Interpretation of magnetic anomalies using the pseudogravimetric transformation and other methods, with application to tertiary volcanic centres in N-W Scotland

Abstract

A method of transforming magnetic anomalies into pseudogravimetric anomalies which uses fast Fourier transforms and a method of interpreting gravity anomalies in three dimensions using a non-linear optimization technique which involves automatic end corrections are developed. These methods have been used to interpret the complicated aeromagnetic anomalies over the Mull, Skye and Blackstones Bank Tertiaryigneous centres of N.W. Scotland. One inch aeromagnetic maps over the complexes have been digitized at 1 km intervals covering 32 x 32 km2 (Mull and Blackstones Bank) or 64 x 64 km (Skye).The strongly magnetized rocks of the Mull complex extends in the preferred model to a depth of 2.1 km with normal magnetization of 6.7 A/m. This body is smaller in depth and a real extent than the gravity model of the complex. The magnetic model of the Skye complex consists of two strongly magnetized bodies of opposite polarity and a weakly magnetized region corresponding to the Red Hills granites. A reversely magnetized body of depth extent in the range 1.5km - 3.0km with a modelling magnetization of 4.9 A/m can be identified with the basic and ultrabasic rocks of the Cuillin centre. Normally magnetized body probably represents more weakly magnetized deep seated body which underlains the whole complex and extends to a modelled depth of 14 km with a modelling magnetization of 1.1 A/m. This composite model has approximately equal dimensions to the dense rocks of the gravity model. The main body of the Blackstones Bank magnetic model extends to a depth of 15 km with reverse magnetization of 0.93 A/m. A shallow normally magnetized body occurs within this to depth of the order of 1.0km - 5.0km with a magnetization in the range O.OA/m-2.47 A/m. The composite model has approximately equal dimensions to the gravity model. Intense negative anomalies over the lavas and over some intrusive rocks demonstrate the strong remanent magnetization of these rocks. The magnetic interpretations show that basic and ultrabasic rocks are strongly magnetized and large in volume and the granitic rocks are small in volume, shallow and weakly magnetized. Generally, the magnetic interpretations agree with previous gravity interpretations.