A NEW METHOD FOR DETERMINING MAGNETIZATION DIRECTION FROM GRAVITY AND MAGNETIC-ANOMALIES - APPLICATION TO THE DEEP-STRUCTURE OF THE WORCESTER-GRABEN (VOL 152, PG 561, 1995)

The aeromagnetic field over the Worcester Graben, a major N-S-oriented, fault-bounded sedimentary basin, shows a large positive anomaly of 100 nT amplitude near the town of Tetbury. A new method for the determination of the magnetization direction of the causative body was employed that searches for the maximum correlation between the observed gravity anomaly and the pseudogravity anomaly computed for a range of orientations of the magnetization vector. The method suggests a vertical magnetization for the causative body of the Tetbury anomaly. A three-dimensional magnetic model of the body has the form of a cone, elongated N-S, whose base is at 6.0 km and upper surface at 3.1 km as indicated by power spectrum analysis of the anomaly. A two-dimensional model derived from the gravity field and constrained by the surface geology, borehole data and existing density measurements cannot simulate the observed anomaly over the centre of the graben. However, when the magnetic body is included in the gravity model, the fit is much improved. The western side of the body is probably imaged on a seismic reflection profile to the north. The body magnetization vector and the correlation of its possible northern extension with exposed Malvernian rocks suggest a Precambrian age.