SS imaging with vertical-force sources

Transcription

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Special section: Multicomponent seismic interpretation
S-S imaging with vertical-force sources
Downloaded 06/04/14 to 129.116.232.233. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
Bob A. Hardage1 and Donald Wagner1
Abstract
We show examples of S-S images created from multicomponent seismic data generated by vertical-force
sources that can be quite useful to seismic interpreters. Two source types are used: vertical vibrators and
shot-hole explosives. We first discuss S-S images made from data generated by a vertical vibrator and recorded
with vertical receiver arrays of 3C geophones. We next show images extracted from surface-based 3C geophones deployed around this VSP well as a 3D seismic grid. The energy sources used to generate these surface
3D seismic data were shot-hole explosives. In all data examples, we observe that each type of vertical-force
source (vertical vibrator and shot-hole explosive) produces abundant direct-S energy on radial and transverse
geophones. We find only minimal amounts of P-wave energy on transverse-receiver data. In contrast, radialreceiver data have significant P-wave events intermingled with radial-S events. The minimal amount of P-wave
noise on transverse-receiver data makes it easier to study S-S wave physics and to create S-S images with transverse-S data. The data examples focus on transverse-S data created by vertical-force sources because interpreters will find it more convenient to process and use this S-mode. Subsequent publications will assign
equal weight to radial-S and transverse-S data.
Introduction
Vertical-force sources produce P and SV displacements at the point where they apply their force to
the earth. Because SV displacement vectors are oriented in every azimuth direction around the point of
vertical-force application, a far-field geophone receives
radial-S and transverse-S displacements regardless of
where that sensor is positioned relative to a verticalforce source station (Hardage and Wagner, 2014).
The attraction of using vertical-force sources to produce direct-S modes is that it will no longer be necessary for interpreters to deploy horizontal-vibrator
sources or inclined-impact sources to generate directS data. The cost of S-S data acquisition will be reduced
because full-elastic wavefield data can be acquired using only a simple, reliable, vertical-force source, and it
will not be required to deploy two sources — a verticalforce source to generate direct-P data and a horizontalforce source to generate direct-S data. Perhaps more
important is the fact that S-S seismic programs can
now be implemented across any area where P-wave
data can be acquired. This capability allows interpreters
to acquire S-S data in swamps, marshes, desert dunes,
dense timber, and across rugged mountainous terrains.
These surface conditions are areas where the use of
horizontal vibrators would not be considered. Although
the source-side of S-S data acquisition is simplified by
using a vertical-force source, the receiver-side of the
data acquisition is unchanged. It will still be necessary
to deploy 3C sensors to acquire S-S data when verticalforce sources are used.
Numerous physical mechanisms have been proposed
to explain why S waves are observed when verticalforce sources are used to generate illuminating wavefields. As will be explained in the following text,
included in some of these proposals are assumptions
that there has to be a subsurface interface local to a
source station that causes P-to-SV mode conversion
to occur near the source position, or that there must
be a steplike variation in topography adjacent to the
source station that creates a P-to-SV mode conversion
close to the source location. Although these conditions
do create converted SV modes, direct-S waves are created by vertical-force sources when these near-source
physical anomalies are not present. The basic physics
we have observed is that direct-S modes are produced
at the point of application of a vertical force to the earth
without the presence of local interfaces or close proximity to topographic variations.
Geyer and Martner (1969) were among the first to
recognize that a shot-hole explosive (one type of vertical-force source) produces S-wave energy. These authors came to this conclusion because they deviated
from common industry practice of deploying only
single-component vertical geophones during the decade
of the 1960s, when they did their work, and use
1
The University of Texas at Austin, Bureau of Economic Geology, Austin, Texas, USA. E-mail: [email protected]; [email protected].
Manuscript received by the Editor 25 June 2013; published online 22 April 2014. This paper appears in Interpretation, Vol. 2, No. 2 (May 2014); p.
SE29–SE38, 10 FIGS.
http://dx.doi.org/10.1190/INT-2013-0097.1. © 2014 Society of Exploration Geophysicists and American Association of Petroleum Geologists. All rights reserved.
