Technical Papers - Airborne Electromagnetics

Identification of Electrical Anisotropy from Helicopter EM Data

Changchun Yin and Greg Hodges
Fugro Airborne Surveys, Mississauga, ON, Canada

Abstract

It has been theoretically proven that through collecting helicopter electromagnetic (HEM) data in several directions, the directional conductivity of an electrically anisotropic earth can be measured. Measurement of electrical anisotropy is potentially useful for determining geological fabric related to fracturing or bedding, which may control groundwater flow. Plotting of the EM responses collected in different directions with VCX (vertical coaxial - horizontal magnetic dipole) and VCP (vertical coplanar - horizontal magnetic dipole) coil arrays distinguishes the orientation of the principal axes of the electrical anisotropy in the earth. However, the EM signal is strongly influenced by the flight altitude, and it is very difficult in practice to maintain the required constant altitude. The ratio of VCX to VCP EM responses is less influenced by altitude, however the calculation requires two orthogonal coil systems operated at the same frequency. This leads to technical problems.

In this paper, we investigate the anisotropy of the apparent resistivity based on the pseudo-layer model for VCX and VCP coil arrays. This parameter has the advantage that it does not require the EM signal from two orthogonal coil systems at the same frequency and is little influenced by the flight altitude. Polar plotting of this parameter, collected in different directions, shows the orientations of the anisotropic principal axes.

A test survey of about km2 was flown in Texas over the Edwards Aquifer recharge area. The Fugro RESOLVE system, with five HCP (horizontal coplanar - vertical magnetic dipole) coils and one VCX coil, was flown in 4 flight directions varying from 0° azimuth to 135° with 5 lines each. The VCX apparent resistivities identify four resistive anisotropic regions in the survey area, where the principal resistivity axes are respectively oriented northwest and northeast. As expected from the theory, the HCP apparent resistivities at the adjoining frequencies show no dependence on flight direction.

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