CHAPTER 3 PETROLEUM EXPLORATION
| Jurisdiction | United States |
(2001)
PETROLEUM EXPLORATION
Professor of Geophysics, University of Houston
Houston, Texas
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THE SEISMIC METHOD IN 2001
Robert E. Sheriff, Professor of Geophysics, University of Houston
Almost all geophysical work today is either seismic or measurements in wells. Non-seismic geophysical data are primarily gravity and magnetic measurements and, for mineral and some environmental work, electrical and electromagnetic. New magnetic acquisition today is almost all done from aircraft. Gravity measurements merely require setting up a small instrument and making a reading, which takes only a very few minutes for one man, before he moves on for a measurement at another location.
Because 99%+ of geophysical work (excluding measurements in boreholes) is seismic, I shall concentrate on this.
Hypothetical experiment
The seismic method is basically sound-in-air echo-location. A noise is created and instruments record the echoes. An individual can determine how far he is from a canyon wall (or something else that reflects sound waves) by measuring the time between a shout and the echo. Utilizing the stereo effect, with his two ears he can also determine the direction of the point from which the echo reflected. If there are reflectors in different directions, he may hear several echoes. If he could listen simultaneously at a number of different locations, he probably could deduce the shape and geometry of the reflectors. Knowledge of the shape might help him postulate tentative ideas about the identity of the reflectors, and perhaps speculate how they came to be where they are. If he were very sensitive to subtle variations in the strength of the reflections he could possibly deduce something about the hardness of the reflectors. We can appreciate that the more places and the more extensive the area over which he can make observations, the more complete his picture will be and the more probable that the inferences he draws will be correct.
The seismic method differs in several important ways from this sound-in-air experiment. With the seismic method only a small portion of the sound is reflected at the first surface it encounters and most of it penetrates deeper into the earth so that portions can be reflected back from many surfaces behind each other and different distances away so that we can receive many reflections from each location as well ass from many directions.. Another important difference is that the speed of seismic waves is not nearly constant, as is the speed of sound waves in air, but
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rather the speed varies considerably depending on the nature of the medium in which the seismic waves travel. Also in the air there is only one kind of sound wave whereas there are different kinds of seismic waves that can travel in the Earth, and, under certain circumstances, wave energy can transfer from one kind of wave to another.
We can idealize some observation conditions from the foregoing. We want quantitative measurements made in considerable spatial detail over as large an area as possible. We want the recording conditions to be the same at all locations, with the same magnitude of input energy and the same responsiveness of all of the "ears" that are listening to the echoes. We process the data, it in a variety of ways to identify all of the patterns of the multitude of different reflectors. This will take considerable analysis, that is, it will require sizeable computing capacity processed and supervised by able scientists whose knowledge and experience include an understanding of how configurations came to be the way they are. We will then interpret the results in the light of our experience and knowledge of possible earth processes. We will then try to postulate possible scenarios that explain the observations. Related knowledge of the area and its history helps us winnow the assortment of possible scenarios and thus improve the probability that any actions we take will be successful.
Our hypothesized seismic experiment is not the only way we could have obtained the information about the earth. For example, we could have excavated the entire region and thus determined the nature of the region with greater confidence, but at much larger cost and with destructive results. Our seismic experiment will be much less expensive than most alternative ways of acquiring information and our seismic experiment is nearly non-destructive. Nevertheless, carrying out our experiment will involve costs, it will not guarantee determining the correct answer, and the value of the answer may not exceed the cost.
Acquisition concepts
Most two-dimensional seismic data today is acquired by the common-midpoint method where various combinations of source and receiver locations involve the same midpoint, the point midway between the source and receiver. The amplitudes of seismic waves of different types change in different systematic ways as the source-to-receiver distance (offset) changes. We can calculate the differences in distance that reflections
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have traveled (making certain assumptions) and adjust their arrival times to compensate for the raypath differences (the normal-...
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