Interpretation / May 2014 SE29
surface-based 3C geophones to record reflection data
produced by shot-hole explosives. Because their data
were analog recordings, not digital data, they are not
able to process the data and develop a robust theory
of direct-S generation. Interpretation results in their paper are shown only as marked reflection events on paper wiggle-trace records produced as camera playouts.
Three papers published in the 1980s deserve citation
and comment. The first paper, by Helbig and Mesdag
(1982), is a theoretical study that summarizes the potential of S-wave technology. What is noteworthy about
this classic paper is that it points out that SV shear energy is produced by a vertical vibrator, a vertical impact, and an explosion in a half-space, which covers
the full range of vertical-force sources that can be used
in S-S data acquisition. The material in this paper is
theoretical, and no real data are exhibited. A second paper by Fertig (1984) emphasizes that shot-hole explosives produce S-wave energy. However, this paper
teaches that S-energy is not produced directly at the explosion point, but at the free surface above a buried explosive at the earth-air interface. The claim that a free
surface is required in order for a subsurface explosion
to generate SV energy resulted from the numerical modeling that was used. Although it is true that an explosion
creates an SV mode at the free surface above a buried
shot, the principle that a buried explosive can create an
SV mode only at a free surface is a matter of debate. A
third paper of the 1980s, by Mazzotti et al. (1989), appears to be the first published example demonstrating
an S-wave reflection profile can be created from data
produced by a vertical vibrator. However, the authors
assume that this S-energy was produced by mode conversion reasonably close to the source point, not directly at the source station.
Four papers published in the 1990s establish that
S-S data are present in data created by vertical-force
sources. The earliest of the four papers (O’Brien,
1992) presents evidence that a downgoing SV mode
is produced directly at the position of a surface-positioned vertical vibrator. However, the only information
extracted from these SV data was S-wave velocity (V S ).
No S-wave images were made, and no S-S applications
were pursued. The second paper (Sun and Jones, 1993)
documents the fact that a downgoing SV wave is produced at the surface coordinates occupied by two
bumper-to-bumper, inline vertical vibrators. These authors promote the idea that these SV waves are the result of operating these vibrators with sweeps that were
180° out of phase. Although a direct-SV mode is produced by this type of vibrator array, a counterphase operation of a vibrator pair is not required to produce
a direct-S wave. Our observations show that robust
S-S-waves can be produced by a single vertical vibrator.
The third paper of this decade (Luschen, 1994) illustrates the principle that a shot-hole explosive generates
SV waves, but the data used in this study were deep
crustal reflections recorded by surface-based receivers,
not exploration data such as we analyze. The fourth
SE30 Interpretation / May 2014
paper (Yokoi, 1996) presents theoretical models and actual data that illustrate a vertical impact also produces
downgoing S-waves. However, the author proposes that
a fundamental requirement for generating the observed
direct SV waves is that there needs to be a steplike
change in the surface topography in the immediate
vicinity of the surface impact. Our position is that a
topographic discontinuity local to a source station will
indeed create a converted-SV mode local to a source
station, but a vertical impact will also produce a downgoing SV wave when there is no local surface discontinuity close to the point of impact.
The most pertinent papers occur after year 2000. The
first (Daley, 2002) is a theoretical analysis of P- and Sradiation from a shot-hole explosive. According to the
model described in this paper, a shot-hole explosive
produces a downgoing S-wave only because there is a
P-to-SV reflection from the free surface above the shot.
As already stated, our position is that an SV mode is
indeed created at the free surface above a shot-hole explosive, but direct-S modes are also created directly at
the explosion point.
Zhou et al. (2005) and Zhao et al. (2005) then
present companion VSP research papers at the 2005
SEG Annual International Meeting where they showed
SV waves are produced directly at vertical-vibrator
source stations. The value of these papers lies in their
use of SV modes to create images local to VSP wells;
a value we share. Following these papers, Yang et al.
(2007) present evidence that downgoing S-waves exist
in VSP data generated by vertical vibrators and shot-hole
explosives. An additional paper by Li et al. (2007) illustrates SV data produced directly at shot holes can be
used to make SV images along a 2D profile.
