When All Data Are Not Created Equally

Contributors: Satinder Chopra

Tidewater areas can be difficult places to acquire consistent-quality seismic data, because different sources have to be used across exposed land surfaces than what are used across shallow-water areas.

Typically, explosives are used in shot holes in the onshore portion of a tidewater prospect, whereas environmental regulations may require that an air-gun source be used in shallow-water areas.

These two seismic sources produce different basic wavelets – and profiles produced with explosives and air guns rarely tie in an optimal manner at common image coordinates without using wavelet-shaping algorithms to create equivalent reflection character across targeted intervals.

An example of using an explosive source and an air-gun source across a Louisiana tidewater area is documented as figures 1 and 2. This shallow-water test line was recorded twice because, at this location, explosive sources were allowed.

For one profile, the source was a 30-pound (13.6-kilogram) charge positioned at a depth of 135 feet (41 meters) at each source station.

For the second data acquisition along the same profile, the source was an array of four air guns with a combined volume of 920 in3, and eight air-gun pops were summed at each source station.

Considerable processing effort was expended to make the final reflection character identical on each test line. The data illustrated as figure 1 show the results of the data processing.

The frequency content of the two profiles is approximately the same, but wavelet character is not identical at the junction point (station 165). In this instance, the interpreter responsible for this prospect decided that the reflection character expressed by the explosive source was preferred rather than the wavelet response shown by the air-gun source.

The challenge was that in neighboring tideland areas, regulations required that an air-gun source be used in water-covered areas – shot-hole explosives could not be used in shallow water as they had been across this initial test site, and a method had to be developed that would allow air-gun-source data to be used in conjunction with explosive-source data acquired across adjacent exposed-land areas.

Said another way, the problem was to create a basic wavelet in air-gun-generated data that was equivalent to the basic wavelet embedded in explosive-source data.

This type of problem has to be solved by data-processing procedures, not by data-acquisition techniques.

An approach used by many data processors to ensure that equivalent basic wavelets exist in two seismic profiles acquired with different sources is to calculate numerical cross-equalization operators that convert the phase and frequency spectra of source A to be equivalent to the phase and frequency spectra of source B.

This technique was applied to the tidewater seismic data illustrated on figure 1 by using data from the image trace at station 153 to calculate cross-equalization operators that converted the phase/frequency spectra of the air-gun data to the spectra of the explosive-source data.

The result is exhibited as figure 2.

The wavelet character of the profiles now agrees better at the tie point so that common horizons, sequence boundaries, and facies character can be interpreted on both profiles with greater confidence.

The example discussed here is from a tidewater area where operating and environmental constraints forced different sources to be used on land-based and water-based seismic lines.

The concept of numerical equalization of the basic wavelets embedded in any grid of intersecting 2-D (or 3-D) data, however, applies to a variety of onshore and offshore areas where people have access to overlapping legacy seismic data that have been acquired by different companies at different times and with different energy sources.

Comments (0)


Division Column-DEG Jeffrey Paine

Jeffrey Paine is DEG President for 2014-15.

Geophysical Corner

Geophysical Corner - Satinder Chopra
Satinder Chopra, award-winning chief geophysicist (reservoir), at Arcis Seismic Solutions, Calgary, Canada, and a past AAPG-SEG Joint Distinguished Lecturer began serving as the editor of the Geophysical Corner column in 2012.

Division Column-DEG David Vance

David Vance is principal scientist, ARCADIS-US Inc., Midland, Texas, and is a member of the DEG CO2 Sequestration Committee.  

Division Column-DEG Doug Wyatt

Doug Wyatt, of Aiken, S.C., is director of science research for the URS Corporation Research and Engineering Services contract to the USDOE National Energy Technology Laboratory. He also is a member of the DEG Advisory Board for the AAPG Eastern Section.

Division Column-DEG Tom J. Temples

Tom J. Temples is DEG President.

Division Column-DEG Bruce Smith

Bruce Smith is a DEG member and is with the Crustal Geophysics and Geochemistry Science Center of the U.S. Geological Survey in Denver.

Geophysical Corner

The Geophysical Corner is a regular column in the EXPLORER that features geophysical case studies, techniques and application to the petroleum industry.


