Geophysical imaging of the base of remobilized salt, and hence, the underlying sediments and basement, can be difficult in certain deepwater salt provinces of the northern Gulf of Mexico. The difficulty arises primarily due to: (1) the contorted shapes that the boundary between salt and the surrounding sediments can take, and (2) the fact that there can be multiply connected and disconnected salt bodies overlying each other in depth and lateral extent. Even though density and acoustic velocity contrasts between salt and sediment are significant, limitations in the gravity and seismic methods can still create difficulties in solving the problem of fully describing the salt geometries and associated exploration lithologies.
Because the large elastic contrast in the physical properties of salt versus sediment does not always provide useful measurements of salt/sediment relationships, one can look for properties other than seismic velocity to exploit. There is a large resistivity contrast between salt and sediment as well; indicating that an electromagnetic method might prove useful. Consequently, the marine magnetotelluric (MMT) method offers promise in being able to differentiate salt/sediment geology, both alone and by integration with seismic (traditional and long offset), and gravity (including higher-resolution full-tensor gravity, FTG).
The images shown to the right are part of the methodology and preliminary results from an ongoing interpretation of a WesternGeco MMT project in the Gulf of Mexico. Seismically measured depth to top of salt and stratigraphic control, along with three-dimensional FTG and MMT measurements are being used to co-constrain models derived from the three different physical property measurements. In this specific case, seismic and gravity interpretations are being used to provide initial trials for a combination of different MMT models, including: 1D, 2D-smooth, 2D-sharp-boundary, and 3D-forward and -inverse geoelectrical modeling. These geoelectrical models are then being used to constrain the bottom of salt and basement positions for an update to the 3D gravity modeling. These constraints can then be used to update seismic interpretations and the resulting velocity models used for prestack depth migration.
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