G&G Data Acquisition and Analysis

Geophysical Surveys

Seismic Surveys play the most significant role in supplying valuable data for oil and gas exploration.

For the past 30 years, BOEM (BOEMRE/MMS) has issued 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 the majority of them being shot in the Gulf of Mexico OCS. Statistically speaking, a large percentage of the geophysical data in the BOEM inventory is two-dimensional (2-D) CDP seismic information. 2-D seismic data has been the basis of evaluation for many of the historic BOEM 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.

BOEM 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 (kilometers) in length, separated by 75 to 150 ft. (~22 to 50 m). The seismic energy is produced by an array of air guns towed behind the vessel, generating highly pressurized air, and 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 in the GOMR, 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 standard pre-requisite for oil and gas exploration during the 1990's. 3-D seismic acquisition technology has developed through constant improvements, which 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 are related to imaging the subsalt and complex geologic structures. In these environments, conventional 3-D seismic may not be able to provide the high quality data needed for more precise imaging and accurate reservoir delineation. To solve these imaging problems, more advanced acquisition techniques such as multi-component, wide-azimuth (WAZ), full-azimuth (FAZ), multi-azimuth (MAZ) are now being used in the GOM to enhance seismic imaging quality to levels not achievable before. In the past 10 years, 3-D surveys have increasingly been repeated, enabling time-lapse reservoir monitoring.

The hardware used for and the design of the seismic surveys has also improved significantly. In addition to the technologies used during data acquisition, new seismic data processing algorithms have been developed and more sophisticated interpretation techniques have been 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 have been created, enabling the processing and interpretation of multi-component seismic data.

In addition to the exploration seismic surveys, 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.

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 lies 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 gravity exploration is to map density differences that may indicate different rock types. Gravity data usually are displayed as anomaly maps.

By regulation, BOEM has the right to acquire any data collected under an OCS permit. BOEM has also actively collected data in the OCS for various reasons. It is possible to look at the types and amount of data BOEM has collected and purchased from industry by location and time. Industry, overall, has acquired more data in the OCS than BOEM, but BOEM has purchased a substantial portion of that data. As an example, of the data shot by industry in the Alaska OCS, BOEM has acquired approximately 90 percent. Alaska remains a large frontier area with limited data coverage by industry, a fact that necessitates BOEM to acquire as much of these data as feasible.

BOEM acquired more data in the Atlantic Region than industry in 1976 and 1983. Before 1976, the BOEM database was limited because industry had shown very little interest in leasing this frontier area, although industry had been acquiring geophysical data.

During the period 1976 to 1984, BOEM not only acquired most of the industry data, but purchased much of the pre-1976 data. Since 1985, there has been less activity, reflecting a decrease in industry interest. Since the mid-1980’s, there has been less activity possibly reflecting less industry interest and a more focused approach to offshore exploration.

In recent years, the Gulf of Mexico has acquired most 3-D surveys and most large 2-D surveys. However, BOEM does not acquire all the volumes that industry obtains to reprocess.

Some of the reasons BOEM does not acquire all the data held by industry are as follows:

• Redundancy of data, because individual companies do not share it,
• Difference in data quality,
• Budgetary constraints,
• Personnel limitations for data reduction and interpretations, and
• Data storage, retrieval and display limitations.

In conclusion, totals for mileage acquired, permits issued, and expenditures reflect the overall trends of oil and gas pricing, limitations of areas due to offshore moratoria, and a shift of industry interest to foreign theatres.