New developments of the magnetic tilt‐depth method to improve structural mapping of sedimentary basins

Abstract

This paper interprets aeromagnetic data for a deep basin section of the Karroo rift in south‐east Tanzania. We use a novel integrated approach involving the application of advanced derivatives to define structure and the tilt‐depth method to determine and map the depth to basement. In the latter case we use the result of both reduced to pole and reduced to equator data to help constrain the shape and depth of the basin. We show that for a reduced to pole aeromagnetic data set, the generalized form of the local phase, called the tilt derivative, is an effective means of providing an initial (first pass) mapping of a sedimentary basin in terms of its fault structure, dip direction of faults and depth to basement. Since the amplitude of the tilt derivative does not contain information on the strength of the geomagnetic field nor magnetization (other than inclination) of the causative body, the susceptibility contrast across faults/contacts is derived from the analytic signal derivative. We also investigate how effective the tilt derivative and tilt‐depth method are for structural and depth to basement mapping in regions close to the magnetic equator, where the reduction to pole transform is often unstable; this is done using the same Tanzania data set transformed to the pole and the equator. We find that the tilt derivative applied to the reduction to equator data cannot be used to map the structure because of the effects of magnetic anisotropy, which results in the magnetic response of structures varying with strike azimuth. To overcome this anisotropy problem the analytic signal and/or local wavenumber derivatives, which are for all practical purposes independent of inclination, provide the best means of defining the major structural trends. We also find that the tilt‐depth method provides coherent depth to basement estimates for both reduced to pole and reduced to equator data. For the deep basin sections of the Karroo rift, there is a sparsity of tilt‐depth results from both the reduced to pole and reduced to equator data sets. However, each set of results has a different spatial coverage, so when combined they provide a better spatial sampling of the long wavelength magnetic character of the basin and thus improve the constraints on the minimum curvature gridding method to map the shape and depth of the basin.

Determining the depth of certain gravity sources without a priori specification of their structural index

“…Different values of N should be chosen for different models before depth estimation (Salem et al, 2007(Salem et al, , 2013Cooper, 2011Cooper, , 2012Fairhead et al, 2011). However, in real potential field interpretation, the buried source type is unknown.…”

“…Salem et al (2007) pointed out that the tilt angle method could be used to determine the vertical contact depth by half the distance between the − 45°and the + 45°contours. Fairhead et al (2011) applied the tilt angle method to interpret fault structure. Roy (2013) interpreted transformed total magnetic intensity data for a dipping contact model using the Tilt-depth method.…”

The horizontal boundary and top depth estimates of buried source using gravity data and their applications

“…The Tilt angle consists of the ratio of the vertical and horizontal derivatives and the "tan −1 " part of the function limits the magnitude of the angle to ±90°and together with the ratio of the vertical and total horizontal gradients operates as an effective automatic gain control filter such that small and large amplitude anomalies now have normalized Tilt amplitudes (Fairhead et al, 2011). The zero contour of the Tilt angle is generally located close to the boundary of the causative body.…”

“…1). Later the method has been discussed in the theoretical precision and application conditions, and was used in the interpretation of aeromagnetic data in Namibia, Tanzania and other regions (Salem and Williams, 2007;Lee et al, 2009;Lahti and Karinen, 2010;Fairhead et al, 2011). This paper firstly introduces the Tilt angle and Tilt-depth methods, then extends the derivative from the order of 1 to 2, and derived the horizontal boundaries and top depth estimates of buried sources for gravity anomaly based on the 2nd derivative.…”

Interpretation of basement structures from aeromagnetic data associated with oil producing fields in parts of the Central Kansas Uplift

“…Aeromagnetic survey is a method of geophysical exploration that is widely used to delineate basement structures (e.g. Fairhead et al, 2011) and targets of interest for hydrocarbon and mineral exploration. Magnetic data dominantly depends on the magnetite content of rocks which tends to be low in most sediments.…”