedy et al.
(2014) concluded that coastal processes were removing debris
and that the amount of erosion appeared to be related to the
spacing of joints within the granite. They also calculated
rugosity for the bathymetry and, in a series of offshore pro-
files, compared it to the jointing pattern present in the rock.
Similarly, Le Gall et al. (2014) used lidar and multibeam
bathymetry techniques to enhance their structural mapping of
the Variscan basement off the coast of Brittany, France, allow-
ing them to trace lineaments offshore and correlate them with
geophysical maps. Covency and Monteys (2011) examined the
integration of topographic lidar and ALB for coastal research
along the Irish coastline. Others have studied the movement of
offshore sediments, including Arifin and Kennedy (2011), who
examined the evolution of large scale crescentic bars before
and after hurricanes within the Gulf of Mexico, and Kennedy
et al. (2008), who examined ephemeral sand waves in response
200 Tim Webster, Kevin McGuigan, Nathan Crowell, Kate Collins and Candace MacDonald
http://www.geosciencecanada.ca
Figure 1. Overview of Atlantic Canada showing the location of the Cape John study site for the topo-bathymetric
lidar survey. The black outline in A shows the location of B; C is the location of Figure 2, and the red rectangle
in C outlines the area shown in Figure 3.
to hurricane forces in the surf zone off the coast of Florida.
Collin et al. (2011a, b; 2012) used the Optech SHOALS system
to survey a section of the coastal zone in the Gulf of St.
Lawrence in Québec, focusing on benthic habitat.
Previous research has utilized large, deep-water lidar sen-
sors such as the LADS Mk II, SHOALS and Hawkeye systems.
A new generation of commercial, relatively lightweight, shal-
low-water sensors are now available, including the Leica
AHAB Chiroptera II, Riegl VQ-820-G, and Optech Aquarius
systems, allowing surveys to be conducted in smaller aircraft
and at lower costs compared to sensors designed for deeper
water. At the time of this study, the Chiroptera II was the only
sensor that utilizes a NIR laser in combination with a green
laser to map the sea surface; the VQ-820 and Aquarius systems
rely only on a green laser for their operation. Wang et al. (2015)
compared a variety of waveform-processing algorithms for
single-wavelength lidar bathymetry systems and pointed out
that the disadvantage of such systems is the lack of a NIR
channel, leading to difficulties in extracting the water surface.
The Applied Geomatics Research Group at the Nova Sco-
tia Community College recently acquired the Chiroptera II
topo-bathymetric lidar sensor equipped with the Leica RCD30
medium-format 60 megapixel digital camera system capable of
RGB (red-green-blue) and NIR image acquisition and motion
compensation. We surveyed the Cape John area on September
26, 2014 and present here the methods used to process the
lidar and seabed reflectance data, and to construct continuous
elevation models, orthophotos, and a new interpretation of the
structural geology based on the lidar data. Water clarity influ-
ences how deep the green laser will penetrate, and turbidity
management is important for successful surveys. Webster et al.
(in press) report on the influence of variable turbidity, and on
advances in topo-bathymetric lidar data-processing techniques
using multiple datasets collected around the Maritimes, includ-
ing Cape John.
GEOLOGICAL SETTING
Cape John, Nova Scotia is located near the southern flank of
the Maritimes (Carboniferous) Basin and is part of the
Appalachian orogen (Fig. 1). The late Paleozoic evolution of
the Appalachian orogen was profoundly influenced by post-
accretionary motion along terrane boundaries (Williams and
Hatcher 1982). The contact between the two most outboard
terranes, the Avalon and Meguma terranes, can be traced
across mainland Nova Scotia as the Minas Fault Zone (Cobe-
quid−Chedabucto Fault System; Fig. 2). The Cobequid–Ched-
abucto Fault is interpreted to have had a history of recurrent
movement that records important episodes of Late Paleozoic
relative motion during the later tectonic phases of the
Appalachian orogen (Keppie 1982; Mawer and White 1987;
Murphy et al. 2011). Upper Devonian to Upper Carboniferous
sedimentary rocks occur on both the Avalon and Meguma ter-
ranes and are generally considered to represent an overstep
sequence across the boundary between the two terranes. The
Cape John study area is located within the Cumberland Sub-
basin of the Maritimes Basin.
