Transient electromagnetic surveys for the measurement of near-surface electrical anisotropy

Structural and petrological anisotropy can cause significant complications during any interpretation of electrical and electromagnetic surveys. New methods are presented here for the determination of the direction and magnitude of lateral anisotropy via measurement of the rate of diffusion for eddy current systems generated in a transient electromagnetic (TEM) sounding. We use the smoke-ring analogy for the current induced in the ground and measure its radial propagation by tracking the change of sign measured in the receiver coil at a range of distances from the transmitter-loop centre. A slower signal migration rate is typically observed for good conductors, therefore by measuring the time dependence of the smoke-ring propagation in this manner for different directions, information on the average horizontal conductivities and subsequent ground anisotropy is provided. In order to test this hypothesis, two locations with different geological conditions were selected. Data were first obtained for a 1-D site, located near Ouyen in northern Victoria, SE Australia, where uniform sedimentary layers dominate the near-surface geology. Additional data were then obtained for two 2-D sites, located close to the Heathcote Fault Zone (central Victoria), where a north–south greenstone belt is responsible for significant lateral anisotropy as observed in previous magnetotelluric (MT) data. Eddy current or smoke-ring expansion rates have subsequently been determined; the results we present here provide the average conductivities along eight axes at each site. A uniform expansion is observed for eddy current systems generated in a 1-D or horizontally layered terrain while strike-dependant distortion is seen for each of the 2-D sites. Hence, static corrections required for MT data in particular, may be further refined using TEM methods to reflect the lateral directional dependence of bulk electrical conductivity.

► Electrical anisotropy is a common problem where geological structure is 2D.
► We present here a procedure to measure electrical anisotropy using the TEM method.
► Mapping of the expanding smoke-ring shows uniform propagation for 1D earth.
► For 2D earth, the eddy current migration rate reflects the direction of strike.
► Orthogonal conductivities and anisotropic coefficients are subsequently determined.