This title appears in the Scientific Report :
2013
Semi-3D Multi-layer Inversion of Calibrated Electromagnetic Induction Data acquired at the Selhausen test site
Semi-3D Multi-layer Inversion of Calibrated Electromagnetic Induction Data acquired at the Selhausen test site
An electromagnetic induction (EMI) system is a geophysical measurement tool to determine structural and hydrological subsurface variations in a non-invasive manner, since the measured apparent electrical conductivity (ECa) can be related to changes in soil moisture, soil water, clay content and/or s...
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Personal Name(s): | von Hebel, Christian (Corresponding author) |
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Rudolph, Sebastian / van der Kruk, Jan / Vereecken, Harry | |
Contributing Institute: |
Agrosphäre; IBG-3 |
Imprint: |
2013
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Conference: | TR-32 International Symposium: Patterns in Soil-Vegetation-Atmosphere-Systems: Monitoring, Modelling & Data Assimilation, Bonn (Germany), 2013-03-11 - 2013-11-14 |
Document Type: |
Conference Presentation |
Research Program: |
Modelling and Monitoring Terrestrial Systems: Methods and Technologies |
Publikationsportal JuSER |
An electromagnetic induction (EMI) system is a geophysical measurement tool to determine structural and hydrological subsurface variations in a non-invasive manner, since the measured apparent electrical conductivity (ECa) can be related to changes in soil moisture, soil water, clay content and/or salinity. The recorded ECa represents an average value of a specific sensing volume, depending on the source-receiver configuration. A change in source-receiver separation or orientation changes the penetration depth and achieves therefore information from different depth levels. Thus, a multi-coil EMI system that embeds one electromagnetic field transmitter and various receivers with different offsets enable the simultaneous measurement of different sensing depths and in principle a better vertical characterization of the subsurface.
However, most EMI measurements do not return quantitative values and are influenced by external conditions such as the operator, GPS, data logger or other metal objects. To eliminate these influences and obtain quantitative apparent conductivity values a calibration is required. Application of a Maxwell-based full-solution forward model using inverted electrical resistivity tomography (ERT) data as input results in predicted apparent conductivities that are used to calibrate the measured EMI apparent conductivities using a linear regression approach.
The calibrated apparent conductivities can discover structural pattern like buried paleo-river channels, which are not directly observable at the land surface. A comparison of the EMI pattern of several fields around the Selhausen test site with RapidEye satellite imagery showed similar structures. A deeper insight into subsurface properties in terms of electrical conductivity and layer thickness can be achieved by an inversion of the calibrated apparent conductivities. A global-local search algorithm, that was formerly used for a two-layer inversion was not able to invert for a reliable three-layered earth. Therefore, a shuffled complex evolution algorithm was implemented that reliably found a three-layer model with the smallest misfit between measured and predicted data at each position.
Many measurements can be made with the EMI system that is pulled behind an all-terrain vehicle. A high spatial resolution survey was conducted at the Selhausen test site that has a size of around 160 x 60 m. Profiles with approximately three meter distance were measured with a sampling rate of 10 Hz and returned around 300000 data points. To be able to invert for each position for a three-layer model separately, the inversion scheme is parallelized and runs on the JUROPA - supercomputer of Forschungszentrum Jülich. All inverted electrical conductivity three-layer models are stitched together to form a three dimensional (3D) image of the subsurface that indicates lateral and vertical changes in conductivity values.
Thus, the combination of fast and efficient high-resolution multi-configuration EMI measurements that are calibrated to obtain quantitative values with a multi-layer conductivity inversion indicate lateral and vertical conductivity changes that are related to structural and hydrogeological properties in the vadose zone. |