Compare the merits

Vertical Wave Testing: Part 2

Vertical wave testing is done by deploying seismic receivers downhole and recording the downgoing wavelet generated by each energy source being considered for surface seismic data acquisition across the area local to the receiver well.

The objectives of a vertical wave test are to determine the frequency bandwidth of the downgoing wavelet that illuminates subsurface geology, and to observe how the energy and frequency content of that wavelet diminishes as the wavelet propagates through stratigraphic intervals that need to be imaged with surface-based seismic data.

Vertical wave testing is a rigorous technique that allows geophysicists to decide which seismic source is optimal for imaging specific sub-surface geology.

One limitation is that the data provide information that helps only in selecting the seismic source that will be used across a prospect. The technique does not provide information that helps in designing surface-based receiver arrays. Horizontal wave testing, described in last month’s Geophysical Corner, has to be done to determine appropriate surface-receiver array dimensions.


The source-receiver geometry used for vertical wave testing is identical to that used for vertical seismic profiling. A downhole receiver is positioned at selected depths by wireline, and the surface sources that are to be tested are stationed at selected distances from the wellhead (figure 1).

The downgoing wavelet generated by each source option proposed for use across a prospect should be recorded at depth intervals of 600 to 1,200 feet (200 to 400 meters), starting close to the Earth’s surface and extending to the deepest interval of interest.

A vertical wave test can compare different sources, such as explosives, weight droppers and vibrators, or it can evaluate the relative merits of only one type of source, say a vibrator, when that source is operated under different conditions. In either type of wave test, the objective is to determine what source, operated in what manner, will generate a downgoing illumination wavelet that detects geology with a targeted thickness at a specified depth.

An example of wave-test data comparing vibrator-source wavelets against explosive-source wavelets is illustrated as figure 2.

In this source test, wavelets generated by a 40,000-pound vibrator are compared against wavelets produced by small 10-ounce (280-gram) directional charges buried at a depth of 10 feet (3 meters). At this prospect, both source options create high-frequency wavelets, and either source would provide the desired illumination of the targeted geology.

The small directional-charge source option was selected for acquiring 3-D seismic data across this prospect because a significant part of the survey area was covered by dense timber that made vibrator operations difficult and expensive (due to timber clearing). However, small drill rigs could wend through the trees and drill shallow holes for deploying explosives without the necessity of clearing any timber for vehicle movement, resulting in more affordable data acquisition.


The frequency content of the explosive-source and vibrator-source test data is exhibited as figure 3. The frequency spectrum of the explosive-source wavelet measured at a depth of 2,000 feet (600 meters) extends to 200 Hz – and at a depth of 5,000 feet (1,500 meters) there is still appreciable energy at frequencies as high as 180 Hz (figure 3a).

The vibrator sweep of 6 to 160 Hz results in a frequency spectrum that exhibits an abrupt onset of energy near 8 Hz and an abrupt energy decrease at 160 Hz at all receiver depths (figure 3b).

These data supported the decision to use small directional explosives as the seismic source at this prospect. To increase the signal-to-noise ratio of the surface-recorded data, three shot holes, each having a 10-ounce (280-gram) directional charge, were shot simultaneously to increase the amplitude of the downgoing wavelet.


Results from a second vertical wave test at a different prospect are illustrated on figure 4. At this prospect there were numerous buried data communication cables (some of them connected to intercontinental missile silos!).

Because of these buried cables, the option of drilling shot holes for explosive charges could not be allowed; the source had to be vibrators. Consequently, the objective of this wave test was to determine what vibrator sweep parameters would create a robust wavelet at a depth of 5,000 feet (1,500 meters) that had frequencies up to – and we hope above – 100 Hz.

As illustrated by the frequency spectra of the recorded vibrator wavelets, a non-linear sweep rate of 3 dB/octave produced a greater amount of energy above 100 Hz than did a linear sweep rate. With these test data, a decision to operate vibrators using a 10 – 120 Hz, 3 dB/octave sweep was made with confidence.

Good quality data were acquired; no buried communication cables were damaged as the production data were recorded; no missiles were launched.


The message: Always execute a vertical wave test if there is any desire to compare the relative merits of seismic sources – and if a well is available for depth deployment of receivers.

Comments (0)

 

Geophysical Corner

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

VIEW COLUMN ARCHIVES

Image Gallery

Part 2 of 2

This month’s column is the second of a two-part series that started in December, dealing with seismic wave tests – vertical wave testing.

See Also: Bulletin Article

The origin of thermogenic natural gas in the shallow stratigraphy of northeastern Pennsylvania is associated, in part, with interbedded coal identified in numerous outcrops of the Upper Devonian Catskill and Lock Haven Formations. Historically documented and newly identified locations of Upper Devonian coal stringers are shown to be widespread, both laterally across the region and vertically throughout the stratigraphic section of the Catskill and Lock Haven Formations. Coal samples exhibited considerable gas source potential with total organic carbon as high as 44.40% by weight, with a mean of 13.66% for 23 sample locations analyzed. Upper Devonian coal is thermogenically mature; calculated vitrinite reflectances range from 1.25% to 2.89%, with most samples falling within the dry-gas window. Source potential is further supported by gas shows observed while drilling through shallow, identifiable coal horizons, which are at times located within fresh groundwater aquifers. Thermogenic gas detected in area water wells during predrill baseline sampling is determined not only to be naturally occurring, but also common in the region.

Desktop /Portals/0/PackFlashItemImages/WebReady/geologic-and-baseline-groundwater-evidence-for.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 5776 Bulletin Article

See Also: CD DVD

Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 4327 CD-DVD
Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 4576 CD-DVD

See Also: Energy Policy Blog

Crude oil and natural gas infrastructure problems, from pipeline oil spills to train derailments and fires, have been in the news recently. Though these problems are not new, public concern is growing. Think tanks and government agencies have been considering the problems and potential solutions for some time and are now reporting the results of their studies. Here are reports of one oil and one natural gas infrastructure study.
Desktop /Portals/0/PackFlashItemImages/WebReady/Oil-and-natural-gas-infrastructure-challenges-03mar-17-2015-hero.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 16886 Energy Policy Blog

See Also: Short Course

Here is an introduction to the tools and techniques that geologists and geophysicists use to locate gas and oil, that drillers use to drill the wells and that petroleum engineers use to test and complete the wells and produce the gas and oil. Exercises throughout the course provide practical experience in well log correlation, contouring, interpretation of surface and subsurface, contoured maps, seismic interpretation, well log interpretation, and decline curve analysis.

Desktop /Portals/0/PackFlashItemImages/WebReady/sc-basic-petroleum-geology-for-the-non-geologist.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 13584 Short Course