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By BOB A. HARDAGE and PAUL E. MURRAY

Hardage and Murray are both with the Bureau of Economic Geology in Austin, Texas.

Click on Figure To Enlarge

FIGURE 1a

FIGURE 1b

FIGURE 2a

FIGURE 2b

The Geophysical Corner is a regular column in the EXPLORER, edited by Bob A. Hardage, senior research scientist at the Bureau of Economic Geology, the University of Texas at Austin. This month’s column, part 2 of a two-part series that began in July, is titled “High Resolution P-SV Imaging of Deepwater Near-Seafloor Geology.” Part 1

P-SV Data Most Impressive Image

In last month’s Geophysical Corner we considered how to improve the seismic resolution of deepwater, near-seafloor geology using P-P data acquired with seafloor-positioned multicomponent sensors.

This month we move to part two: We show how P-SV (converted-shear) data acquired with these same sensors provide even greater resolution of deepwater, near-seafloor strata.

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 seafloor. 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 seafloor and illuminates seafloor 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-seafloor 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 1a. The image makes the same traverse across a targeted seafloor expulsion chimney that was illustrated in last month’s article.

These high-frequency P-P signals penetrate only 40 or 50 meters into the seafloor, 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-seafloor 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-seafloor 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 seafloor, P-SV data will have wavelengths much shorter than P-P wavelengths.

Shown as figure 1b is a P-SV image constructed from 4C data acquired with seafloor sensors deployed along the same profile as the AUV data in figure 1a. 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-seafloor 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 1 shows the spatial resolutions of kilohertz-range P-P data and low-frequency P-SV data are equivalent in deep-water, near-seafloor geology.

The same data are shown again in figure 2, 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 seafloor in these seafloor-flattened data (Horizon B).

Unfortunately, these high-resolution P-SV images cannot be extended to great sub-seafloor depths. P-SV wavelengths increase and P-SV resolution then decreases with increasing depth below the seafloor 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-seafloor depths of several kilometers, P-P and P-SV data have approximately the same resolution. However, for deepwater strata close to the seafloor, the spatial resolution of P-SV data is most impressive (figures 1b and 2b).

Additional information about deep-water applications of multicomponent seismic data is available at www.beg.utexas.edu/indassoc/egl/.

WesternGeco provided the 4C OBC data used in this study.

Research funding was provided by Minerals Management Service.

(Editor’s note: Figures 1a and 1b in the July “Geophysical Corner” were incomplete; missing was the labeling for the explusion chimney, which was located in the lower left-hand corner of both figures. Also, the symbol for the incident angle should have been a “f” instead of a “°”. The correct versions are available with the column online.)