Table Definitions, Sources, and Explanatory Notes

Definitions, Sources and Explanatory Notes

Category:	Exploration & Reserves

Topic:	Maximum U.S. Active Seismic Crew Counts


Definitions

Key Terms	Definition
Dimensions	In two-dimensional (2D) reflection seismic surveying both the sound source and the sound detectors (numbering up to a hundred or more per shot) are moved along a straight line. The resultant product can be thought of as a vertical sonic cross-section of the subsurface beneath the survey line. It is constructed by summing many compressional (pressure) wave reflections from the various sound source and sound detector locations at the halfway sound path points beneath each location (common depth point stacking). In three-dimensional (3D) reflection seismic surveying the sound detectors (numbering up to a thousand or more) are spread out over an area and the sound source is moved from location to location through the area. The resultant product can be thought of as a cube of common depth point stacked reflections. Advantages over 2D include the additional dimension, the fact that many more reflections are available for stacking at each point, which provides greatly improved resolution of subsurface features, and elimination of the "ghost" or "side swipe" reflections from nearby offline features that 2D surveys are prone to (except, of course, along the outer faces of the cube). Four dimensional (4D) reflection seismic surveying is the exact repetition of a 3D survey at two or more time intervals. The primary application of 4D is mapping the movement of fluid interfaces in producing oil and gas reservoirs.
Seismic Crew	A group of people, of varying number, engaged in a seismic surveying job.

Key Terms

Definition

Dimensions

In two-dimensional (2D) reflection seismic surveying both the sound source and the sound detectors (numbering up to a hundred or more per shot) are moved along a straight line. The resultant product can be thought of as a vertical sonic cross-section of the subsurface beneath the survey line. It is constructed by summing many compressional (pressure) wave reflections from the various sound source and sound detector locations at the halfway sound path points beneath each location (common depth point stacking).

In three-dimensional (3D) reflection seismic surveying the sound detectors (numbering up to a thousand or more) are spread out over an area and the sound source is moved from location to location through the area. The resultant product can be thought of as a cube of common depth point stacked reflections. Advantages over 2D include the additional dimension, the fact that many more reflections are available for stacking at each point, which provides greatly improved resolution of subsurface features, and elimination of the "ghost" or "side swipe" reflections from nearby offline features that 2D surveys are prone to (except, of course, along the outer faces of the cube).

Four dimensional (4D) reflection seismic surveying is the exact repetition of a 3D survey at two or more time intervals. The primary application of 4D is mapping the movement of fluid interfaces in producing oil and gas reservoirs.

Seismic Crew

A group of people, of varying number, engaged in a seismic surveying job.

For definitions of related energy terms, refer to the EIA Energy Glossary.

Sources


	"World Geophysical News", IHS Energy Group, Denver, CO, used with permission.

Explanatory Notes



	"48 States" is the United States excluding Alaska and Hawaii.
	"48 States, Offshore" are Federal and State Jurisdication waters of the Gulf of Mexico.
	Alaska is all onshore.
	Total crews includes crews with unknown survey dimension.
	Data are reported on the first and fifteenth of each month, except January when they are reported only on the fifteenth.
	When semi-monthly values differ for the month, the larger of the two values is shown here. Consequently, this table reflects the maximum number of crews at work at any time during the month.