Click to view article in PDF format.
Bob Hardage1
Search and Discovery Article #40503 (2010)
Posted January 19, 2010
*Adapted from the Geophysical Corner column, prepared by the author, in AAPG Explorer, January, 2010, and entitled “Vertical
Wave Testing: Part 2”. Editor of Geophysical Corner is Bob A. Hardage
([email protected]). Managing Editor of AAPG Explorer is Vern Stefanic;
Larry Nation is Communications Director. Please refer to closely related article by Horizontal Wave Testing, Search and Discovery article #40502.
1 Bureau of Economic Geology, The University of Texas at Austin ([email protected])
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 Search and Discovery Article #40502, has to be done to determine appropriate surface-receiver array dimensions.
|
The source-receiver geometry used for
A 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
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
Results from a second
The message: Always execute a
Copyright © AAPG. Serial rights given by author. For all other rights contact author directly. |