Reflection models and ramp response for downhole TEM data

Abstract

Very conductive ore bodies are difficult to detect using standard TEM techniques. Eddy currents are established at the surface of these bodies to oppose the flux associated with the primary field in accord with Faraday's law of induction. These currents persist with negligible losses through resistivity when the primary field is removed. Consequently during the off-time dB/dt remains close to zero and signal levels at the surface are negligible. However an examination of the primary field, or ramp response, rather than the secondary field, will often show sudden changes in amplitude consistent with interference effects from source multiples. Ramp response data obtained for highly conductive targets have now been interpreted using models based on multiple current filaments. The results are consistent with conventional interpretations based on filament inversion theory and also with models based on a reflection of the primary field at the conductor surface.