Overview
	Geophysical Surveys
	Geological Surveys
	Maps of Surveys in the Gulf of Mexico
	G&G Publications

Geophysical Surveys

Seismic Surveys play the most significant role in supplying valuable data for oil and gas exploration. MMS issues permits and obtains data from 2-D and 3-D marine seismic surveys on the OCS. Marine seismic surveys differ from land surveys in the sources of seismic energy, types of receivers (hydrophones), survey geometries and the manner of acquisition. During a typical 3-D marine seismic acquisition one or more vessels   tow a number of parallel streamers, several miles in length, separated by 75 to 150 ft. The seismic energy is produced by an array of air guns placed behind the vessel, which are fed with high-pressure air, and are fired every 10 to 20 seconds. Depending on the survey design, the separation between the source and the streamers can be varied. Up until the mid 1980-s the marine seismic surveys were predominantly 2- D, i.e. imaging just one single vertical section at a time. The need for more precise imaging of the subsurface drove the technology into the domain of  3-D surveys, which became a standard pre-requisite for  oil and gas exploration during  the 1990-s. The 3-D seismic acquisition technology has sustained constant improvements and has allowed for some of the biggest oil and gas discoveries to be made in the past 20 years. However a number of pitfalls still exist, some of which are related to the subsalt imaging and the imaging of complex geologic structures. The conventional 3-D seismic surveys sometimes are unable to provide high quality, when more precise imaging and accurate reservoir delineation is necessary. To solve those problems, more advanced acquisition techniques such as multi-component, wide-azimuth (WAZ), full –azimuth (FAZ), multi-azimuth (MAZ) have moved from the research labs into the practical acquisition process. These techniques can enhance seismic imaging quality to levels not achievable before. In the past 10 years increasingly repeat 3-D surveys have been carried out enabling time-lapse reservoir monitoring. A number of high-resolution seismic surveys are being shot in order to provide analysis of the shallow layers and identify potentially hazardous conditions such as surface faulting, potential slope failure areas, or shallow gas accumulations. The hardware and the design of the seismic surveys have marked significant progress too. In addition to the purely acquisition technologies, new seismic data processing algorithms have been developed and more sophisticated interpretation techniques were created. Advanced pre-stack time and depth migration techniques have greatly enhanced the quality of the acquired data. Amplitude Versus Offset (AVO) analysis has further allowed better hydrocarbon indication. New software packages were created enabling the processing and interpretation of multi-component seismic data.

For the past 25 years MMS has issued a number of permits for conventional 2-D and 3-D seismic surveys as well as for multi-component, high resolution, wide-azimuth and other advanced types of seismic surveys with majority of them being shot in the Gulf of Mexico OCS. Still, statistically speaking, a large percentage of the geophysical data in the MMS inventory is two-dimensional (2-D) CDP seismic information. 2-D seismic data has been the basis of evaluation for many of the historic MMS lease sales. It is still one of the main sources of information in the Alaska tract evaluations, although 3-D is increasingly being acquired in that region.

Magnetic surveys measure the magnetic field or its vertical component at a series of different locations over an area of interest. Aeromagnetic data consist of magnetic measurements made from an aircraft, and they offer measurements of larger areas. Magnetic and aeromagnetic acquisitions delineate anomalies caused by the changes in physical properties of the subsurface that lie beneath a thick layer of sediments

Gravity surveys produce measurements of the gravitational field at a series of different locations over an area of interest. The objective in exploration work is to map density differences that may indicate different rock types. Gravity data usually are displayed as anomaly maps.