Evaluation And Design Of A Large Spacing Loop-Loop Electromagnetic Tool

This paper investigates the potential use of a large spacing loop-loop electromagnetic logging tool to detect resistivity anomalies as far as 25 meters away from the borehole wall. A three-dimensional whole space electromagnetic modeling code was used to evaluate the responses of such a tool to various transmitter and receiver loop orientations, operating frequencies, loop spacings, resistivity contrasts, anomaly sizes and anomaly distances. Anomalies were modeled as finite extent sheets of various thicknesses which terminated before intersecting the borehole wall. It was found that both coplanar and coaxial loop orientations provide strong anomaly responses, but the coaxial loop orientation has superior depth penetration and is a less complex tool to design and build. For such a coaxial tool, anomalies could be detected as far from the borehole wall as half the loop spacing. For rock resistivities on the order of 100 ohm-m, contrasts greater than 2:1 and loop spacings of 75 meters the optimal operating frequency is shown to be 10 kHz. At these loop spacings, anomalies must be thicker than 10 meters to be detected. It is further shown that for coaxial loops the nature of the response curves allows one to estimate the distance of an anomaly from the borehole wall although no angular location is possible. A coaxial loop-loop electromagnetic tool design that is currently being built is presented. This tool provides for a downhole power supply to a transmitter loop with a magnetic moment between 100 and 1000 A/m2 that is electrically isolated via a fiber optic link from the variably spaced receiver located above. Signal detection is provided by a high sensitivity lock-in amplifier. The errors are conservatively estimated for this design at 5%. Thus anomalies resulting in-a secondary field less than 5% of the primary transmitted field are deemed undetectable.