Sensitivity analysis of crosswell electromagnetics

Abstract

Sensitivity studies for low‐frequency crosswell electromagnetics (EM) show that the region contributing to the response is quasi‐ellipsoidal in shape and encompasses both source and receiver. The strongest response originates from the immediate vicinity of the source and receiver, but measurable contributions are also produced in the interwell region. Side lobes of opposite sign are present outside the interwell region. The interwell contribution increases markedly for elongated (2-D) geological features and for increasing frequency. The horizontal magnetic‐field component as a result of a vertical source is sensitive to vertical changes in conductivity in the vicinity of the receiver well and, when source and receivers are aligned, is relatively insensitive to any conductivity distribution at the transmitter well. Spatial resolution of crosswell electromagnetics, as inferred from the sensitivity patterns, is highest near the source and receiver and decreases away from the wells. Interpretation is improved by combining measurements at different frequencies and by using multicomponent receivers. The highest resolution is obtained at the highest frequencies, when the crosswell sensitivity pattern approaches the shape of a relatively narrow, focused beam.