3D Seismic Explanation

THE VALUE OF 3D SEISMIC IN TODAY'S EXPLORATION ENVIRONMENT 
-- IN CANADA AND AROUND THE WORLD
Cooper, N.M.1
1Mustagh Resources Ltd., Calgary, 400 604 -- 1st Street SW, Calgary, Alberta T2P 1M7, e:mail: ncooper@mustagh.com
OVERVIEW
Exploration for oil and gas involves the evaluation of a variety of information. Well logs provide detailed information at specific locations, usually an area lessthan one meter surrounding the well bore. We use our knowledge of formation signatures to recognize depositional environments and try to project this information between wells.
Reflection seismic methods can be used to create images of the geologic changes between wells. These images can help complete a picture of the subsurface that should enhance the ability of the explorationist tosuccessfully select future well locations. Seismic is an effective tool as long as it provides meaningful and helpful images for the desired objectives. The seismic tool must provide a cost efficient alternative to additional drilling.
3D seismic has become a common exploration and production tool. This year, about 1200 3D programs will be recorded in Canada. In fact, Canadian geophysicists areinternationally recognized as experts in the design, acquisition and processing of 3D seismic programs. 3D has also taken a strong hold on seismic operations in the United States. In every country where the author has consulted (18 countries on 6 continents), 3D techniques are either being used or are under serious consideration. The following map indicates the locations of some of the more intense onshore3D activity. It is by no means a complete record of activity levels.

What is it about 3D that is making it such a popular tool? What determines the cost of 3D seismic and how does it compare to 2D?
REVIEW OF 2D SEISMIC
Reflection seismic is a method that allows us to image changes in the subsurface geology by inducing an acoustic wave from near the surface of the earth and listening for theechoes from deeper stratigraphic boundaries (much like ultra-sound is used to create pictures of unborn babies in their mother’s wombs).
  
2D seismic is recorded using straight lines of receivers crossing the surface of the earth. Acoustic energy is usually provided by the detonation of explosive charges or by large vibroseis trucks. The sound spreads out through the subsurface as a sphericalwave front. Interfaces between different types of rocks will both reflect and transmit this wave front. The reflected signals return to the surface where they are observed by sensitive microphones known as geophones. The signals detected by these devices are recorded on magnetic tape and sent to data processors where they are adjusted and corrected for known distortions. The final processed data isdisplayed in a form known as "stacked" data.
COMPARISON OF 3D AND 2D METHODS
In the 3D seismic method, we record many lines of receivers across the earth’s surface. The area of receivers we record is known as a "patch". Often, we employ lines of source points laid out orthogonally to the receivers. By sequentially recording a group of shots lying between two receiver lines (referred to as a"salvo") and centered within the patch, we obtain uniform, one-fold reflection information from a subsurface area that is one quarter of the useful surface area of the patch. Although we usually record a large square or rectangular patch, the useful data at our zone of interest is offset limited by several geophysical factors. Therefore, we often consider the useful area of coverage as a circle with aradius equal to our maximum useful offset. By moving the patch and recording more salvos of source points, we accumulate overlapping subsurface coverage and build statistical repetition over each subsurface reflecting area (bin).
The quality of the sub-surface image obtained can be related to the statistical diversity of the information recorded for each cell of sub-surface coverage (known as a...