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GCExploring Beneath High-Velocity
Surfaces*
Bob Hardage1
Search and Discovery Article #40334 (2008)
Posted February 13, 2009
*Adapted from the Geophysical Corner column, prepared by the author, in AAPG Explorer, August, 2008, Part 1 entitled “Getting Under Surface Challenges”, and September, 2008, Part 2 entitled “Options Exist for Surface Problems”. Editor of Geophysical Corner is Bob A. Hardage. Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director.
1Bureau of Economic Geology, The University of Texas at Austin ([email protected])
In general, the quality of conventional P-wave seismic data
is poor when
data
are acquired across areas where high-
velocity
rocks (primarily carbonates and basalts) form the exposed, first-layer of the Earth. Some basins that have high-
velocity
rocks exposed at the surface have deeper layers with good oil/gas potential. Examples would include:
· Large areas of Argentina, Paraguay and Brazil (basalt outcrops).
· The Val Verde Basin and other areas of West Texas (carbonate outcrops).
Numerous other carbonate-covered and basalt-covered exploration areas could be listed. Explorationists working in these high-velocity
outcrop areas are frustrated by their inability to acquire seismic
data
that have signal-to-noise character sufficient to see and map deeper hydrocarbon plays.
Here we examine some principles of seismic imaging in areas where the seismic propagation velocity
in the shallowest Earth layer is greater than the
velocity
in the layers immediately below the surface layer. We consider the question “Does the downgoing compressional (P) wave successfully penetrate a high-
velocity
surface layer and illuminate deeper targets?” and then the cause of poor
data
quality before one option for resolving the imaging dilemma.
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Example of Surface Problem
A generalized picture of the geology that needed to be imaged in one basalt-covered area is shown as Figure 1. The Earth surface here was covered by a thick basalt layer characterized by a fast seismic
Oil production had been established across this particular area by random drilling, without the aid of seismic
VSP
· A robust downgoing P wave (center panel), as well as a strong downgoing SV wave (left and right panels), travels through the deep, slower-
· Good-quality upgoing P-wave (left panel) and converted-shear (SV) reflections (center panel) are generated at several deep interfaces, including interfaces associated with critical reservoir intervals.
At this point we know that the deep geology has been illuminated and that reflection events from our primary targets head back toward the Earth’s surface. Yet these reflections cannot be recognized by surface-positioned receivers.
Why not? We appear to have isolated the imaging problem to something that occurs in the local vicinity of the surface receivers.
Cause of Surface Problem
Because good-quality reflections head upward toward the earth’s surface, why do we not capture these reflections with earth-surface receivers? The culprit that prevents the capture of good-quality reflection events often seems to be severe, unorganized ground-roll noise. The earth model in Figure 3 will be used to illustrate the wave physics. There are two kinds of surface waves that travel horizontally away from a source station and spread across the earth-air interface:
· One surface wave is the Rayleigh mode, created by any surface-based source that produces a vertical displacement. Almost all onshore seismic sources (vertical vibrators, explosives in shotholes, weight droppers, etc.) create a vertical displacement and thus produce a Rayleigh wave. The common term used for a Rayleigh wave is “ground roll.” The particle motion associated with a Rayleigh wave is a vertical, retrograde, elliptical motion as shown in Figure 3.
· The second surface wave that can propagate along the earth-air interface is a Love wave, which can be generated only by an SH shear source that creates pure horizontal displacement, and the wave propagates horizontally as a pure SH shear mode that produces no vertical displacement Figure 3. Of these two surface waves, the Rayleigh mode is the “bad” noise mode when the surface layer has a fast seismic propagation
Why is the Rayleigh ground roll so troublesome across outcropping basalts and carbonates? For most poor-
Because this noise is unorganized (i.e., it does not arrive from a fixed direction, and its components have variable time origins), it is difficult – and usually impossible – to remove from the
SolutionHow then can geology beneath a high-
In this equation, ω is the frequency (Hz) of the Love wave, H is the thickness of the high-
Concluding Example
One test of this principle – work done years ago by researchers at Arco – is shown in Figure 4 to illustrate the physics. The P-wave
References
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