Using S-S technology in VSP programs
We show in this section that vertical-force sources
allow valuable S-S images to be created from vertical
seismic profile (VSP) data to assist interpretation projects. The energy sources used in these VSP studies were
vertical vibrators. We show data-processing efforts for
zero-offset VSP data and a brief example of far-offset
data. We wish to allow research colleagues from Halliburton to publish the far-offset VSP data examples that
we have used. It is particularly important to concentrate
on zero-offset VSP data because theoretical models of
direct-S radiation patterns produced by a vertical-force
source imply no direct-S modes propagate vertically
from a vertical-force station (Hardage and Wagner,
2014). We find this physics model to not be correct
for the VSP data we evaluated.
The well in which downhole receivers were deployed
had a deviation of less than 2°. Receiver stations
spanned a depth interval extending from 2003 m
(6570 ft) to 67 m (220 ft). A vertical vibrator was positioned only 79 m (252 ft) from the well head. Thus,
travel paths from source to receivers were, for all
practical purposes, true vertical except for receiver stations above a depth of 305 m (1000 ft). The VSP data
that were processed are shown in Figure 1. At this datadisplay stage, the downhole receivers have been rotated
so horizontal geophones X and Y are oriented at constant azimuths at all receiver stations. At each receiver
depth, one horizontal phone was rotated so it was
oriented in the vertical plane passing through the
receiver well and the vertical-vibrator source station.
The response of this horizontal geophone is labeled
SR (S-radial) in Figure 1b. The companion horizontal
geophone was oriented perpendicular to the sourceto-receiver vertical plane, and the responses of these latter geophones are labeled ST (S-transverse) in Figure 1c.
The data are presented with a T1.6 gain applied,
where T is VSP time as labeled on the vertical scale
of the wiggle-trace displays. An important point is that
the amplitudes of the downgoing direct-S modes are significant compared to the amplitudes of the downgoing
P-wave first arrivals. A second point is that the leading
peak of the long direct-S wavelet extrapolates upward
to the same point of origin as the direct-P first arrival
wavelets, as indicated by the extrapolation lines drawn
on Figure 1b and 1c. Thus, in this case, direct-S modes
originate at the same earth coordinate as the direct-P
mode, and have amplitudes that compare favorably
with the amplitudes of the direct-P mode.
This VSP is particularly valuable for documenting the
origin point of direct-S modes because the shallowest
receiver was only 76 m (220 ft) below the elevation
of the vertical-vibrator source. This receiver deployment allowed downgoing first arrivals to be tracked
back almost to the earth surface. Thus, the common origin points of direct-P and direct-S modes produced by
the vertical-vibrator source can be identified as illustrated in Figure 1b and 1c.
Even before VSP wavefield separation is done, P-P
and S-S reflections can be seen in the raw data displayed
in Figure 1. Observing reflections in unprocessed data is
a harbinger that a high-quality image can be made for
interpretation use. The creation of an S-S image is illustrated in Figure 2a–2c. The procedure used for image
construction is the front-corridor stack approach that
has been used by VSP data processors and seismic interpreters for several decades. The construction of the P-P
front corridor stack is illustrated in Figure 2d–2f.
The S-S front-corridor stack image is compared with
its companion P-P front-corridor stack in Figure 3.
Figure 1. Zero-offset VSP data after receiver rotation. (a) Vertical geophone response. (b) Radial-S geophone response. (c) Transverse-S geophone response. The leading peak of the direct-S mode (green) extrapolates upward to intersect the origin point of the
direct-P mode (red) at the earth surface in (b) and (c). When processing these S-wave data, the data processor used the next robust
peak in the long direct-S wavelet as the first arrival of the S mode rather than the true first arrival.
leave no doubt as to how the P-P and S-S reflections
should be correlated to each other. Although formation
names are assigned to selected reflection events, we
These images are displayed in their respective imagetime coordinates and also in depth. The depth images
(Figure 3a) are particularly important because they
Figure 2. (a) Upgoing S-S reflections extracted from the VSP data (Figure 1). (b) Front-corridor data window of S-S reflections.
(c) Front-corridor S-S stack. (d) Upgoing P-P reflections extracted from the VSP data (Figure 1). (e) Front-corridor data window of
P-P reflections. (f) Front-corridor P-P stack.
will not delve into a geological interpretation of the
data. A key point we wish to emphasize is that these
images show that the direct-S data produced by this vertical-vibrator source image the same major-formation
interfaces that the direct-P data image. A second point
is that these images involve vertical travel paths, which
is a take-off angle for which a common assumption is
that there will be no direct-S illumination.
