Multi-fold TDEM Experiment Design for Near Surface Conductivity Mapping

Multi-fold Time Domain Electromagnetics (TDEM) is a novel experimental approach that couples elements of traditional land-based TDEM survey designs to obtain a robust data set. This design inherently accommodates a broad range of possible Earth models through a rich combination of analysis opportunities making it ideally suited for reconnaissance. Kentland Farms, VA was chosen as the test site, for its ease of access and interesting geologic features such as river terraces and karstic landscape. Three independent methods of analyzing the 3-component data set each provided unique insights into the subsurface electrical structure through a complementary interpretation. Synthesis of log-normalized ∂tB_z pseudo-sections provided a first-order analysis of the lateral and vertical heterogeneities of the profile. A Zero-Crossing Moveout (ZCMO) analysis used a brute-force grid-search inversion to estimate the two-layer Earth model that best-fit the observed moveout times for a range of interface depths. By using the ZCMO result as an initial model, regularized 1D Occam inversions determined a 3-layer electrical structure consisting of a 3.5 m – 5 m thick resistive upper layer, over an 12.5 m – 15 m thick conductive layer, overlying a resistive half-space. From correlation of the inverse solutions with ZCMO derived conductivity models and prior resistivity information, the depth to the limestone bedrock was approximated to be 16 – 20 m. The delineation of the bedrock depth provided additional support for the fill-cut terrace formation model (Ward et al., 2005), as well as possible evidence of groundwater drainage on the 40 m terrace at Kentland Farm.