Alfonso Benavides

Mutual induction and the effect of host conductivity on the EM induction response of buried plate targets using 3-D finite-element analysis

A finite-element analysis of electromagnetic induction (EMI) in the presence of multiple buried metal targets is undertaken for the purpose of unexploded ordnance (UXO) detection and discrimination. The effects of mutual coupling between metal targets and the host conductivity are shown to be important. At high frequencies, mutual coupling is strong, and effects of host conductivity are relatively minor. At lower frequencies near the resistive limit, EMI responses are very small, but the effect of host conductivity becomes important. This is due to the galvanic current flow in the host medium that dissipates charge accumulations on the host/target interfaces. Qualitative analysis of induced current patterns in metal targets demonstrates that mutual coupling is strongly affected by target orientation and skin depth. Rigorous forward modeling of EMI responses is essential to understanding UXO sensor signatures so that discrimination between live UXO items and harmless fragments and clutter may become possible.

An experimental study of the time-domain electromagnetic response of a buried conductive plate

It is important to understand the effects of a buried metal object on electromagnetic data, whether the object is a source of cultural noise or a target of interest. The time-domain electromagnetic response of a buried metal plate exhibits several remarkable properties. An experimental study has been undertaken to confirm these properties. The spatial response of a shallow-buried plate is temporally self-similar and exhibits a late-time dipolelike response. Clutter-generated noise can be significant if the plate is poorly coupled to the primary transmitter flux. A vertical plate exhibits a transition from a horizontal to a vertical mode of eddy current induction.

Fractal properties of background noise and target signal enhancement using CSEM data

Controlled-source electromagnetic (CSEM) spatial profiles and 2-D conductivity maps were obtained on the Brazos Valley, TX floodplain to study the fractal statistics of geological signals and effects of man-made conductive targets using Geonics EM34, EM31 and EM63. Using target-free areas, a consistent power-law power spectrum (|A(k)| ~ k ^-β) for the profiles was found with β values typical of fractional Brownian motion (fBm). This means that the spatial variation of conductivity does not correspond to Gaussian statistics, where there are spatial correlations at different scales. The presence of targets tends to flatten the power-law power spectrum (PS) at small wavenumbers. Detection and localization of targets can be achieved using short-time Fourier transform (STFT). The presence of targets is enhanced because the signal energy is spread to higher wavenumbers (small scale numbers) in the positions occupied by the targets. In the case of poor spatial sampling or small amount of data, the information available from the power spectrum is not enough to separate spatial correlations from target signatures. Advantages are gained by using the spatial correlations of the fBm in order to reject the background response, and to enhance the signals from highly conductive targets. This approach was tested for the EM31 using a pre-processing step that combines apparent conductivity readings from two perpendicular transmitter-receiver orientations at each station. The response obtained using time-domain CSEM is influence to a lesser degree by geological noise and the target response can be processed to recover target features. The homotopy method is proposed to solve the inverse problem using a set of possible target models and a dynamic library of responses used to optimize the starting model.

UXO Detection Improvements Using EM63 Synthetic Multi-receiver Array Geometries

The detection and discrimination of buried metal targets is an important aspect of geophysical electromagnetic induction (EMI) techniques. A Geonics EM63 time domain metal detector was modified from its standard configuration to permit variable transmitter—receiver (TX—RX) offsets. Several aluminum and steel objects were buried in shallow ground. Transient electromagnetic responses were measured and are presented in profile and map forms. The responses from different target arrangements show distinctive spatiotemporal RX induced—voltage patterns. The results indicate that pattern recognition methods may be useful for buried metal target discrimination using multi—RX EMI array configurations.

Geological noise and target signals in controlled source electromagnetic data

Summary Controlled-source electromagnetic spatial profiles were obtained on the Brazos Valley, TX floodplain to study the fractal statistics of geological noise and effects of manmade conductive metal targets on the wavenumber spectrum. Targets tend to flatten the power-law power spectrum at small wavenumbers. Detection and localization of targets can be achieved using short-time Fourier or, preferably, continuous wavelet spectrogram techniques. A simple pre-processing step that combines responses from different transmitter-receiver orientations greatly enhances the localization and resolution of targets.