Far-offset VSP data were also recorded in this study.
We show data from one of the far-offset source stations
in Figure 4 after receivers have been rotated to segregate the data into the downgoing-P wavefield (and any
upgoing events associated with that wavefield), the
downgoing radial-S wavefield (and any upgoing events
associated with that wavefield), and the downgoing
transverse-S wavefield (and any upgoing events associated with that wavefield). Examples of downgoing and
upgoing wave modes embedded in each of these segregated wavefields are labeled to illustrate the abundance
of S-S events in the data and the high quality of these S-S
reflections even in this raw-data stage. An image constructed from the transverse-S wavefield is displayed
on Figure 5. The outlined data window extending from
1.3 to 2.3 s on this image will be used later to compare
with S-S images constructed from surface-recorded 3C
3D data local to this VSP well.
Surface-recorded vertical-force data and S-S
imaging
A modest size 3C 3D seismic data volume was recorded around the well where the VSP data presented
in Figures 1 to 5 were acquired. The energy sources
used in this 3D seismic program were shot-holes having
a 1 kg explosive positioned at a depth of 6 m. Such shothole explosives are another type of vertical-force
source. The combination of these VSP data and their
associated 3C 3D data provide a valuable opportunity
to create and compare S-S images produced by two different vertical-force sources — a vertical vibrator
and a shot-hole explosive. It is important for interpreters to know if direct-S modes produced by two
different vertical-force sources are equivalent or significantly different.
We show as Figure 6 a vertical slice through a brute
stack constructed from the transverse-S data extracted
from the surface-recorded data. We refer to this image
Figure 3. (a) Comparisons of P-P and S-S front-corridor stacks in the depth domain. (b) P-P front-corridor stack in P-P image time.
(c) S-S front-corridor stack in S-S image time.
as a brute stack because (1) it was constructed with a
single velocity function (no lateral velocity variations
were allowed), (2) only one estimate of S-wave static
corrections was calculated and applied to the data,
and (3) we made no effort to rotate the data to coordinates that would allow fast-S and slow-S modes to be
separated. This brute-stack data volume was created
in late 2010 and presented to the sponsors of the Exploration Geophysics Laboratory in April 2011 as a proofof-concept effort of S-S imaging with vertical-force
sources. The data example was then shown to a few
selected seismic data-processing companies in the latter part of 2011 because we wished to let professional
data processors complete the job of refining our simple
velocity analysis, replacing our one pass of static estimations with improved static corrections, and segregating the data into fast and slow modes. We mention
these dates because we think the S-S data exhibited
in Figure 6 represent the first 3D S-S data volume created from direct-S modes generated by a vertical-force
source. We welcome feedback from readers regarding
our assumption about the uniqueness of these data.
In Figure 7 we compare this early brute stack version
of a transverse-S image with the VSP transverse-S image
from Figure 5. The VSP image is time shifted (not
squeezed or stretched) to account for the fact that a different depth datum was used when processing the VSP
data and the 3D data. The comparison between the two
images is reasonable and implies that our quickly constructed brute stack not only demonstrates S-S images
can be produced from data acquired with vertical-force
sources, but is also reasonably accurate considering
that we have made no effort to equalize the basic wavelets in the two images to improve the phase alignments
Figure 4. Far-offset VSP data. (a) Downgoing direct-P wavefield and its associated upgoing events. (b) Downgoing radial-S wavefield and its associated upgoing events. (c) Downgoing transverse-S wavefield and its associated upgoing events.
of events. An important point is that this image comparison shows that equivalent S-S-images can be created
by different types of vertical-force sources — shothole explosives (surface data) and vertical vibrator
(VSP data).
Figure 5. Transverse-S image extracted from the data exhibited in Figure 4c are shown on the right. Data inside the outlined image window will be compared with transverse-S
images constructed from surface-recorded 3C 3D data. The
transverse-S data that generated the image on the left have
not been shown or discussed. The zero-offset VSP in the
center is discussed in Figure 3c. The front-corridor stack
(center) is shifted down approximately 100 ms from the image
coordinates used in Figure 3c.
