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GCHigh Resolution P-P Imaging of Deepwater Near-Previous HitSeafloorNext Hit Geology*

AND

GCHigh Resolution P-SV Imaging of Deepwater Near-Previous HitSeafloorNext Hit Geology**

By
Bob A. Hardage1 and Paul E. Murray1
 

Search and Discovery Article #40200 (2006)

Posted July 4, 2006 (Part 1), September 20 (Part 2)

 

*Adapted from the Geophysical Corner column, prepared by the authors and entitled “Technique Improves Deep Imaging,” in AAPG Explorer, July, 2006, as Part 1 of a two-part series.

 

**Adapted from the Geophysical Corner column, prepared by the authors and entitled “P-SV Data Most Impressive Image,” in AAPG Explorer, August, 2006, as  Part 2 of a two-part series.

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, University of Texas, Austin, Texas ([email protected] )

 
P-P Imaging (Part 1)

 

Introduction 

Multicomponent seismic data have unique value for studying near-Previous HitseafloorNext Hit geology in deepwater environments. When properly processed, P-P (compressional) and P-SV (converted-shear) images made from 4-C seismic data acquired in deep water with Previous HitseafloorNext Hit sensors show near-Previous HitseafloorNext Hit geology with amazing detail.

 

This is the first of two parts that describe how improved imaging of near-Previous HitseafloorNext Hit, deepwater strata can be achieved with conventional multicomponent seismic data.

 

This part focuses on P-P imaging; Part 2 focuses on P-SV imaging.

 

P-P imaging

uIntroduction

uFigures 1-2

uAcquisition / processing

uApplications

P-SV imaging

uComment

uFigures 3-4

uWave length

uIncreasing frequency

uReducing velocity

uAppendix

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

P-P imaging

uIntroduction

uFigures 1-2

uAcquisition / processing

uApplications

P-SV imaging

uComment

uFigures 3-4

uWave length

uIncreasing frequency

uReducing velocity

uAppendix

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

P-P imaging

uIntroduction

uFigures 1-2

uAcquisition / processing

uApplications

P-SV imaging

uComment

uFigures 3-4

uWave length

uIncreasing frequency

uReducing velocity

uAppendix

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

P-P imaging

uIntroduction

uFigures 1-2

uAcquisition / processing

uApplications

P-SV imaging

uComment

uFigures 3-4

uWave length

uIncreasing frequency

uReducing velocity

uAppendix

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

P-P imaging

uIntroduction

uFigures 1-2

uAcquisition / processing

uApplications

P-SV imaging

uComment

uFigures 3-4

uWave length

uIncreasing frequency

uReducing velocity

uAppendix

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

P-P imaging

uIntroduction

uFigures 1-2

uAcquisition / processing

uApplications

P-SV imaging

uComment

uFigures 3-4

uWave length

uIncreasing frequency

uReducing velocity

uAppendix

 

Figure Captions

Figure 1. Standard production processing of deepwater 4-C OBC seismic data along a profile that traverses a Previous HitseafloorNext Hit gas-expulsion chimney. 1(b) [right] Improved resolution of near-Previous HitseafloorNext Hit geology using VSP-style concepts for processing deepwater OBC data. Both images are flattened to the Previous HitseafloorNext Hit.

Figure 2. Illustration of similar source receiver geometries used for acquiring (a) VSP data and (b) deepwater OBC/OBS seismic data.

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Acquisition and Processing 

In deepwater multicomponent seismic data acquisition, there is a large elevation difference between source stations (an air gun at the sea surface) and receiver stations on the Previous HitseafloorNext Hit. Conventional processing of deepwater 4-C seismic data involves a wave-equation datuming step that transforms the data to a domain in which sources and receivers are on the same depth plane. This step effectively removes the water layer and allows the data to be processed as if the source was on the Previous HitseafloorNext Hit. This adjustment of source-receiver geometry also allows deepwater multicomponent data to be processed with software already developed for shallow-water environments where marine multicomponent data acquisition technology was originally developed and applied. 

An example of a good-quality, deepwater P-P image of near-Previous HitseafloorNext Hit geology made with this wave-equation datuming approach is shown as Figure 1a. This image shows local geology associated with a fluid-gas expulsion chimney that extends to the Previous HitseafloorNext Hit. 

