Electromagnetic sounding in the Columbia Basin, Yakima, Washington

Abstract

A controlled‐source electromagnetic (CSEM) survey was conducted near Yakima, Washington, to map the thickness and resistivity of a thick volcanic sequence overlying a sedimentary section and to infer structure in the sediments. The survey was conducted with a frequency‐domain system employing loop transmitters 400–500 m on a side and three‐component SQUID magnetometer receivers separated from the loop by 1 to 5 km. Data collected along a 30 km profile orthogonal to regional strike were interpreted initially with 1-D layered models, which were then pieced together to make a geoelectric section. Induction logs in a 5000 m exploration hole at one end of the profile agree very well with the CSEM soundings made around the hole. The geoelectric section reveals a smoothly varying thickness of volcanics with a pronounced anticlinal structure approximately concordant with a surface topographic ridge. To assess the validity of inferring lateral structure from 1-D interpretations, we made scale models of an anticlinal structure and of a surface inhomogeneity and conducted CSEM measurements over the scale models. Layered‐model inversions of these data show that the anticlinal structure and its location are very well determined by 1-D inversion, but its height and width are not accurately determined. CSEM sounding over the surface inhomogeneity model shows that this feature does not significantly degrade the interpretation of a deep target layer. In this setting, a geoelectric section made up from 1-D interpretations provides good qualitative measures of subsurface structure and also provides excellent starting models for detailed 2-D or 3-D modeling.