In 2012, Gaiser and Verm (2012) reprocessed the
same 3D 3C data and created a much-improved image
compared to our hasty brute stack. They demonstrated
the data should be segregated into fast and slow modes
and made an image for each mode. Their imaging efforts are displayed in Figure 8. Their fast-S image is
compared with the S-S image created from the VSP data
in Figure 9. Again, as is the case for our brute stack
comparison (Figure 7), there is some phasing difference
between shallow and deeper reflection events in the
two images, but in general, there is a rather good correlation between the VSP data (created by a vertical vibrator) and the surface-recorded data (created by shothole explosives).
Comparison of P-P and transverse-S VSP images
The transverse-S VSP image presented in Figure 5
will now be compared with P-P VSP data. The comparison of these VSP images is shown in Figure 10. The vertical vibrator that was positioned at the far-offset VSP
source station at this study site generated downgoing
direct-P and downgoing direct-S modes. The P-P image
made from the downgoing direct-P mode propagating
from this far-offset source station is displayed as
Figure 10a. As stated previously, a vertical vibrator
was also positioned only 79 m (252 ft) from the receiver
well to generate zero-offset VSP data (Figure 1). P-P and
S-S VSP front-corridor stacks made from these zero-offset VSP data are shown in Figures 2 and 3. The zero-offset P-P image exhibited in these earlier figures is
repeated as Figure 10b.
The transverse-S image made from the downgoing
direct-S mode that propagated from the same far-offset
source station (Figure 5) is also repeated as Figure 10c
after the S-S image-time scale is compressed to agree
with the P-P image-time scale. The agreement between
Figure 6. Brute stack of surface-recorded
transverse-S data generated by shot-hole
explosives.
Figure 7. Comparison between the VSP S-S image created by a vertical vibrator
(Figure 5) and the shot-hole explosives S-S image exhibited in Figure 6.
the P-P and S-S images is good. A particularly impressive aspect of the transverse-S image is the robust nature of the
S-S reflections associated with the geology in the time window between 0.9 and
1.1 s. The bolder reflections in this data
window are, respectively, the tops of the
Upper and Lower Marcellus Shale and
the Onandaga, the major targets of interest at this study site. The faint downlapping S-S events across the top of the
Onandaga (1.05 s) are important stratigraphic features that are also seen in
3D seismic data across this study area.
These downlapping events are not
present in the P-P image, nor do the
far-offset P-P data image the top of
the Onandaga at 1.05 s. In this instance,
offset imaging with direct-S modes produced by a vertical vibrator provides a
better depiction of geology than does
offset imaging with direct-P modes generated by a vertical vibrator. To our
Figure 8. Improved images of the surface-recorded direct-S-data exhibited in Figure 6 after the data are segregated into fast and
slow modes. From Gaiser and Verm (2012).
Figure 9. Comparison between the VSP S-Simage (Figure 5) and the reprocessed fast-Simage of the surface-recorded direct-S-modes
shown in Figure 8.
common-conversion-point concepts. Even in cases
where there is no justification for questioning P-SV data
processing, it is a wise strategy to have a second S-wave
image to integrate into an interpretation effort. The S-S
images we have shown provide such a backup strategy.
Almost all VSP data are acquired with 3C geophones;
thus, the option for creating S-S images from VSP data
acquired with vertical-force sources will be available in
most VSP projects. The possibility of reprocessing legacy VSP data to create S-S images should be particularly
attractive to interpreters. At the Exploration Geophysics Laboratory, our interpretation strategy is to create
VSP-based S-S images at every opportunity to provide
a first look at the quality and value of S-S data in areas
of interest.
Figure 10. (a) P-P VSP image made from direct-P modes produced at far-offset station A (source station location not
shown). (b) P-P image made from zero-offset VSP data when
a vibrator was positioned only 79 m (252 ft) from the receiver
well. (c) Transverse-S VSP image made from direct-S modes
produced simultaneously with the direct-P modes at the same
far-offset station A. The time scale of the S-S data is adjusted
to P-P image time.
knowledge, the images in Figure 10 are the first published example in which VSP images made from direct-P and direct-S modes generated simultaneously
by the same vibrator have ever been compared.