The Cumberland Subbasin is underlain by a thick accumu-
lation of Lower Carboniferous to Lower Permian strata
assigned to the Cumberland and Pictou groups in northwest-
ern Nova Scotia and southeastern New Brunswick (Ryan and
Boehner 1994) (Figs. 2, 3). It is bordered to the north by the
Northumberland Strait and to the south by the Cobequid
Highlands, which in turn is bordered to the south by the Cobe-
quid−Chedabucto Fault (Fig. 2). The internal structure of the
Cumberland Basin is generalized as a broad east- to northeast-
trending synclinorium that is bounded by the parallel, diapiric,
Claremont and Scotsburn anticlines; major synclines in the
basin include the Tatamagouche and Wallace synclines (Ryan
and Boehner 1994) (Figs. 2, 3). The Tatamagouche syncline
and Claremont anticline (Figs. 2, 3) are genetically related and
caused by tectonically driven diapirism (halotectonic; Ryan and
Boehner 1994). The axis of the Tatamagouche Syncline trav-
erses the Cape John study area, which is underlain by Upper
Carboniferous rocks of the Pictou Group, consisting predom-
inantly of continental clastic material deposited in fluvial and
lacustrine settings (Gibling and Martel 1996) (Figs. 2, 3).The
Pictou Group rests either disconformably or with angular
unconformity over older Carboniferous strata of the Cumber-
land Group (Fig. 3). Younger rocks that may have overlain the
Pictou Group are not preserved.
The major structural features of the region are the Cobe-
quid−Chedabucto Fault and related faults such as the North
Fault, which forms the boundary between the Cobequid High-
lands and the Cumberland Basin (Ryan and Boehner 1994)
(Fig. 2). The Cobequid−Chedabucto Fault is characterized by
recurrent dextral strike-slip movement between the middle
Devonian and early Permian (Mawer and White 1987; Webster
and Murphy 1998; Murphy et al. 2011). Late Carboniferous
activity along the fault zone was responsible for polyphase
deformation (Nance 1987; Waldron et al. 1989) and the gener-
ation of local pull-apart basins (Bradley 1982; Yeo and Ruixi-
ang 1987; Murphy and Keppie 1998). The North Fault has a
complex history; it includes a series of splays that displaced
Upper Carboniferous strata in the basin, whereas other units
overlap the fault, whose trace has been interpreted through the
use of remote sensing and geophysics (Ryan and Boehner
1994). The Malagash Mine Fault is west of the Cape John
study area, on the west side of Amet Sound (Fig. 3). Faulting
in the Malagash area is commonly associated with salt flow
(Ryan and Boehner 1994). The Cape John Fault trends north-
south at the end of the Cape John peninsula (Ryan and Boehn-
er 1994) (Fig. 3). Fault movement here is predominantly nor-
mal dip-slip with a minor strike-slip component. Reidel shear
sets adjacent to the Cape John Fault collectively indicate exten-
sional stress oriented northwest-southeast, and associated with
diapirism north of Cape John (Ryan and Boehner 1994). The
Malagash−Claremont anticline has a normal south limb and a
faulted north limb of overturned fault blocks having high-
angle reverse or thrust geometry (Fig. 3). Movement on some
of these faults is difficult to constrain and some may reflect
later events such as Triassic rifting, which has been document-
ed around the Bay of Fundy (Ryan and Boehner 1994; Webster
et al. 2006).
GEOSCIENCE CANADA Volume 43 2016 201
http://www.dx.doi.org/10.12789/geocanj.2016.43.099
The region was affected by fluctuations in Late Wisconsi-
nan ice dynamics until ca. 12 ka (14C yr) (Stea and Mott 1998).
The earliest ice flows were eastward and southeastward from
an Appalachian or Laurentide ice source ca. 75−40 ka (Caledo-
nia ice flow phases 1A and 1B; Stea et al. 1998). The Hartlen
Till was deposited by southeastward ice flow, and typically con-
sists of 40% gravel, 40% sand, and 20% mud (silt and clay)
(Lewis et al. 1998). The second major ice-flow was southward
and southwestward from the Escuminac Ice Centre in the
Prince Edward Island region (Escuminac ice flow phase 2, ca.
