Bosm, W., EBERLE, D. and REHLI, H.-J. 1988. A gyro-oriented 3-component borehole magnetometer for mineral prospecting, with examples of its application. Geophysical Prospecting 36, A triple axis borehole magnetometer is described that consists of a Forster-probe (fluxgate) triplet (sensitivity 1 nT), a Forster-probe gradiometer (sensitivity 2 nT/40 cm), a gyro unit (mean angular drift approx. 0.5"/h) which is equipped with accelerometers (sensitivity 1/100"), and a data transmission unit (with multiplexer and 16-bit AD converter). The sensitive fluxgate-magnetometer can detect weakly magnetic or small source bodies. Data from the gyro and the accelerometers allow the 3-component magnetic field values to be transformed to north, east and vertical components. Since they do not rely on magneticallydetermined directional data, the results are not disturbed by local anomalies of the magnetic declination. Furthermore, the magnetometer can also be used in vertical boreholes. 3component measurements are carried out at discrete points in the neighbourhood of a source body to locate its position, and within the source body to determine the direction of magnetization. The strength of magnetization and information on magnetic classification are obtained by continuous measurement of one or more components within the source body. Calculation algorithms and computer programs are available to simplify data processing and interpretation. Survey examples are discussed. 933-961.Paper read at the 45th EAEG meeting

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Borehole measurements of the Earth's magnetic field and the magnetic susceptibility of rocks penetrated by Hole 418A were made on Leg 102 with a gyroscope-oriented three-axis borehole magnetometer probe (Bundesanstalt fur Geowissenschaften und Rohstoffe, Federal Republic of Germany) and a magnetic susceptibility probe (U.S. Geological Survey). Measurements made with the two probes were analyzed together to obtain the in-situ magnetic properties of pillow and massive basalts, which compared favorably with laboratory-measured equivalent properties of basement cores recovered from Hole 418A on DSDP Leg 53. The logs provide a continuous record of the magnetic characteristics of basalts penetrated by the borehole, which makes them useful for locating boundaries of lithologic units, for detecting polarity reversals, and for delineating zones with different magnetic inclinations. In addition, estimates of paleomagnetic pole positions interpreted from the Leg 102 borehole magnetic data compare favorably with Apparent Polar Wander Paths from shore-based paleomagnetic studies of oriented cores.

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During Leg 109, the Bundesanstalt fur Geowissenschaften und Rohstoffe (BGR) three-axis borehole magnetom eter was used for magnetic measurements in Hole 395A. First, measuring procedures and data evaluation are explained. Then, we discuss the application of a newly developed centralizer unit that yielded precise determina tions for the azimuth of the gyro, by which the three-axis fluxgate system is controlled, improving data quality considerably. Although three-component measurements were not conducted during Leg 109 because of a breakdown of the probe, a detailed continuous log of the vertical magnetic intensity was made and interpreted to obtain a magnetization profile and a statistical classification of rocks penetrated by the borehole. The main results are (1) proof of two reversals of the Earth's magnetic field, one of which (as should be stressed) occurred inside a petrographic unit, (2) correlation of rough petrographic classification with significant differences in magnetic anomaly patterns, (3) correspondence of detailed statistical analysis of the magnetic log in one section to the detailed lithologic profile. In other profiles, necessary additional information is not yet available. The authors recommend that research during future legs prove the correlation of detailed statistical log analysis with petrography.

D. R,~DULESCU --Uber die Anwesenheit einer Tiefenbruchzone in Rum~inien blieben w~iren, da man in dieser Zeit keine Beweise ffir ihre Existenz hat. Die Frage, ob nieht in vorneogenen Zeiten ein Emporkommen der Magmen und Bildung von heute unbekannten Strukturen erfolgt ist, die wegen ihres plutonisehen Charakters oder durch die Bedeckung mit neueren Ablagerungen unerkannt blieben, kommt uns daher gereehtfertigt vor.Die Best~tigung dieser Annahme wiirde zur Notwendigkeit ffihren, dab man im Rahmen des subsequenten Vulkanismus der Ostkarpaten diese letzte basische Phase von den /ibrigen Gesteinen mit ghnliehen Eigensehaften absondern mug, die organiseh mit der ganzen tleihe der vorhergehenden andesitisehen Gesteine verbunden ist und welehe das Ergebnis einer weitl~iufigen Differentiation darstellen.

Results of rock magnetic investigations and magnetic field measurements in the Logar Valley, Afghanistan, are presented. Rock magnetic investigations on samples of the ore and the country rock have demonstrated that the chromite is strongly magnetic, whereas the country rock has proved predominantly non‐magnetic, partly weakly magnetic. The isanomalic maps show strong anomalies of some 1000 γ above the known chromite occurrences. Anomalies of the country rock amounting to some 100 γ have been observed only above basic dikes. The measured anomalies have been interpreted quantitatively by 2‐ and 3‐dimensional models. This interpretation is demonstrated and the question of chromite detection discussed. Model graphs are used to investigate in detail whether the extension in depth of ore bodies can be estimated.

The magnetic fields in the deepest oceanic drill hole, DSDP/ODP Hole 504B, and in the neighboring Hole 896A, have been logged continuously with a calibrated three-axis magnetometer so that horizontal and vertical field anomalies could be determined. The extrusive basalts produce the largest anomalies (up to 5000 nT) but the amplitudes vary strongly with short wavelengths, whereas the sheeted dikes are associated with 500 to 1500 nT anomalies. The horizontal anomalies are predominantly negative and confirm the inferred inverse polarity of the drilled oceanic crust. At the base of the extrusives (oceanic Layer 2A) there are positive anomalies, presumably from basalts with normal magnetization. However, steeply dipping layers with inverse polarity could also cause positive anomalies. The vertical extent of negative anomalies in Layer 2A is only 200 m for Hole 896A and 475 m for Hole 504B, indicating a strong lateral variability in the thickness of this source layer. The fine structures of the anomalies in both holes bear little resemblance to each other but the average amplitudes of the horizontal anomalies are both around 2000 nT in the upper parts. Simple forward models as horizontal cylinders allow us to determine average magnetization values of 3 A/m for the extrusive basalts and 1.7 A/m for the sheeted dikes (Layer 2B). The former is only half the average measured rock magnetization, whereas sheeted dike values agree. At depths that correspond to the transitions in drilling from Legs 111/137 and 140/148 there are clear changes in the horizontal anomalies. These are inteΦreted as a result of drilling-induced magnetizations caused by strongly magnetic drill pipes used during Legs 137 and 140. The deduced average magnetization intensities have been used as parameters to model surface anomalies. Layers 2A and 2B contribute approximately equally to the surface anomalies with maximum amplitudes of 160 nT that agree well with the measured anomalies; thus gabbros are unlikely to contribute to marine lineated magnetic anomalies.