Marine magnetotellurics for base-of-salt mapping: Gulf of Mexico field test at the Gemini structure

A sea-floor magnetotelluric (MT) survey was conducted over the Gemini subsalt petroleum prospect in the Gulf of Mexico (GOM) to demonstrate that the base of salt can be mapped using marine magnetotelluric (MMT) methods. The high contrast in electrical resistivity between the salt and the surrounding sediments provides an excellent target for MMT. The Gemini salt body, located at 28 degrees 46' N 88 degrees 36' W, is a relatively complex shape buried 2-5 km below the sea floor in I-km-deep water. Its geometry has been previously determined using 3-D seismic prestack depth migration with well log control. In order not to confuse limitations in interpretation technique with limitations in data acquisition, numerical forward and inverse modeling guided the survey design to locate a profile that would be amenable to 2-D inversion, even though the body was clearly 3-D. The seismic imaging of the base of salt along the chosen profile is considered good, thus providing a good control for testing the MT method. In many other areas of the GOM, and indeed other portions of the Gemini structure itself, seismic imaging of the base of salt is problematic. Data were collected using autonomous sea-floor data loggers equipped with induction coil magnetic sensors and electric field sensors consisting of silver-silver chloride electrodes connected to an ac-coupled amplifier originally designed for sea-floor controlled-source studies. Nine sites of excellent quality MT responses were obtained. Smooth 2-D inversion of the data produce a confined resistive anomaly at the comet location and depth, and recently developed sharp-boundary 2-D inversion recovers base of salt in excellent agreement with the seismic models. Simple perturbation analysis shows that base of salt has been resolved to within 5-10% of burial depth.