Improvement In The Azimuthal Em Method - The Value Of Signal Processing

The azimuthal EM method has recently been proposed as an alternative to the well-applied azimuthal<br>resistivity survey, primarily for the investigation of fracture-induced electrical anisotropy. The advantages<br>of the EM approach include (1) reduced data acquisition time, (2) simple field acquisition procedure, and<br>(3) a reduced data acquisition area. In addition, the problem of electrode contact resistance is overcome<br>with the EM method.<br>Signal processing techniques were developed to assist quantification of noise in azimuthal EM datasets and<br>to enable noise reduction. Comparison of the energy in the even and odd coefficients of the frequency<br>spectra of azimuthal EM datasets allows the signal-to-noise ratio to be identified. Convolution of the<br>azimuthal data with linear phase low pass filters allows for significant noise removal. These processing<br>techniques were applied to azimuthal EM datasets collected at two study sites. Following data processing,<br>the EM datasets revealed apparent resistivity lobes consistent with the orientation of fracture strike mapped<br>at these locations. These results suggest that, with the application of data processing, structural information<br>obtained from an azimuthal EM survey is likely to be comparable to that obtained from an azimuthal<br>resistivity survey.<br>Azimuthal resistivity surveys have been previously performed with array configurations that allow for<br>misinterpretation of apparent resistivity variability caused by lateral heterogeneity as evidence for the<br>presence of anisotropy. Use of an asymmetric azimuthal EM array, combined with analysis of the energy of<br>the even and odd coefficients of the frequency spectra, provides a means to differentiate heterogeneity from<br>anisotropy. The value of this approach is described and the results of a test at a site of known lateral<br>heterogeneity are presented. Analysis of the frequency spectrum for an asymmetric array allowed<br>identification of the lateral heterogeneity at this site.<br>These examples illustrate that analysis of the frequency spectra of azimuthal EM data greatly assists<br>interpretation. The ability to (1) characterize and reduce noise, and (2) differentiate lateral heterogeneity<br>from anisotropy, significantly improves the value of the azimuthal EM technique in site characterization.