DOWNHOLE MAGNETOSTRATIGRAPHY IN SEDIMENTS - COMPARISON WITH THE PALEOMAGNETISM OF A CORE

The magnetostratigraphy of weakly magnetized sediments is determined f or the first time from direct downhole measurements. Experimental resu lts were obtained in both scientific and oil-exploration drillholes pe netrating Jurassic sediments of the Paris basin, France. The magnetic polarity of the remanence is deduced from the total magnetic induction , combined with the magnetic susceptibility. The magnetic induction is measured in depth by a Proton Precession Magnetometer with Overhauser effect designed by the Laboratoire d'Electonique de Technologie et In strumentation (Commissariat a l' Energie Atomique). This method gives a continuous record with an accuracy of 0.1 nT, and an excellent stabi lity. In order to cancel out the transient component of the Earth's ma gnetic field, the magnetic induction is computed as the difference bet ween the induction measured in the borehole and that obtained at a sur face reference point located near the drillhole. The dipole-dipole sus ceptibility probe gives a continuous record with an accuracy of a few 10(-6). The four total magnetic induction logs recorded in oil explora tion holes show a tight correlation, although distant from up to 80 km . This technique is therefore excellent for precise stratigraphic corr elation. Data processing allows us to isolate the component of inducti on due to remanent magnetization and to find the magnetic polarity. A sequence of magnetic polarity is proposed for Kimmeridgian and Oxfordi an stages. The variation of the sedimentation rate is analyzed with a very high resolution in time, and compared with the mean sedimentation rate deduced from a lithologic log. Cores were collected in the lower most part of two holes. The correlation between the magnetostratigraph y deduced from paleomagnetic measurements performed on 240 standard pl ugs and that obtained by logging is good. A study of the components of magnetization found in the core has shown that the remagnetization oc curred mainly during the coring operation itself or during the storage of the core. Consequently, downhole magnetostratigraphy proves to be reliable, besides providing the advantage of a continuous investigatio n performed over a large volume of rock scrutinized in situ.