OBSERVATIONS OF GEOMAGNETIC-PULSATIONS AND VARIATIONS WITH A NEW BOREHOLE MAGNETOMETER DOWN TO DEPTHS OF 3000-M

A triaxial magnetometer has been developed for investigating the in si tu skin effect of horizontal geomagnetic pulsations and variations in deep boreholes. The observations were carried out in the pilot borehol e of the Continental Deep Drilling Program of Germany (KTB) down to de pths of 3000 m and up to temperature of 90-degrees-C. A weak skin effe ct, due to the known very low conductivity of the penetrated crystalli ne rocks, of 90 to 95 per cent in amplitude and -5-degrees to -10-degr ees rotation in phase has been observed at periods of 10 s and magneto meter depth of 2400 m. An essential prerequisite for all calculations is the accurate determination of the orientation of the downhole magne tometer. It is demonstrated how oriented record samples of temporal va riations at depth correlate precisely with those from the surface. Res ults from surface magnetotelluric (MT) investigations show strong loca l distortions of the telluric field. The distortion of the MT tensor r esponse has been determined by means of newly introduced skin-effect t ransfer functions, which are assumed to be undistorted.