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◦ 46′ N 88◦ 36′ W, is a relatively complex shape buried 2–5 km below the sea floor in 1-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 correct 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. Presented at the 68th Annual International Meeting, Society of Exploration Geophysicists. Manuscript received by the Editor August 17, 1998; revised manuscript received March 24, 2000. ∗Lawrence Berkeley National Laboratory, One Cyclotron Road, MS 90-1116, Berkeley, California 94720. E-mail: GMhoversten@lbl.gov. ‡Scripps Institution of Oceanography, IGPP 0225, La Jolla, California 92093-0225. E-mail: sconstable@ucsd.edu. ∗∗University of California at Berkeley, 577 Evans Hall, Berkeley, California 94720. E-mail: hfmengeo@socrates.berkeley.edu. c © 2000 Society of Exploration Geophysicists. All rights reserved. INTRODUCTION The magnetotelluric (MT) method has been used for many decades on land to assist in exploration for petroleum (Vozoff, 1972; Orange, 1989). It is particularly useful for basin reconnaissance, and in areas where seismic methods perform poorly, such as carbonate and volcanic terrain. In areas where sediments are obscured by rock units that scatter and reflect most normal incident seismic energy, electrical methods may be very helpful in determining the structural relationships and thickness of the various units. It is generally true that rocks with high seismic velocities and impedance contrasts are also higher in electrical resistance than surrounding sediments. For these reasons, Hoversten and Unsworth (1994) and Hoversten (1992) carried out model studies to test the feasibility of using MT methods to map base of salt in the Gulf of Mexico (GOM) and other regions. Salt has a high acoustic contrast with surrounding sediments, which make seismic sections difficult to interpret, and it is much more resistive than water-saturated clastic sediments. It was shown that with MT data of reasonable quality in the 0.001–1 Hz band, the base of salt could indeed be mapped with accuracy approaching that of seismic methods. Unfortunately, however, most marine MT up to that time involved use of long-period instrumentation in deep ocean waters to probe the mantle at depths of 100 km and deeper (e.g., Filloux, 1983). It was generally believed that attenuation of the natural EM fields by seawater precluded the use of MT in the ocean at any significant depth (Chave et al., 1991). An early attempt to use MT in very shallow water in the GOM met with limited success (Hoehn and Warner, 1960); however, data processing techniques at the time were unable to remove the large wave-motion noise resulting from the experiment’s depth of only 10 m. In addition, the equipment was far too bulky to be commercially viable. And because it was a moored system, it would not have been practical in modern prospects where the sea-floor depth approaches 2000 m. Traditionally, controlled-source methods have been used on the sea floor to replace the electromagnetic (EM)