Improvement of measuring accurary of magnetic fields in borehole drilling

Sammanfattning: To increase measuring accuracy in magnetotellurgic measurements anelectrode can be lowered into a borehole to create constraints forthe inversion. For this method a long cable is need to connect tothe electrode. This creates a new type of problems with parasiticeffects when the cable is placed on a winch made of metal. Toaccount for this, the behavior of the cable while on the winchwas measured. It was considered to be a multilayered multi rowcoil with a resistance of 2,4Ohm. The inductance of the winch wasmeasured for different frequencies and with different amount ofcable on the winch. With this data the physical properties of amultilayered multi row coil was numerically fitted. To explain thefrequency dependency of the inductance two different models wherecreated. Model one described the frequency dependency as randomand fitted the physical properties of the multilayered multi rowcoil once for every frequency. The second model (cftool-model)described the frequency dependency of the coil as a power-functionand fitted this behavior numerically in MATLABs curvefittingtoolbox (cf-tool). Both models predicted an inductance which increased with more cable on the winch and with lowerfrequencies. The models fitted the measured data points well insome areas. But for the measurements made with 135m of cable ofthe winch both models fitted poorly with relative errors of up to43%. This can be because of a systematic error made in how thecable was wound on the winch. To help to improve furthermeasurements the error needed to be within 10% which it overallfails to be. It is uncertain if more data and a better model wouldallow the error to reach tolerable levels or if the dependency ofthe winding of the cable onto the winch is too large.