Abstract: A method for determining resistivity anisotropy of subsurface rock formations includes imparting a transient electromagnetic field into the subsurface rock formations. Electromagnetic response of the formations is measured at a plurality of offsets from a position of the imparting. For each offset, an arrival time from the imparting is determined of a peak of an impulse response such that the response is related to subsurface horizontal and vertical resistivities. For each offset, a step response of the formations is determined at a time from the imparting selected such that the step response is related substantially only to mean resistivity. The arrival time of the peak of the impulse response and the late time value of the step response are used to determine the resistivity anisotropy.

Abstract: A method for determining resistivity anisotropy of subsurface rock formations from measurements of response to a transient electromagnetic field imparted into the subsurface and measured at a plurality of distances from a position of the imparting includes that for each offset, a step response of the formations is determined at a time from the imparting selected such that the step response is related substantially only to horizontal resistivity and at a time from the imparting selected such that the step response is related substantially only to mean resistivity. The horizontal resistivity step response and the mean resistivity step response are used to determine the resistivity anisotropy.