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.
The Geophysical Corner is a regular column in the EXPLORER, edited by Bob A. Hardage, senior research scientist at the Bureau of Economic Geology, the University of Texas at Austin.