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.