22−18 ka; Stea et al. 1998). The younger Lawrencetown Till
(Stea et al. 1998) is a reddish muddy till unit that has a higher
clay content than the Hartlen Till because of the incorporation
of red Carboniferous sediment derived from Prince Edward
Island, and typically consists of 20–30% gravel, 30–40% sand,
and 30–50% mud (silt and clay) (Lewis et al. 1998). This
deposit is locally known as the Eatonville−Hants Till. Ice then
flowed northwestward and southward from the Scotian Ice
divide across the axis of Nova Scotia (Scotian ice flow phase
3, ca. 18−15 ka; Stea et al. 1998). The study area is dominated
by thick (ca. 3–5 m), red, clay-rich Lawrencetown Till.
METHODS
Topographic−Bathymetric Lidar System Specifications
The Chiroptera II topo-bathymetric lidar system incorporates
a 1064 nm NIR laser for topographic returns and assisting in
defining the water surface, and a green 515 nm laser for bathy-
metric returns (Fig. 4). The lasers utilize a Palmer scanner,
which forms an elliptical pattern with angles of incidence of
14° forward and back and 20° to the sides of the flight track
(Fig. 4). This scan pattern enables more returns from a single
target from different angles, which reduces shadow effects and
increases the number of points on vertical faces such as cliffs
along the coast (Fig. 5). The elliptical scan pattern results in an
increased likelihood that the target will be surveyed twice,
from different angles a few seconds apart, and thus is less sen-
sitive to ocean wave interaction whereby the air bubbles of a
breaking wave will attenuate the green laser pulse (515 nm) and
prevent penetration to the seabed.
The beam divergence of the topographic laser is 0.5 milli-
radians, and for the bathymetric laser is 3 milliradians. The
topographic and bathymetric lasers have pulse repetition fre-
202 Tim Webster, Kevin McGuigan, Nathan Crowell, Kate Collins and Candace MacDonald
http://www.geosciencecanada.ca
Figure 2. Regional structural setting and features of the Cumberland Basin, northern mainland Nova Scotia, after Ryan and Boehner (1994); the outline shows the extent of
Figure 3.
quencies up to 500 kHz and up to 35 kHz,
respectively. The operational altitude of
the bathymetric laser is between 400–600
m above ground level and for the topo-
graphic laser is up to 1600 m above ground
level. The GPS has a sample rate of 1 Hz
and the Inertial Measurement Unit has a
sample rate of 200 Hz for positioning. The
bathymetric accuracy of the bathymetric
lidar is stated to be within 0.12 m at 2 stan-
dard deviations (95% confidence interval).
The topographic laser is reported to have a
ranging accuracy of 0.02 cm at 1 standard
deviation (68% confidence interval) and a
horizontal accuracy of 0.2 m at 1 standard
deviation, not including GPS-Inertial
Measurement Unit error. The system is
equipped with a standard 5 megapixel
RGB camera capable of exposures at 1
frame per second for quality assurance
purposes, and is linked to the timing of the
laser points. The Leica RCD30 camera col-
lects co-aligned RGB+NIR motion-com-
pensated photographs that can be
orthorectified using direct georeferencing.
The RCD30 is capable of exposures at 0.8
frames per second with a distortion-free
lens having a focal length of 53 mm, and produces images of
6732 and 9000 pixels in the across- and along-track directions,
respectively. The across-track field of view is 54°, which is
slightly wider than the 40° across-track lidar field of view. At
400 m altitude the RCD30 produces imagery with a 5 cm pixel
resolution.
The sensor was installed in a Beechcraft A90 King Air air-
craft and calibration flights were conducted at altitudes of 400
m and 1000 m. The coastal bathymetric survey was acquired at
an altitude of 400 m with 30% overlap between flight lines,
and a flying speed of 55 m/s, resulting in a swath 291 m wide.
