Quantitative Two-Layer Inversion and Customizable Sensor-Array Instrument for Electromagnetic Induction based Soil Conductivity Estimation
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Beschreibung
Electromagnetic (EM) measurement methods oer the great potential to non-invasively and
contactlessly obtain geological and hydrological soil properties of the uppermost six meters of
the subsurface with an areal resolution in the sub-meter range. The presented work is focused
on small-sized frequency domain `electromagnetic induction' (EMI) systems which combine
the transmitter (Tx) and receiver (Rx) unit in one portable construction and obtain the apparent
electrical conductivity (a) of the sensed soil volume by inducing electrical currents
and measuring the responding electromagnetic eld. The sensing depth of EMI instruments
depends on the sensor conguration and in particular the coil orientation and Tx{Rx separation.
In principle, multi-conguration EMI data can be inverted for the electrical conductivity
distribution over depth. However, there is a demand for ecient inversion algorithms and
high-quality EMI data from dierent sensing depths to perform such an inversion.
Here, a novel one-dimensional global-local inversion approach is implemented which evaluates
the mist between EMI data and forward modeled data for a two-layer soil using a L1-norm
objective function. The global approach is based on a grid search for reasonable model
parameters in combination with the local-sensitivity forward model. The two soil models
with the smallest mist are rened using the (local) simplex search algorithm with the more
precise full solution electromagnetic forward model. The algorithm is analyzed using synthetic
EMI data. Applying the inversion on quantitative EMI transect data from two commercial
devices with eight dierent sensor congurations results in a two-layer electrical conductivity
model with lateral and vertical conductivity changes that are in good agreement with a
collocated electrical resistivity tomography data set.
To improve the depth-resolution beyond available xed congurations, a novel EMI prototype
system (ElMa1) with customizable sensor-array is developed, containing multiple modular
sensor units which can be
exibly arranged by the operator for each survey, ensuring optimal
depth-sensitivity (i.e. coil orientations and Tx{Rx separations) for the specic investigation.
The sensor units consist of coil-based transmitter and receiver circuits which allow for the
measurement of the magnetic
ux and the sensor impedance in a frequency range between
3 and 33 kHz, respectively. To allow for
exible sensor congurations, data processing and
signal optimization, the transmitter current and the receiver voltages are separately digitized
using 24-bit analog-to-digital converters (ADC's) which provide a high dynamic range and
phase stability. For a measurement time of 0.5 s, the ElMa1 system achieves an instrumental
a-accuracy of 1 mS/m at 20 kHz for the intended Tx{Rx separation of 1.0 m and an
accuracy of 10 mS/m for a less favorable conguration with smaller Tx{Rx separation of
0.3 m and smaller measurement frequency of 5 kHz, both observed under stable temperature
conditions.
Autor*in
Achim Thomas Mester