Quantification of clay content using the transient electromagnetic and spectral induced polarization method

Abstract

<p>Detailed information on the clay content of the subsurface and its spatial distribution plays a critical role in the interaction between surface- and groundwater. In this study, we investigate a new methodology to integrate data measured with electromagnetic and electrical geophysical methods, namely, the transient electromagnetic (TEM) and the spectral induced polarization (SIP) to quantify subsurface clay content in an imaging framework. The methodology is tested in data sets collected at a quarry close to Vienna and consists of a ca. 10 m thick clay layer below a ca. 8 m thick overburden of sandy silts. Our data set includes SIP data collected along a 315 m long profile with an electrode separation of 5 m in a frequency range from 0.1 to 225 Hz. Along this profile, we measured 26 TEM soundings using a 12.5 m loop with 24 windows recording in a time range between 4 and 140 μs. Ground truth information corresponds to grain size analysis conducted in 25 soil samples collected in a depth from 5 to 28 m. SIP inversion results at a single frequency provided structural a-priori information to improve the inversion of the TEM data. The inverted TEM conductivity model, nearest to the position of soil sample collection, was correlated to the grain size distribution and the resulting positive exponential relationship was used to obtain vertical 1D variations of clay content with depth. All sounding positions were interpolated to obtain a 2D image of subsurface clay content. This clay content variations were then compared to images of the Cole-Cole parameters, describing the frequency dependence of SIP imaging results. To evaluate the uncertainty in our clay estimations, we applied the Bayesian evidential learning 1D imaging (BEL1D). We obtained uncertainties of layer thickness, resistivity, and clay content by integrating the clay-conductivity relationship derived from TEM data into the BEL1D framework.</p>