Quantitative Two-Layer Inversion and Customizable Sensor-Array Instrument for Electromagnetic Induction based Soil Conductivity Estimation

摘要

Electromagnetic (EM) measurement methods oer the great potential to non-invasively andcontactlessly obtain geological and hydrological soil properties of the uppermost six meters ofthe subsurface with an areal resolution in the sub-meter range. The presented work is focusedon small-sized frequency domain `electromagnetic induction' (EMI) systems which combinethe transmitter (Tx) and receiver (Rx) unit in one portable construction and obtain the apparentelectrical conductivity (a) of the sensed soil volume by inducing electrical currentsand measuring the responding electromagnetic eld. The sensing depth of EMI instrumentsdepends 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 conductivitydistribution over depth. However, there is a demand for ecient inversion algorithms andhigh-quality EMI data from dierent sensing depths to perform such an inversion.Here, a novel one-dimensional global-local inversion approach is implemented which evaluatesthe mist between EMI data and forward modeled data for a two-layer soil using a L1-normobjective function. The global approach is based on a grid search for reasonable modelparameters in combination with the local-sensitivity forward model. The two soil modelswith the smallest mist are rened using the (local) simplex search algorithm with the moreprecise full solution electromagnetic forward model. The algorithm is analyzed using syntheticEMI data. Applying the inversion on quantitative EMI transect data from two commercialdevices with eight dierent sensor congurations results in a two-layer electrical conductivitymodel with lateral and vertical conductivity changes that are in good agreement with acollocated electrical resistivity tomography data set.To improve the depth-resolution beyond available xed congurations, a novel EMI prototypesystem (ElMa1) with customizable sensor-array is developed, containing multiple modularsensor units which can be exibly arranged by the operator for each survey, ensuring optimaldepth-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 themeasurement of the magnetic ux and the sensor impedance in a frequency range between3 and 33 kHz, respectively. To allow for exible sensor congurations, data processing andsignal optimization, the transmitter current and the receiver voltages are separately digitizedusing 24-bit analog-to-digital converters (ADC's) which provide a high dynamic range andphase stability. For a measurement time of 0.5 s, the ElMa1 system achieves an instrumentala-accuracy of 1 mS/m at 20 kHz for the intended Tx{Rx separation of 1.0 m and anaccuracy of 10 mS/m for a less favorable conguration with smaller Tx{Rx separation of0.3 m and smaller measurement frequency of 5 kHz, both observed under stable temperatureconditions. In addition, experimental data were corrected for temperature-induced systemdrifts by simulating the electrical circuit of the sensor system using spectral measurements ....