Mapping the depth to magnetic basement using inversion of pseudogravity: Application to the Bishop model and the Stord Basin, northern North Sea

摘要

Magnetic depth estimation methods are routinely used to map the depth of sedimentary basins by assuming that the sediments are nonmagnetic and underlain by magnetic basement rocks. Most of these methods generate basement depth estimates at discrete points. Converting these depth estimates into a grid or map form often requires the application of qualitative methods. The reason for this is twofold: first, in deeper parts of basins, there is generally a scarcity of depth estimates and those that have been determined tend to be biased toward the shallower basement structures close to the basin edge; and second, depth estimates intrinsically relate to magnetic anomalies that emanate from the top edges of basement faults/contacts resulting in a shallow depth bias. Thus, simple grid interpolation of these depth estimates often forms a shallower and structurally unrepresentative map when evaluated in detail. To overcome these problems of qualitative and/or simple grid interpolation of these point-depth estimates into a regular grid, we use the pseudogravity field transform response of the magnetic field to constrain this interpolation using inversion methods together with the relationship between the point-depth estimates and their pseudogravity values. The pseudogravity transformation converts a grid of magnetic data such that the resulting grid has the same simple relationship to magnetic susceptibility that a gravity grid has to density. The pseudogravity map is thus straightforward to visualize in terms of basement structure, but it only maps the magnetic properties of the subsurface and is not related to the gravity anomaly or the density. We describe a practical approach to invert pseudogravity grids using gravity inversion software to produce a 3D basin model assuming a constant susceptibility basement. The approach is initially tested on the Bishop 3D model and then applied to an example from the northern North Sea. This approach can be considered complementary to 3D gravity inversion and has the advantage that the pseudogravity response is not affected by structure within the sediments or effects such as sediment compaction, inversion, or isostatic compensation, all of which often complicate the gravity response of sedimentary basins.