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| Titel |
Recognition of magnetic anomalies in Ground Conductivity Meter soil surveys: a high-resolution field experiment |
| VerfasserIn |
Niels Emil Søe, Jesper Bjergsted Pedersen, Esben Auken, Mogens Humlekrog Greve, Henrik Nørgaard, Anna K. E. Tjelldén, Søren Munch Kristiansen |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250079729
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| Zusammenfassung |
| 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. |
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