American Oil and Gas Reporter - July 2015 - 96

SpecialReport: Seismic Acquisition & Reservoir Modeling
First, the cost of S-S data acquisition is reduced if data are acquired using simple P-wave sources. It is no longer necessary to
deploy two sources to acquire multicomponent data: a verticalforce source (e.g., vertical vibrators) to generate direct-P data and
orthogonal horizontal-force sources (e.g., horizontal vibrators)
to generate direct-S data. Using one source rather than three
sources lowers the cost of acquiring S-wave data by a factor of
three.
Second, S-S seismic programs can be implemented across any
area where P-wave programs can be conducted. Specifically, Pwave sources allow S-S data to be acquired across surfaces covered by swamps, marshes, desert dunes, dense timber, and rugged
mountain terrains. These surface conditions are areas where using a horizontal vibrator (the present gold standard direct-S source)
would rarely be considered. However, some type of P-wave source
can always be deployed across these earth surfaces.
Although the source-side of S-S data acquisition is simplified
by utilizing a P-wave source, the receiver-side of S-S data acquisition is unchanged. It is still necessary to deploy three-component sensors to acquire S-S data when P-wave sources are used.
However, when adequate S-wave information can be provided
by SV-P (converted-P) data, S-wave data acquisition uses exactly the same vertical geophones used to acquire P-wave data.
An SV-P mode acquired in this manner provides the same Swave image, the same S-wave information, and the same S-wave
attributes as does the P-SV (converted-SV) mode that is often
recorded with P-wave sources and three-component geophones
to evaluate subsurface geology.
Wolfberry Case Study

The technology to enable surface-based direct-S imaging with
P-wave sources was used to acquire 3-D data across the Wolfberry play in the Midland Basin. The data were generated by an
array of three in-line vertical vibrators and were recorded by surface-based 3-C geophones. Figure 1 compares four images constructed from the dataset. Two images (P-P and P-SV, shown as
Figures 1A and 1B, respectively) were made from the direct-P
mode. Common practice has been to create only these two imFIGURE 1
Comparison of Surface-Based Direct-P
And Direct-S Images

ages with 3-C, 3-D data generated by a P-wave source.
However, when attention is focused on the direct-S modes emanating from the vertical-vibrator source stations, two additional images can be made: SV-P and S-S, shown as Figures 1C and
1D, respectively. To our knowledge, these two figures show the
first-ever direct-S images constructed from data generated by a
P-wave source.
The data windows used in these image comparisons span deep
geology that starts in the lower part of the Wolfberry interval and
extends into basement beneath the Ellenburger (the depth of reflection "E" in the images is more than 15,000 feet).
Identifying key formations in the P-P image (Figure 1A) was
accomplished by using a local synthetic seismogram. Transferring these P-P reflection horizons to the three companion image
spaces (P-SV, SV-P and S-S) utilized the equations written on
each respective data panel. These equations specify how depth
intervals in P-P image space are related to equivalent depth intervals in P-SV, SV-P and S-S image spaces.
The ratio of P-wave velocity to S-wave velocity (VP/VS) involved in these data space transition equations is an average of
VP/VS ratios observed in three local wells. While the resulting
formation tops identified in each image space are not precise, they
are sufficiently accurate to justify image comparisons.
Turbidite Reservoir Units

Wolfberry reservoirs are a massive stack of turbidite units and
debris flows. Investigations show that SV-P images constructed from
vertical-geophone data provide excellent definitions of these
stacked Wolfberry turbidite systems. One example is the three turbidite depositional elements located approximately 500 feet below
the top of the Second Spraberry, which are labeled A, B and C in
the SV-P stratal slice images displayed in Figure 2. Each of these
small-scale depositional elements is represented as an area of SVP image space enclosed by a blue, low-semblance boundary.
FIGURE 2
Stratal Slices through Small-Scale Wolfberry
Turbidites A, B and C at Geologic Time Steps
(SV-P Reflectivity Semblance Volume)
2A

Base

10 ft above base

20 ft above base

30 ft above base

VP/VS = 1.65
1A = P-P Image
(Direct-P)
4

1.4 A

1B = P-SV Image
(Direct-P)
29

rTPSV = 1.325 rTPP

1C = SV-P Image
(Direct-S)
29

rTSVP = 1.325 rTPP

1.6 B
1.8 C

Time (s)

2.0 D
2.2 E
2.4
2.6

3.2
3.4
3.6

A: 2nd Spraberry
B: Atoka Lime
C: Woodford
Devonian (?)
D: Near Ellenburger
E: Intra-Basement

rTSS = 1.65 rTPP
rTSS = rTPP (VP/VS)

B
A
C
B
C

3.0

2.8

1D = S-S Image
(Direct-S)
4

E
No bulk shift from refraction statics

550-ft bins

rTPSV = rTPSV = 0.5 rTPP (1 + VP/VS)

96 THE AMERICAN OIL & GAS REPORTER

x = Spraberry producer
QAe2314(d)

Min.

Semblance

Max.

Turbite depositional element approximately 500 ft below Top of 2nd Spraberry

American Oil and Gas Reporter - July 2015

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