Recognition of magnetic anomalies in Ground Conductivity Meter soil surveys: a high-resolution field experiment

Abstract

Ground conductivity measurements are widely used in soil surveys, where
the objective is to map an element or property, which gives a strong
conductive signal compared to the surroundings. It can be used in
mapping of soil contamination, mineral exploration and soil mapping,
where properties like porosity, clay-content and salinity of groundwater
are explored. However, interpretations get poor, when too many
variables, e.g. metals, affect the measurements. To improve
interpretation of the GCM dataset, we investigated confounding signals
from buried metals as magnetic anomalies by a magnetometer. The small
field test site in Illerup Ådal, Denmark (2 ha) was situated on
peat and clayey soil, where buried metal was expected due to previous
archaeological investigations. Both GCM and magnetometer measurements
were on-the-go behind an ATV and logged together with DGPS positioning.
Instruments were a DUALEM-21 and a Geometrics G-858 Caesium
magnetometer. Data were collected in separately runs, since close
proximity of the instruments can affect the magnetometer data. Data were
collected on 12 lines, which were spaced 5 m apart. The frequency of
readings was 4 times s-1 at a speed of approximately 12 km h-1. A 1D
multi-layer model was used for the inversion of EM data, providing
detailed information of the resistivity structure in the upper 2-3 m of
the soil. All 12 lines were driven in both directions during sampling of
magnetic data, to check if measurements are influenced by the direction
of the magnetometer. Time for collecting both datasets was 90 minutes.
The combined dataset showed one area (200 m2) with a magnetic anomaly,
which correlated with a relatively low apparent resistivity
(approximately 27 Ohm m), while the adjacent areas had a higher apparent
resistivity (>50 Ohm m). The inversion model showed that a relatively
low resistivity (20-30 Ohm m) was present at all depths in the area with
the magnetic anomaly. However, the model showed even lower resistivity
in other areas of the site (10-20 Ohm m) in all of the modelled layers.
Therefore, this area would easily be interpreted wrong in GCM surveys,
since it does not appear as an outlier in the EMI dataset. By making
a combined survey with both EMI and magnetic susceptibility
measurements, it is possible to identify small areas with high magnetic
anomalies. Here caution should be taken in interpretation of GCM survey
in relation to the element or property of interest.

Full text

Geophysical Research Abstracts
Vol. 15, EGU2013-8596, 2013
EGU General Assembly 2013
© Author(s) 2013. CC Attribution 3.0 License.
Recognition of magnetic anomalies in Ground Conductivity Meter soil
surveys: a high-resolution field experiment
Niels Emil Søe (1), Jesper Bjergsted Pedersen (1), Esben Auken (1), Mogens Humlekrog Greve (2), Henrik
Nørgaard (2), Anna K. E. Tjelldén (3), and Søren Munch Kristiansen (1)
(1) Aarhus University, Department of Geoscience, Denmark, (2) Aarhus University, Department of Agroecology, Denmark,
(3) Moesgaard Museum, Højbjerg, Denmark
Ground conductivity measurements are widely used in soil surveys, where the objective is to map an element
or property, which gives a strong conductive signal compared to the surroundings. It can be used in mapping of
soil contamination, mineral exploration and soil mapping, where properties like porosity, clay-content and salinity
of groundwater are explored. However, interpretations get poor, when too many variables, e.g. metals, affect the
measurements.
To improve interpretation of the GCM dataset, we investigated confounding signals from buried metals as magnetic
anomalies by a magnetometer. The small field test site in Illerup Ådal, Denmark (2 ha) was situated on peat and
clayey soil, where buried metal was expected due to previous archaeological investigations. Both GCM and mag-
netometer measurements were on-the-go behind an ATV and logged together with DGPS positioning. Instruments
were a DUALEM-21 and a Geometrics G-858 Caesium magnetometer. Data were collected in separately runs,
since close proximity of the instruments can affect the magnetometer data. Data were collected on 12 lines, which
were spaced 5 m apart. The frequency of readings was 4 times s−1at a speed of approximately 12 km h−1. A 1D
multi-layer model was used for the inversion of EM data, providing detailed information of the resistivity structure
in the upper 2-3 m of the soil. All 12 lines were driven in both directions during sampling of magnetic data, to
check if measurements are influenced by the direction of the magnetometer. Time for collecting both datasets was
90 minutes.
The combined dataset showed one area (200 m2)with a magnetic anomaly, which correlated with a relatively low
apparent resistivity (approximately 27 Ohm m), while the adjacent areas had a higher apparent resistivity (>50
Ohm m). The inversion model showed that a relatively low resistivity (20-30 Ohm m) was present at all depths
in the area with the magnetic anomaly. However, the model showed even lower resistivity in other areas of the
site (10-20 Ohm m) in all of the modelled layers. Therefore, this area would easily be interpreted wrong in GCM
surveys, since it does not appear as an outlier in the EMI dataset.
By making a combined survey with both EMI and magnetic susceptibility measurements, it is possible to identify
small areas with high magnetic anomalies. Here caution should be taken in interpretation of GCM survey in relation
to the element or property of interest.