A Practical Approach to 3D Inversion of Pseudo-gravity for Depth to Basement Mapping - A Test Using the Bishop Model

The major use of magnetic data in hydrocarbon exploration is in mapping depth to basement. The available magnetic interpretation methods are generally good at mapping basement lineaments and structures, but generally less good at mapping the morphology of the deeper parts of the basement surface. This is due to the dominance of the magnetic response coming from the top edges of magnetic bodies compared to the bottom edges. The method proposed here applies 3D inversion to pseudo-gravity (the vertical derivative of the RTP magnetic field). This provides accurate depths for the complete basement surface as we are using all the magnetic field, rather than isolated parts of it. To test the method we use the simple 3D Bishop model with constant susceptibility basement. We observe a strong linear relationship between the pseudo-gravity and the true depth of the model. We show how the parameters for the 3D inversion can be constrained using results from conventional depth estimation techniques Our inversion using GM-SYS 3D software produces a basement surface that closely matches the actual depths of the Bishop model. Development of the technique is on-going including application to variable magnetisation basement and examples using real data.