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GC
Passive Seismic
Techniques*
By
Bob Hardage1
Search and Discovery Article #40319 (2008)
Posted August 1, 2008
*Adapted from the Geophysical Corner column, prepared by the author, in AAPG Explorer, July, 2008, and entitled “Is the Future of Seismic
Passive?”. Editor of Geophysical Corner is Bob A. Hardage. Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director.
1 Bureau of Economic Geology, The University of Texas at Austin ([email protected])
Passive-seismic
technology encompasses any procedure by which
seismic
data are recorded without the use of an active
seismic
source. When passive-
seismic
data are acquired, there is no vibrator vehicle, no shothole explosive, no impact source, and no air gun. Instead,
seismic
wavefields are generated by natural phenomena such as wind, microseisms, ocean waves, or by human-made noises such as moving vehicles, passing aircraft, or mechanical vibrations of operating machinery.
One passive-seismic
application that is gaining attention is the acquisition and analysis of low-frequency natural
seismic
wavefields that seem to indicate the presence of subsurface oil and gas accumulations. In this application, data are acquired using high-sensitivity three component geophones deployed across the earth’s surface. Data are recorded for time periods of many minutes to days in order to have data that are appropriate for analysis.
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An example of this low-frequency, passive- Restricting data analysis to this narrow frequency band:
Ocean-wave energy is ubiquitous and can be observed in the interior of continents far from coastlines. The amount of anthropogenic energy varies from site to site, depending on the nature of human culture from area to area. To emphasize a hydrocarbon response in this narrow, low-frequency band, one data-analysis procedure is to integrate the frequency spectrum to amplify and smooth the data signal. An example of a frequency-spectrum integration of data acquired by vertical geophones is displayed as Figure 2.
Although the data behaviors illustrated in these figures seem to imply the presence of hydrocarbons, the data do not provide information about depth to the hydrocarbon accumulation or about the size of the reservoir. These shortcomings are now being addressed by reverse time
An example of one such reverse-time model is displayed as Figure 3. When shown in this format, passive-
What is the source of the hydrocarbon signal? The big question is why are hydrocarbon reservoirs associated with these natural-source, low-frequency responses? This is an active area of research at ETH Zurich, where both meso-scale scattering and pore-scale rock physics mechanisms are being considered. What is known is that empirical evidence is accumulating from the Middle East, North Africa, Europe, Brazil, and North America that implies a relationship between low-frequency passive-
There is nothing wrong with utilizing empirical rules in hydrocarbon exploitation. Empirical relationships between
At present, I have no experience in using passive-
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