Bathymetric lidar spot spacing was 1.56 m in the forward lat-
eral direction and 0.78 m in the forward and backward scan
direction, producing an average point density of 1.65
points/m2. The green bathymetric laser spot diameter on the
water surface is approximately 1.2 m at 400 m altitude, and the
near-infrared topographic laser spot diameter is 0.2 m. Flight
lines were planned parallel to the coastline, except for one
additional line perpendicular to the coastline, intersecting the
parallel lines. The specifications used for this survey were
selected to maximize the resolution and point density of the
lidar on land and submerged areas, and the photo resolution of
5 cm was a result of the flying height of 400 m above ground
level. Specifications of the sensor and typical configurations
can be found on the Leica Geosystems website (http://leica-
geosystems.com/products/airborne-systems/bathymetric-
hydrographic-sensors/leica-chiroptera-ii).
GEOSCIENCE CANADA Volume 43 2016 203
http://www.dx.doi.org/10.12789/geocanj.2016.43.099
Figure 3. Geology of the Cumberland Basin, after Ryan and Boehner (1994). The black outline indicates the loca-
tion of the topo-bathymetric lidar survey and the approximate extent of Figure 7. The Malagash Mine Fault,
Golden Brook Fault and Cape John Fault are highlighted.
Figure 4. A. The elliptical scanning pattern of the Chiroptera II with GPS and
Inertial Measurement Unit navigation system components. B. Illustration of the
reflection of near-infrared topographic laser (red arrows) off the sea surface, and
the green bathymetric laser (green and white arrows) penetrating the water column
and reflecting off the sea bed. (Figures adapted from Leica Geosystems).
Lidar Survey
Depth penetration of the lidar sensor is limited by water clar-
ity, as particles in the water column limit the laser’s ability to
travel through the water. The manufacturer suggests using a
Secchi depth measurement to estimate the depth penetration
of the laser under any given conditions. The Secchi depth is
defined to be the depth at which a black and white Secchi disk
is no longer visible as it is lowered into the water; clear water
will have a large/deep Secchi depth, whereas turbid water will
have a small/shallow Secchi depth. The Chiroptera II has a
depth penetration limit of roughly 1.5 times the Secchi depth
(Leica AHAB, personal communication 2014).
The shoreline at Cape John dominantly consists of sedi-
mentary bedrock (red sandstone and mudstone) covered with
a thick blanket of glacial till that is rich in red clay. Erosion of
this material produces nearshore sediments that have a high
clay content; this may cause increased turbidity when the sedi-
ments are mobilized by onshore wind and nearshore waves,
preventing good laser penetration. Precipitation events can
also cause runoff and increase turbidity. A weather station was
installed at Cape John and the data measured there were broad-
cast through a cellular modem and used to monitor weather
conditions remotely in order to conduct the survey during the
clearest water possible.
The Cape John lidar survey was planned for September 24,
2014, but the weather data indicated that a significant storm
event had occurred, accompanied by a drop in barometric
pressure, wind speeds exceeding 40 km/hr from the north-
west, and rainfall from September 18-22, causing the survey to
be delayed. Following the storm, a high pressure system moved
into the region, providing clear skies, so the survey was
attempted on September 25. However, persistent winds of 20
to 40 km/hr during the time of the survey led to high turbidity
levels and poor bathymetric laser returns, causing the survey to
be aborted and delayed further. Good data were acquired dur-
ing the second survey attempt on September 26 after suspend-
ed sediment had settled.
In-Situ Sampling
Ground-truth data acquisition is another important aspect of
ALB data collection, especially since this was the first survey in
the region with the Chiroptera II system. A Leica GS14 GPS
system was used to set up a base station for the aircraft over a
monument that was tied into the provincial High Precision
Network. Real Time Kinematic GPS elevation validation
checkpoints were collected along hard flat surfaces at the Cape
John wharf to validate the topographic lidar, and Secchi
depths were acquired along with underwater photographs of
the seabed using a 1 m x 1 m quadrat (sampling area) to deter-
mine seabed cover. A Reson T-20 multibeam system was
deployed in October after the ALB survey was completed so
that additional depth validation points could be acquired. The
vessel was also equipped with an MDL Dynascan mobile laser
scanner that has a dual survey grade GPS antenna configura-
tion for improved heading measurements and an Inertial
Measurement Unit for direct georeferencing. The Dynascan
204 Tim Webster, Kevin McGuigan, Nathan Crowell, Kate Collins and Candace MacDonald
http://www.geosciencecanada.ca
Figure 5. Example of dense point-spacing along coastal cliffs at Cape John, from the elliptical scan pattern of the Chiroptera II lidar. The colours in this perspective view
represent differences in lidar intensity values from the topographic laser.
supplied the real-time navigational corrections for the multi-
beam, and data from both sensors were acquired using QIN-
SyTM acquisition software.
