Resolution studies in airborne resistivity surveying at VLF

Abstract

Airborne resistivity surveys, which involve the wavetilt phenomena of radiowaves, are used as a reconnaissance exploration technique in both engineering and geologic investigations. Survey results are usually presented as resistivity flight‐line profiles or as contour maps from which the interpretation or site selection process must be initiated. Attempts to interpret the airborne results and correlate them with similar measurements made on the ground have indicated a need to understand better the relationship between ground and airborne resistivity measurements. To aid in this understanding, an analysis was performed to determine a very‐low‐frequency (VLF) airborne system’s response to modeled resistivity anomalies. The analysis considers a VLF plane wave to be incident at a near‐grazing angle upon a model flat earth. The earth is characterized by a uniform surface impedance, except for a circular area upon which the surface impedance assumes a different but uniform value. This change is assumed to result from some unspecified subsurface resistivity inhomogeneity. The change in the surface impedance is manifested by an amplitude change in the earth tangential electric field component. The perturbed two‐dimensional (2-D) plane‐wave fields are then computed at a standard survey altitude and are used to derive the airborne resistivity patterns above the change in surface impedance. The effects of survey altitude, anomaly size, and average ground resistivity upon the airborne values are analyzed. The results show that surface impedance changes representing resistive inhomogeneities are more prominent than those representing conductive inhomogeneities at survey altitudes. Curves are presented to separate the effects of resistivity contrast and radius of the anomalous surface area. The results also demonstrate the large attenuation observed at altitude for some ground resistivity anomalies.