Y. M. Zhang, C. Liu and L. C. Shen

A coil-type cross-borehole electromagnetic sensing system operating at relatively low frequency (10 Hz to 1 MHz) is simulated using the transmission line matrix (TLM) method and a theoretical equivalent magnetic current source representing a magnetic dipole in an azimuthally symmetrical layered formation. The transmitting coil is simulated in the borehole and the equivalent source concept is developed and applied to the TLM simulation in the frequency domain. We verify this formulation by comparing simulated results with analytical solutions for the amplitude and the phase of the received signal in a homogeneous formation. The system response to a layer sandwiched between two shoulder formations is used to evaluate the spatial resolution and the anomaly detection capability of the EM sensing system. The simulated system response also compares favorably with experimental field measurements at the Devine Test site in Texas as reported by Lawrence Livermore National Laboratory. The phase of the magnetic field at the receiver is shown to be more sensitive than the amplitude to layer contrasts and anomalies in the formation conductivity. These simulations are useful in characterizing the performance of electromagnetic sensing systems operating in subsurface geological formations.