Lidar Processing
Once the aircraft GPS-Inertial Measurement Unit trajectory
was processed utilizing the GPS base station, the navigation
data were linked to the laser returns and georeferenced using
Lidar Survey StudioTM (LSS). The lidar data were then
processed within LSS, which classified the laser waveforms
into discrete points (the LSS software computes the water sur-
face from the lidar returns of both the topographic and bathy-
metric lasers). In addition to classifying points as land, water
surface, or bathymetry, the system also computed a modelled
water surface that ensured the entire surface area of water was
covered, regardless of the original lidar point density. Thresh-
old parameters were set within LSS to classify the bathymetry
points from the waveforms. The point cloud can be viewed in
cross-section or in a perspective view, allowing the land to be
separated from the bathymetry points (Fig. 6). Once the wave-
forms were processed, the resultant points were displayed
using a variety of attributes (flight line, elevation, intensity)
within LSS, and the waveform examined with the 5 megapixel
quality assurance airphoto, which was linked to the lidar scans.
LAS (an open standard file format for the interchange of lidar
data) version 1.2 files were exported from LSS for further clas-
sification and filtering in TerraScanTM, where the separation of
bathymetric points and noise was refined. As an example of
the variation in files and volume of data, the raw waveform
data are stored in 200 MB files, reduced to 70 MB files when
converted to discrete point files in LAS format, and further
reduced to 3 MB as a 2 m-resolution raster grid.
The refined, classified LAS files were then read into an
ArcGISTM LAS dataset, and PythonTM scripts were written to
produce a variety of raster surfaces at a 2 m spatial sampling
interval. Three main data products are derived from the lidar
point cloud. The first two are based on the elevation and
include: 1) the digital surface model (DSM), which incorpo-
rates valid lidar returns from vegetation, buildings, ground and
bathymetry returns; and 2) the digital elevation model (DEM),
which incorporates ground returns above and below the water
line. The elevation attribute of the lidar point cloud is relative
to the WGS84 ellipsoid, since the point reference is based on
the GPS aircraft trajectory. However, once the surface models
(DSM and DEM) were constructed using different combina-
tions of the point class elevations, the data were converted to
orthometric heights relative to the Canadian Geodetic Vertical
Datum of 1928. The geoid-ellipsoid separation model, HT2,
available from Natural Resources Canada, was used for this
conversion of the surface models.
The third data product is the amplitude of the lidar returns
of the bathymetric laser. The amplitude values were depth-
normalized by taking samples of the amplitude values of a
common cover type (such as sand) over depth ranges, and
using these data to establish a relationship between depth and
the amplitude value; the inverse of this relationship was used
to depth-normalize the amplitude data. The amplitude or
reflectance of the green laser can be interpreted for seabed
cover, e.g. sand, submerged aquatic vegetation, rock or poten-
tial bedrock structures. In order to easily interpret the lidar sur-
face models, colour shaded relief models were constructed
from the DSM and DEM for the study site. Figure 7 shows a
DEM of the 2014 topo-bathymetric lidar survey combined
with topographic lidar collected in 2006 and 2007. The missing
strips of data in the southern portion of the study area (Fig. 7)
occurred because fog was present during this part of the sur-
vey, which caused the lasers to reflect back towards the aircraft
at close range, triggering a safety shut off. This happened on a
few flight lines (Fig. 7) before it was resolved and continuous
data were re-acquired.
