Three-Dimensional Response Maps for a New Wide-Bandwidth Induction System

Three-dimensional response maps are displayed and interpreted in both time and frequency domains. The wide-bandwidth induction system utilizes pseudonoise and cross-correlation techniques to characterize subsurface conducting bodies. In contrast to more typical single-frequency systems, both horizontal-profiling and depth-sounding information are present in these maps. Consequently, interpretation of model surveys is prerequisite to interpretation of field data. Experimental model surveys were performed in the laboratory over a conducting dike and a conducting semi-infinite horizontal sheet. The real part, imaginary part, magnitude, and phase of normalized frequency responses are plotted versus frequency and system location. Raw time signatures are converted to frequency response on 0-50 kHz with the discrete Fourier transform (DFT) and normalized to the response without a conductor present. Normalized transient-time responses (impulse responses) of the conductors are obtained by the inverse DFT and compared with the raw time signatures. Windowing effects in both domains are discussed. The raw-time-signature peak value is quite sensitive to conducting anomalies and can be used to directly delineate conductors in the field. However, more detailed information about size and depth must be obtained by additional signal processing.