See Also: Book

Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 4386 Book
Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 4476 Book

See Also: Bulletin Article

Interpretation of seismic data from the Sorvestsnaget Basin, southwest Barents Sea, demonstrates gradual middle Eocene basin infilling (from the north) generated by southward-prograding shelf-margin clinoforms. The basin experienced continued accommodation development during the middle Eocene because of differential subsidence caused by the onset of early Eocene sea-floor spreading in the Norwegian-Greenland Sea, faulting, salt movement, and different tectonic activity between the Sorvestsnaget Basin and Veslemoy high. During this time, the margin shows transformation from an initially high-relief margin to a progradation in the final stage. The early stage of progradation is characterized by the establishment of generally oblique clinoform shifts creating a flat shelf-edge trajectory that implies a gentle falling or stable relative sea level and low accommodation-to-sediment supply ratio (lt1) in the topsets. During the early stage of basin development, the high-relief margin, narrow shelf, stable or falling relative sea level, seismicity, and presumably high sedimentation rate caused accumulation of thick and areally extensive deep-water fans. Seismic-scale sandstone injections deform the fans.

A fully prograding margin developed when the shelf-to-basin profile lowered, apparently because of increased subsidence of the northern part. This stage of the basin development is generally characterized by the presence of sigmoid clinoform shifts creating an ascending shelf-edge trajectory that is implying steady or rising relative sea level with an accommodation-to-sediment supply ratio of greater than 1, implying sand accumulation on the shelf. This study suggests that some volume of sand was transported into the deep water during relative sea level rise considering the narrow shelf and inferred high rates of sediment supply.

Desktop /Portals/0/PackFlashItemImages/WebReady/evolution-of-shelf-margin-clinoforms-and.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 7965 Bulletin Article

The presence of hydrocarbon-bearing sandstones within the Eocene of the Forties area was first documented in 1985, when a Forties field (Paleocene) development well discovered the Brimmond field. Further hydrocarbons in the Eocene were discovered in the adjacent Maule field in 2009. Reservoir geometry derived from three-dimensional seismic data has provided evidence for both a depositional and a sand injectite origin for the Eocene sandstones. The Brimmond field is located in a deep-water channel complex that extends to the southeast, whereas the Maule field sandstones have the geometry of an injection sheet on the updip margin of the Brimmond channel system with a cone-shape feature emanating from the top of the Forties Sandstone Member (Paleocene). The geometry of the Eocene sandstones in the Maule field indicates that they are intrusive and originated by the fluidization and injection of sand during burial. From seismic and borehole data, it is unclear whether the sand that was injected to form the Maule reservoir was derived from depositional Eocene sandstones or from the underlying Forties Sandstone Member. These two alternatives are tested by comparing the heavy mineral and garnet geochemical characteristics of the injectite sandstones in the Maule field with the depositional sandstones of the Brimmond field and the Forties sandstones of the Forties field.

The study revealed significant differences between the sandstones in the Forties field and those of the Maule and Brimmond fields), both in terms of heavy mineral and garnet geochemical data. The Brimmond-Maule and Forties sandstones therefore have different provenances and are genetically unrelated, indicating that the sandstones in the Maule field did not originate by the fluidization of Forties sandstones. By contrast, the provenance characteristics of the depositional Brimmond sandstones are closely comparable with sandstone intrusions in the Maule field. We conclude that the injectites in the Maule field formed by the fluidization of depositional Brimmond sandstones but do not exclude the important function of water from the huge underlying Forties Sandstone Member aquifer as the agent for developing the fluid supply and elevating pore pressure to fluidize and inject the Eocene sand. The study has demonstrated that heavy mineral provenance studies are an effective method of tracing the origin of injected sandstones, which are increasingly being recognized as an important hydrocarbon play.

Desktop /Portals/0/PackFlashItemImages/WebReady/constraining-the-origin-of-reservoirs-formed.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 7966 Bulletin Article

See Also: DL Abstract

Carbonate rock typing provides a vehicle to propagate petrophysical properties through association with geological attributes and therefore is critical for distributing reservoir properties, such as permeability and water saturation, in the reservoir model. The conventional approaches to rock typing have significant gaps.

Desktop /Portals/0/PackFlashItemImages/WebReady/carbonate-petrophysical-rock-typing-road-map-hero.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 11683 DL Abstract