RESULTS
Elevation Validation
GPS checkpoints collected at the end of the Cape John wharf
were compared to the topographic lidar-derived elevation sur-
face (DEM). The mean difference between the GPS elevation
and the DEM was –0.02 m, with a standard deviation of 0.04
m from 13 checkpoints. The topographic sensor was well with-
in the manufacturer’s specifications of 15 cm vertical accuracy.
Rough weather and technical challenges limited the quantity of
depth validation data collected during the multibeam survey;
therefore, more depth validation studies are planned for the
future. The validation of the depths at Cape John was accom-
plished by qualitatively comparing the multibeam echosounder
depths to the lidar bathymetry points where overlap exists.
Results indicate that the lidar bathymetric points match the
multibeam points within the 15 cm specification of the Chi-
roptera II.
Geological Interpretation
The 2014 topo-bathymetric lidar data reveal that ocean cur-
rents have eroded the glacial till cover offshore and exposed
the bedrock geology around Cape John (Fig. 8). These data
allow the coastal outcrops to be traced laterally and more
details on the structural geology to be interpreted. The off-
shore structures thus revealed include bedding planes of the
Cape John Formation, which generally strike northeast but
curve to the north-south toward the west end of Cape John
(Fig. 8B). Ryan and Boehner (1994) mapped north-northeast-
and north-south-trending faults at the end of Cape John (Fig.
9); the north-northeast trending fault corresponds to where
the 2014 lidar bathymetry begins to show more intense defor-
mation (Fig. 8). Bedding appears to be folded and faulted in an
arcuate shape around the point at Cape John, changing from a
northeast trend to an east-west trend, then back to a northeast
trend farther west. Large blocks are dextrally offset by faults,
which also appear to be folded (Fig. 8B). The elevation and
apparent deformation of the submerged outcrop diminishes
south of the point on the west side of Cape John (Fig. 8B).
Near the wharf south of Cape John bedding trends east-west,
parallel to two major onshore lineaments that are expressed as
subtle depressions and wetland locations (Fig. 8B, C). The lin-
eaments are broad, low-lying, and may have been preferentially
GEOSCIENCE CANADA Volume 43 2016 205
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206 Tim Webster, Kevin McGuigan, Nathan Crowell, Kate Collins and Candace MacDonald
http://www.geosciencecanada.ca
Figure 6. Point clouds for Cape John, colour-coded for elevation. A. Perspective view of the topographic laser points highlighting the exposed land. B. Perspective view of
the combined topo-bathymetric lasers; the green features represent submerged, folded fault blocks.
eroded by glaciers. These lineaments are evident on a World-
view 2 satellite image acquired at low tide in September 2010,
which was examined and interpreted for lineaments and
exposed and submerged outcrop (Fig. 8C). However, the satel-
lite image contains little information on exposed or submerged
outcrops compared to what is visible in the lidar DEM (Fig.
8A, B). The lineaments are aligned with breaks in the strike of
the offshore bedding planes and also with extrapolated posi-
tions of the Malagash Mine and Golden Brook faults, mapped
less than 10 km to the west across Amet Sound (Fig. 9A). The
GEOSCIENCE CANADA Volume 43 2016 207
http://www.dx.doi.org/10.12789/geocanj.2016.43.099
Figure 7. Colour shaded relief of topo-bathymetric lidar Digital Elevation Model for Cape John, Nova Scotia.
Figure 8. A. Close-up of offshore structural geology revealed by topo-bathymetric lidar Digital Surface Model (DSM). B. Map of lidar DSM with the geological interpretation
of bedding attitude (yellow lines) and faults (red lines). Bedding symbols are from Ryan and Boehner (1994). C. Example of a low tide Worldview 2 image true colour com-
posite, September 2010 (image copyright© DigitalGlobe).
extrapolated faults also line up with lineaments identified off-
shore on the east side of Cape John (Fig. 9), separating north-
northeast-striking beds on the west side of Megs Cove to the
northwest (Fig. 9).
