Calibration case study of natural source electromagnetic array data recorded over a well in Oregon

Experimental results are described for a data acquisition method originally developed by F.X. Bostick (1986) that measures the electric field continuously along a profile. A three step sequence is used in the automated interpretation of electromagnetic (EM) array data. Following some preliminary steps in which an initial model is constructed, each interpretive refinement of the model consists of (1) forward computation of the array response and the numerically evaluated Jacobian, or matrix of first partial derivatives of the response with respect to model parameters; (2) data processing for static removal making use of the relatively short spatial wavelength behavior of the static effect in an analogous fashion to Bostick´s method and tuning the data to the desired exploration objective; and (3) a linear correction is made to the model parameters. This three-step sequence is repeated, possibly varying the model definition or processing as interpretive judgement dictates, until the quality of fit of synthetic and field data is less than the typical data uncertainty. These data can be processed to remove shallow effects while maintaining the deep penetration characteristics of the natural electromagnetic (EM) field. Processing and modeling techniques for such data are demonstrated on both synthetic and field data. Very good agreement with actual borehole data is demonstrated.<>