If a person wishes to study near-Previous HitseafloorNext Hit strata, a new approach to P-P imaging of deepwater multicomponent seismic data is to not eliminate the large elevation difference between sources and receivers but to take advantage of that elevation difference. The objective is to process deepwater multicomponent data similar to the way vertical seismic profile (VSP) data are processed, because VSP data acquisition also involves large elevation differences between sources and receivers (Figure 2). 

Users of VSP technology know VSP data provide high-resolution images of geology near downhole receiver stations. That same logic leads to the conclusion that deep-water multicomponent seismic data processed with VSP-style techniques should yield higher resolution images of geology near deep Previous HitseafloorNext Hit receivers. 

The P-P processing illustrated here can be done with either 2-C or 4-C Previous HitseafloorNext Hit sensors. The fundamental requirement is to acquire data with a sensor having a hydrophone and a vertical geophone. The Previous HitseafloorNext Hit hydrophone response (P) and the Previous HitseafloorNext Hit vertical-geophone response (Z) are combined to create downgoing (D) and upgoing (U) P-P wavefields as:

 

D=P+Z/cos(F)

U=P--Z/cos(F)

 

“F” defines the incident angle at which the downgoing compressional wave arrives at the Previous HitseafloorNext Hit. Once this wavefield separation is done, deepwater multicomponent seismic data are defined in terms of downgoing and upgoing wavefields, just as are VSP data. 

Having access to downgoing (D) and upgoing (U) wavefields means sub-Previous HitseafloorNext Hit reflectivity can be determined by taking the ratio U/D. This reflectivity wavefield is then segregated into stacking corridors, and data inside these corridors are summed to create image traces just like VSP data have been processed for the past 20-plus years. 

Figure 1b shows a P-P image made with this technique using the same deep-water data displayed in Figure 1a. The improvement in resolution is obvious.

 

Applications and Constraints 

Applying this VSP-style imaging technique to deepwater multicomponent seismic data is proving to be invaluable for gas hydrate studies, geomechanical evaluations of deepwater seafloors and other applications where it is critical to image near-Previous HitseafloorNext Hit geology with optimal resolution. 

Every seismic data-processing technique, however, has constraints and pitfalls. Two principal constraints of the technology described here are:

  • There has to be a significant difference between the elevations of sources and receivers. The technique is not appropriate for multicomponent seismic data acquired in shallow water.

  • The improvement in image resolution over that of production processing of marine multicomponent seismic data diminishes as the image space extends farther (deeper) from the receivers. At significant sub-Previous HitseafloorNext Hit depths, production-style, wave-equation-datuming-based, P-P imaging (Figure 1a) is equivalent or superior to the VSP-style imaging described here.

 

 

P-SV Imaging (Part 2)

 

General Comment

 In Part 1, we considered how to improve the seismic resolution of deepwater, near-Previous HitseafloorNext Hit geology using P-P data acquired with Previous HitseafloorNext Hit-positioned multicomponent sensors.

In Part 2, we show how P-SV (converted-shear) data acquired with these same sensors provide even greater resolution of deepwater, near-Previous HitseafloorNext Hit strata.

 

Figure Captions

Figure 3. (a) High-frequency (2-10 kHz) AUV P-P image of near-Previous HitseafloorNext Hit strata across a fluid expulsion chimney. (b) Low-frequency (10-100 Hz) P-SV image along the same profile. Visual comparisons show the images have equivalent spatial resolutions and thus equivalent wavelength spectra. The south end of the P-SV profile starts at about AUV chirp-trace number 700. These images have been flattened to the Previous HitseafloorNext Hit, which causes small-throw faults (throws of one meter and less) to appear as chevron-shaped patterns.

Figure 4. The same images as in Figure 3 with depth-equivalent horizons defined. A P-SV horizon equivalent to AUV horizon A is not labeled because it is quite faint in this P-SV display format. Interval values of VP/VS velocity ratio between the P-SV horizons are labeled on the right margins. P-P and P-SV image times are labeled on opposing sides of the images.

 

Wave Length 

To achieve better resolution of geologic targets with seismic data, it is necessary to acquire data that have shorter wavelengths. The wavelength l of a propagating seismic wave is given by:

l = V/f

where V is propagation velocity and f is frequency.

 

This equation shows there are two ways to reduce an imaging wavelength l: either increase f, or reduce V.