CONCLUSIONS
The topo-bathymetric lidar provides greater detail both on the
land and in the submerged nearshore compared to previously
available information from geological and hydrographic sur-
veys. Conventional topographic lidar surveys completed
between 2006 and 2011 reveal that the land is covered by a
blanket of glacial till, and that landforms appear to be domi-
nated by the late movement of ice (Fig. 7). However, because
the topographic NIR laser does not penetrate the water col-
umn, no information of the submerged terrain is available
from these older surveys. The topo-bathymetric lidar data also
provide far greater detail to a depth of 6 m compared to a
hydrographic chart of the area that was compiled in 1945
(Canadian Hydrographic Service 1945) (Fig. 8). Bedrock geol-
ogy can only be mapped along the coast, and is only accessible
during low tide or by boat, therefore the interpretation of the
structural geology is limited by what is seen at the coastal sec-
tions. Conversely, the topo-bathymetric lidar study identified
nearshore lineaments representing bedding planes and possi-
ble faults, which we have correlated with known onshore faults
and lineaments. This interpretation has resulted in extending
several faults, such as the Malagash Mines and Golden Brook
faults, farther east to Cape John, where they explain some of
the nearshore deformation in that area. In Brittany, Le Gall et
al. (2014) similarly used lidar and multibeam to enhance their
structural mapping of the offshore Variscan basement, allow-
ing them to trace lineaments, correlate them with geophysics,
and interpret the deformation. Unfortunately, the regional
geophysics, including magnetics, in the Cape John area is of
low resolution and does not reveal any structural details.
208 Tim Webster, Kevin McGuigan, Nathan Crowell, Kate Collins and Candace MacDonald
http://www.geosciencecanada.ca
Figure 9. A. Projection of the Malagash Mine and Golden Brook faults to Cape John, with lidar map location outlined in black. Geology is from Ryan and Boehner (1994).
B. Lidar Digital Surface Model (DSM) of the area enclosed by the black rectangle in A. C. Lidar DSM with interpreted fault extensions (black dashed lines) following the
onshore lineaments.
This paper provides an example showing how ALB data
can be used to examine and interpret structural geology and to
extend onshore mapping into the nearshore. The study has
revealed a significant amount of exposed outcrop in the
nearshore. The glacial till, which obscures outcrop on land, has
been eroded in the nearshore, clearly revealing bedding planes
in the high-resolution elevation model derived from ALB. The
structural details that can be interpreted from the nearshore
bathymetry far exceed the details that can be seen on land,
which is covered by a thick blanket of glacial till from the Lau-
rentian ice sheet. Because of the till blanket, the only available
outcrop on land is along the coast, which does not provide suf-
ficient lateral extent to allow the full complexity of the geolog-
ical structures within the study area to be identified. The
results of previous mapping, combined with the newly inter-
preted nearshore bathymetry, have been used to extrapolate
several faults and link onshore wetlands and marshes with off-
shore breaks in the attitude of bedding planes. These tech-
niques can be applied elsewhere and may, for example, allow
the extension of known mineral deposits to be traced into the
nearshore or, as demonstrated in this study, improve our geo-
logical maps and understanding of local tectonics.
ACKNOWLEDGEMENTS
The Applied Geomatics Research Group at Nova Scotia Community College would
like to thank the Canada Foundation for Innovation (CFI) and our CFI project part-
ner, Leading Edge Geomatics (LEG) who assisted in the operation of the survey
and arranging the aircraft for the sensor. Staff from Leica AHAB and Leica Geosys-
tems were on site for training and assisted us with the installation, calibration and
initial processing of the data. We would like to thank the following project partners
who assisted in funding the equipment and processing of the data for this paper:
Nova Scotia Research Innovative Trust, Atlantic Canada Opportunities Agency,
GeoNova, Nova Scotia government departments: Natural Resources, Fisheries and
Aquaculture, Agriculture and Inland Waters, as well as Federal Government depart-
ments: Public Works Government Services Canada, Fisheries and Oceans Canada
(Gulf Region and Maritimes Region), and the Natural Science and Engineering
Research Council. Additionally, the authors would like to thank the reviewers of this
manuscript, Dr. Brendan Murphy and Dr. Jeff Harris. We would also like to thank
the copyeditor, Reginald Wilson, for his helpful comments on the manuscript.
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Received January 2016
Accepted as revised May 2016
First published on the web May 2016
210 Tim Webster, Kevin McGuigan, Nathan Crowell, Kate Collins and Candace MacDonald
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