A marine magnetotelluric (MMT) survey was performed to assist seismic and gravity methods in mapping the base of salt in a complex system in the Gulf of Mexico. For the salt complex investigated, 2D interpretation of MMT data provided an incorrect base of salt, when compared with well-derived data. When 3D inverse modeling was employed, not only the correct base of salt was determined, but additional detail of the subsurface was produced. Resistivity variation within the sedimentary host provides indication of lithologic variation that can be interpreted further. A preliminary correlation between low-resistivity zones and low velocity was also indicated. Separation of data-required subsurface resistivity of the model and resolution effects of the method, was also investigated.
WesternGeco has an ongoing project to apply marine magnetotelluric, full-tensor gravity (FTG), and seismic interpretation to refine the base of allochthonous salt bodies in the Gulf of Mexico. The overall goal is to improve the resolution of seismic images of possible hydrocarbon-bearing sedimentary structures beneath the salt.
In this paper, we present some detail of the interpretation of a complex of salt bodies in the Garden Banks region of the Gulf. To map this salt, a total of 171 MMT seafloor stations were occupied in 2006, making it the largest MMT survey collected worldwide at that time.
Magnetotellurics (MT) is a natural-source, electromagnetic method of imaging structures in the subsurface. Natural variations in the earth's magnetic field induce electric currents (or telluric currents) under the earth's surface. Measurements of orthogonal components of the electric and magnetic fields at a receiver site constitute the data that are used to image the resistivity structure of the subsurface.
The MMT method generally refers to an MT survey in which the receiver sites are located at the seafloor beneath the seawater. The overlying seawater significantly changes the available signal strength at higher frequencies, and creates a current flow path above the receiver positions.
Electromagnetic investigations of the earth beneath the seafloor have a long history (Bannister, 1968 [active source]; Wannamaker et al., 1989; Heinson et al., 1993). Recently seafloor MT instruments have evolved to be capable of reliably measuring higher frequencies associated with structures shallower than the oceanic mantle and crust (Key and Constable, 2002).
Applying the MT method to mapping salt bodies for hydrocarbon exploration has been discussed in the literature. For example, Watts, et al. (2003) described an onshore example of how MT could be applied where the seismic method had difficulty because of velocity inversion associated with both salt and carbonate structures.