 

Option 1: Increasing the Frequency 

If deepwater strata are illuminated with conventional air gun seismic sources towed at the sea surface, there is really no way to cause a significant increase in the frequency content of the illuminating wavefield that reaches the Previous HitseafloorNext Hit. A different data-acquisition strategy has to be used to acquire shorter-wavelength marine P-P data. 

An approach now used for acquiring deepwater, short-wavelength P-P data is to use an Autonomous Underwater Vehicle (AUV) system. 

An AUV travels only 50 meters or so above the Previous HitseafloorNext Hit and illuminates Previous HitseafloorNext Hit strata with chirp-sonar pulses having frequency bandwidths of 2-10 kHz. This increase in signal frequency shortens P-P wavelengths by about a factor of 100 compared to the wavelengths of an air gun signal. The result is an illuminating wavefield having wavelengths of less than a meter when P-wave velocity VP is 1500 to 1600 m/s, a common range of VP for deepwater, near-Previous HitseafloorNext Hit sediments across the Gulf of Mexico (GOM). 

An example of an AUV chirp-sonar image acquired in water depths of approximately 900 meters in one area of the GOM is shown in Figure 3a. The image makes the same traverse across a targeted Previous HitseafloorNext Hit expulsion chimney that was illustrated in last month’s article.

These high-frequency P-P signals penetrate only 40 or 50 meters into the Previous HitseafloorNext Hit, but they image bedding and fault throws of meter-scale dimensions across this image space.

 

Option 2: Reducing the Velocity 

It is not possible to acquire shorter-wavelength P-P data by reducing VP in a seismic propagation medium. The value of VP within a system of targeted strata is fixed and cannot be altered. 

A seismic imaging effort, however, can switch from the conventional approach of using the P-P seismic mode and focus on using another wave mode that does have reduced velocity within a targeted interval. That logic has great benefit for imaging deepwater, near-Previous HitseafloorNext Hit geology when the imaging effort focuses on P-SV data rather than on P-P data. 

Across most deep-water areas, S-wave velocity VS in near-Previous HitseafloorNext Hit sediments tends to be 20 to 50 times less than P-wave velocity VP. Thus, if P-P and P-SV data have equivalent frequency content, which they do for shallow penetration distances  of an illuminating P-P wavefield into the Previous HitseafloorNext Hit, P-SV data will have wavelengths much shorter than P-P wavelengths. 

Shown as Figure 3b is a P-SV image constructed from 4C data acquired with Previous HitseafloorNext Hit sensors deployed along the same profile as the AUV data in Figure 3a. The illuminating wavefield that created these  P-SV data was a 10-100 Hz P-P wavefield produced by a conventional air gun array positioned at the sea surface. 

Because VS in near-Previous HitseafloorNext Hit sediment along this profile is less than 100 m/s, the  P-SV data have many wavelengths less than one meter in length, just as do the high-frequency chirp-sonar data. Visual inspection of the images in Figure 3 shows the spatial resolutions of kilohertz-range P-P data and low-frequency P-SV data are equivalent in deep-water, near-Previous HitseafloorNext Hit geology. 

The same data are shown again in Figure 4, with depth-equivalent horizons superimposed to emphasize the amazing resolution of the low-frequency P-SV data. Horizon A shown on the AUV image is not easily seen on this particular P-SV image, so no P-SV equivalent horizon is labeled. 

Note the large magnitudes of the interval values of the VP/VS velocity ratio. Also note how easy it is to identify where stratigraphy first becomes unconformable to the Previous HitseafloorNext Hit in these Previous HitseafloorNext Hit-flattened data (Horizon B).

 

Unfortunately, these high-resolution P-SV images cannot be extended to great sub-Previous HitseafloorNext Hit depths. P-SV wavelengths increase and P-SV resolution then decreases with increasing depth below the Previous HitseafloorNext Hit because:

·        VS increases with depth.

·        Higher frequencies attenuate more rapidly with depth for P-SV wavefields than for their companion P-P wavefields.

 

At sub-Previous HitseafloorNext Hit depths of several kilometers, P-P and P-SV data have approximately the same resolution. However, for deepwater strata close to the Previous HitseafloorTop, the spatial resolution of P-SV data is most impressive (Figures 3b and 4b).

 

Appendix/Acknowledgments 

Information about this technology is available at www.beg.utexas.edu/indassoc/egl/. 

WesternGeco provided the seismic data used in this research. Research funding was provided by Minerals Management Service (Contract 0105CT39388) and DOE/NETL (Program DE-PS26-05NT42405).

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