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.


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: Book

Desktop /Portals/0/PackFlashItemImages/WebReady/book-M84-The-Carpathians-and-their-Foreland-Geology-hero2.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 4450 Book

See Also: Bulletin Article

Transfer zones in rift basins are classified into convergent, divergent, and synthetic, based on the relative dip directions of adjacent faults within the transfer zone. Experimental models were constructed to determine the geometry, evolution, and fault patterns associated with each of these transfer zones. In addition, basement faults with initially approaching, laterally offset, and overlapping geometries were modeled. The models consisted of two layers, with stiff clay representing basement and soft clay representing the sedimentary cover. Laser scanning and three-dimensional surface modeling were used to determine the map geometry to compare the models with examples of natural structures. The experimental models showed many similarities with conceptual models but also showed more details and a few significant differences. Typically, divergent transfer zones are narrower than convergent transfer zones, for the same initial spacing between basement faults. The differences between the different initial fault configurations (approaching, laterally offset, or overlapping) are the degree of interaction of the secondary faults, the amount of overlap between the fault zones, and in some cases, the width of the transfer zone. The main faults propagate laterally and upward and curve in the direction of dip of the faults, so that the faults curve toward each other in convergent transfer zones, away from each other in divergent transfer zones, and in the same direction in synthetic transfer zones. A primary difference with schematic models is the significant component of extensional fault propagation folding (drape folding), accompanied by secondary faulting within the sedimentary cover, especially in the early stages of fault propagation. Therefore, all three types of transfer zones are characterized by significant folding and related variations in the shapes of structures. The transfer zones are marked by a progressive change in relief from the footwall to the hanging wall, resulting in a saddle-shaped geometry. The hanging walls of the faults are marked by a gentle flexure or rollover into the fault, with the amount of flexure increasing with fault throw away from the fault tip. The geometries and fault patterns of the experimental structures match some of the observations in natural structures and also provide predictive analogs for interpretation of surface and subsurface structures and the delineation of structural traps in rift basins.
Desktop /Portals/0/PackFlashItemImages/WebReady/Experimental-models-of-transfer-zones-in-rift.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 3723 Bulletin Article

See Also: DL Abstract

Reservoir characterization is an exercise in constraining uncertainty that arises from sparse sampling of the subsurface by widely spaced wells at lengthscales below seismic resolution. Outcrop analogs are an invaluable complement to well and seismic data in this context, because they provide qualitative concepts and quantitative spatial data to guide interpretations of lithology distribution in inter-well volumes. However, analog-driven interpretations of reservoir architecture are not straightforward to compare with dynamic data that describe fluid flow during production – the acid test of any interpretation of reservoir geology. The value of outcrop reservoir analogs is most fully realized when they are used to construct outcrop-based reservoir models that enable explicit predictions of flow patterns in a form that can be compared with routine reservoir-monitoring data.

Desktop /Portals/0/PackFlashItemImages/WebReady/outcrop-based-reservoir-models-hero.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 11400 DL Abstract

See Also: Energy Policy Blog

And, it is more complicated than we first thought! It is not just a shortage of workers that the industry is facing, it is going to be a shortage of managers. In a recent analysis, Russell Reynolds Associates, a global recruiting firm, pointed out that 61 percent of senior energy leaders are over 52 and nearing retirement. The next generation is much smaller: 27 percent of senior leaders are 44 to 52.

Desktop /Portals/0/PackFlashItemImages/WebReady/the-great-crew-change-is-getting-closer-2013-06jun-20.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 12902 Energy Policy Blog

See Also: Learn! Blog

Update your knowledge on world shale plays and take advantage of new opportunities using new technologies / techniques which you will learn with us at the AAPG International Shale Plays Workshop, 28-29 April 2015.
Desktop /Portals/0/PackFlashItemImages/WebReady/ws-International-Shale-Plays-hero.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 17432 Learn! Blog