Evaluating Transient Electromagnetic Data in a High Noise Environment

In 2011, the Summer of Applied Geophysical Experience (SAGE) students acquired transient electromagnetic (TEM) sounding data at 20 sites in the Caja del Rio (CDR) area, west of Santa Fe, New Mexico to assess groundwater conditions and the potential for future geophysical work. The CDR area is reported to have geothermal potential, so TEM data were collected to constrain the structure and water quality of the local aquifer. Unfortunately, the CDR survey area also has multiple noise sources present including a power transmission substation, a series of high-voltage transmission lines, and gas and water pipelines. It is necessary to assess the impact of the noise sources on TEM data from study areas like the CDR before inverting and interpreting the data. We analyzed the maximum amplitude of background noise relative to the measured transient decay and used the deviation of the sounding response from a power-law decay to empirically estimate the extent of electromagnetic (EM) coupling to anthropogenic structures at each site along the profile. The theoretical effects of EM coupling as a function of distance were also calculated for an infinite line source. A correlation between potential noise sources and measured background noise was found, but no evidence of EM coupling was observed in the data, which we attribute to the conductive environment and relatively small transmitter loop size. In a more resistive environment the background noise would have been more problematic and a larger transmitter moment would increase coupling with the infrastructure. Our analysis indicates that the TEM data are valid, and for this data set the estimated signal-to-noise ratios are sufficient for reliable interpretation of TEM inverse models. The electrical resistivity of the CDR subsurface was modeled to a depth of approximately 150 meters. Results of two- and three-layered one-dimensional TEM inverse models are in agreement with well-log data in the CDR area. Porosity variations along the top of the aquifer were not resolvable due to high clay content in the sediments. The results of this study leave room for further investigations of the local aquifer structure and chemistry.