Conclusions
The data produced in this study show interpreters
that S-S images can be created using direct-S data generated by vertical-force sources. Our study has particular value because it provided the opportunity to
compare S-S images collected at the same prospect using two different vertical-force sources and to compare
an S-S VSP image with an S-S image constructed from
surface-recorded 3C 3D data.
Currently, when interpreters work with 3C seismic
data acquired with vertical-force sources, imaging options are confined to P-P and P-SV data. We show that
the direct-S modes produced by vertical-force sources
allow imaging options for 3C data to be expanded to
include S-S images. Prospect interpretations will be
more robust if S-S images are included in the list of data
that interpreters examine to analyze rock and fluid
properties. In particular, the availability of S-S data
can be quite important when a seismic propagation
medium is anisotropic. Because S-S data are processed
using common-midpoint concepts, there will be some
data-acquisition geometries where S-S images may
be more reliable than P-SV data processed using
Acknowledgments
We thank Chesapeake Energy for providing access
to valuable test data and Halliburton for providing
VSP data-processing support. The lead author and
The University of Texas at Austin are part owners
of VertiShear, a company that licenses technology
based on concepts discussed in this paper. The terms
of this VertiShear ownership have been reviewed
and approved by The University of Texas at Austin in
accordance with the University’s policy on objective
research.
References
Daley, P. F., 2002, S-shear energy from a P-wave source:
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— The Earth surface as a generator of converted P-S
waves: Geophysical Prospecting, 32, 1–17, doi: 10
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P-wave sources in azimuthally anisotropic media:
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Abstracts, doi: 10.1190/segam2012-1293.1.
Geyer, R. L., and S. T. Martner, 1969, SH waves from explosive sources: Geophysics, 34, 893–905, doi: 10.1190/1
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modes with simple, low-cost, widely available seismic
sources: Interpretation, 2, this issue, doi: 10.1190/INT2013-0095.1.
Helbig, K., and C. S. Mesdag, 1982, The potential of shearwave observations: Geophysical Prospecting, 30, 413–
431, doi: 10.1111/j.1365-2478.1982.tb01314.x.
Li, Y., P. Sun, D. Tang, Y. He, H. Chen, and Y. Yue, 2007,
Imaging through gas-filled sediments with land 3C
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from multicomponent P and S wave measurements in
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at near-vertical incidence from a counterphase source:
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Yang, L., Z. Qinghong, B. Leiying, and W. Xiucheng, 2007,
Pure S-waves in land P-wave source VSP data: Applied
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Yokoi, T., 1996, Numerical study on the generation of
downgoing S-waves by a vertical force acting close to
a step-like topography: Geophysics, 61, 192–201, doi:
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Zhao, X., R. Zhou, Y. Li, P. Janak, and D. Dushman, 2005,
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Bob A. Hardage received a Ph.D. in
physics from Oklahoma State University. He worked at Phillips Petroleum
for 23 years followed by management
positions at Western Atlas. He then established a multicomponent seismic
research laboratory at the Bureau of
Economic Geology, where he has
been since 1991. He has published
four books on VSP, crosswell profiling, seismic stratigraphy, and multicomponent seismic technology. He has been
a member of SEG for 47 years and has served SEG as assistant editor, editor, first vice president, president-elect,
president, past-president, chairman of the technical program committee for annual meeting (twice), honorary lecturer, short course instructor, and chair of numerous
committees. SEG has awarded him a special commendation, life membership, and an honorary membership. He
wrote the monthly Geophysical Corner column for the
AAPG Explorer magazine for six years. AAPG honored
him with a distinguished service award for promoting geophysics among the geologic community.
Don Wagner received a B.S. in physics
from the University of Tulsa and a
Ph.D. in geophysics from Saint Louis
University. He is a senior research
fellow at the Bureau of Economic
Geology. His fields of expertise are
multicomponent seismic analysis, migration, depth imaging, 3D seismic
processing, and seismic software development (he is coauthor of the FreeUSP Seismic
Processing System). He worked at the Cities Service Research Center from 1971 to 1974 and at the Amoco
Research Center from 1974 to 2003, and he became a
member of the research staff of the Exploration Geophysics Laboratory at the Bureau in 2005. He has been an SEG
member for 45 years and has served as president of the
Geophysical Society of Tulsa and associate editor for
GEOPHYSICS.

SS imaging with vertical-force sources

Transcription

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