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A Practical Approach to Seismic Imaging of Complex Geology*
By
Matthew Brzostowski1 and Todd Jones2
Search and Discovery Article #40057 (2002)
*Adapted for online presentation from the article by the authors in AAPG Explorer (April, 2000), entitled “Complexities Can Be Mind-Bending.” Appreciation is expressed to the authors and to M. Ray Thomasson, former Chairman of the AAPG Geophysical Integration Committee, and Larry Nation, AAPG Communications Director, for their support of this online version.
1Houston, Texas (713-469-1311)
2Houston, Texas
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General StatementThe biggest distinction between geology and geophysics can probably be broken down into the different domains from which they both start their work. The geologist works in terms of spatial coordinates and depth, with perhaps the roadcut epitomizing the best example of his world view. Depth is also how he makes use of his interpretation results, i.e. a well is drilled to a certain depth. The geophysicist, however, deals with information recorded in time. His job in seismic processing is to transform this information in time into depth for the geologist to make his maps and calculate where to drill. The geophysicist works in time because of the nature of seismic exploration. A source is initiated at some location and sensors record the subsequent reflections as a function of time.
Figure 1b. Complications of translating time into distance where the well is not vertical.
Analogies and ExamplesThe problem is not unlike the scenario depicted in Figure 1a: Here we have a person determining the depth of his water well by dropping a rock into it and recording the time for the splashing sound to come back. After some mathematical manipulation - and knowing the speed of sound in air - the person can translate time into depth. This sounds simple enough, but we made assumptions about the rock traveling straight down and the sound traveling straight back up to our ears. If the well is not vertically straight but deviated (Figure 1b), then we have a more complicated problem to solve. This is the nature of seismic exploration: We record the strength of seismic reflections, and we can assume they all come from directly below the surface, but more likely the reflections come from anywhere in some three dimensional subsurface location around our surface position.
Figure 2a shows more clearly the issue.
Reflected energy from a subsurface point will travel to our surface
receivers in a straight line if the This ray bending is not unlike light bending as it travels through water and air as depicted in Figure 3. The resultant bent rays can lead to a gross misinterpretation of what is in the glass if we do not account for it. That is the goal of seismic imaging; accounting for the complicated velocities in the subsurface that will distort our interpretation, especially in terms of where features are actually located.
Figure 4 shows this distortion due to
Specification of
One of the means we have for controlling the
processing of seismic data and the eventual placement of events comes
from the specification of a
There are two broad classes of imaging
algorithms available to the geophysicist. One class has historically
been referred to as time
Time
The
price for the accuracy, however, is more expense - and there is a
greater need to determine the |