Marine seismic acquisition gets improvements

Marine geophysical contractors are continuing to push the technology envelope to find ways to improve the imaging of the subsurface and increase oil and gas exploration and development success. Wide-azimuth (WAZ) marine surveys have become commonplace in areas requiring subsalt images, and today's surveys are now using full-azimuth shooting using ocean-bottom nodes/cables or coil shooting with towed streamers.

But those areas not hampered by salt or other imaging obstructions are in need of improved seismic data as well. Two new technologies are allowing us to dramatically improve the quality. The first is broadband marine seismic, which produces a broader frequency spectrum in the recorded signal and improves stratigraphic and structural detail in the final processed and imaged section. The second, simultaneous shooting, produces higher quality data through increased fold without a linear increase in acquisition costs.

Broadband marine seismic

FIGURE 1. Amplitude-frequency reponses differ for receivers at different depths. (Image courtesy of Apache Corp.)

Conventional marine 3-D seismic acquisition uses multiple cables pulled behind a marine vessel. Each of these cables or streamers contains hundreds of groups of hydrophones that record the pressure changes as seismic energy returns from reflections deep within the earth's surface. This energy also reflects off of the ocean's surface, creating what is called a "ghost." The combination of the primary reflection plus the ghost removes the very low frequencies and creates notches in the frequency spectrum of the recorded energy that are a function of the depth of the streamer.

Figure 1 shows the response for the receiver ghosts for different towing depths. Note the notches are at different frequencies for the different depths.

Streamer depth in marine survey design is based on the tradeoff between shallow tow depths to obtain higher frequencies but typically more noise and deeper tow depths that are less noisy but have notches at lower frequencies. It would be helpful to have the benefits of both.

Today geophysical contractors are getting those benefits through novel ways of implementing different tow depths. By towing a combination of shallow and deep streamers, the output can be combined to fill in the notches and produce a flatter frequency spectrum or, in the time domain, a higher resolution time section.

There are other ways to fix ghost notches and broaden the spectrum of the data. One method is to use both hydrophones and geophones as receivers. PGS has implemented this approach in its GeoStreamer, combining hydrophones and geophones in a towed streamer to improve resolution. Since hydrophones measure pressure and geophones measure velocity, the two can be combined to remove the ghost notch and produce a higher resolution image.

In addition to the receiver ghost, there also is a source-related ghost. Marine sources radiate energy in all directions, not just down, so energy from the source is reflected off of the ocean's surface, creating notches in the spectrum as well. PGS also has addressed the source ghost. Figure 2 shows an example of the improvement obtained by removing the receiver and the source ghosts. Again, there is a marked improvement in the stratigraphic and structural detail on the broadband image.

Other companies use this approach in ocean-bottom cables or nodes, combining hydrophone and geophone outputs to remove unwanted ghosts and other types of multiple reflections. Ocean-bottom node surveys, although typically more costly than towed streamer surveys, have their advantages. They can be deployed in areas that streamers dare not tread – shallow water, shipping channels, around platforms, etc. Most nodes record four components, a hydrophone and a three-component geophone/accelerometer, allowing subsequent processing technology to remove ghosts and other energy reflected in the water layer. Like the broadband towed-streamer data, nodes are pushing the frequency spectrum at the high and low ends of the spectrum, even recording signals below 1Hz.

Simultaneous shooting

Simultaneous shooting in land acquisition has been used for several decades. Hundreds of land surveys around the globe have and are successfully employing these techniques. Now marine acquisition is getting into the game.

FIGURE 2. With source and receiver ghosts removed, the broadband image on the right shows considerably more detail than the conventional image on the left. (Image courtesy of PGS)

FIGURE 3. Marine simultaneous shooting data (right) looks very similar to conventional data (left). (Image courtesy of Apache)

Since most marine sources are impulsive, there is little that can be varied to encode the source signature. There are, however, other ways to differentiate simultaneous, or nearly simultaneous, sources. Many of these techniques rely on a known but random variation in the timing of source detonations that allows subse- quent separation of the nearly time-coincident shots during the processing. Figure 3 shows an example from offshore Australia where the initial results from simultaneous shooting are similar to conventional acquisition. Denser spatial shooting in simultaneous shooting can eliminate the spatial aliasing artifacts that have plagued conventional shooting.

Combining forces

The obvious path forward is to combine these technologies to reap the benefits that each of them bring to the table. Simultaneous shooting could produce large benefits in ocean-bottom node surveys since the cost of these surveys is directly tied to battery life. Doubling or tripling the number of shots that can be recorded in a given deployment of the nodes can increase data quality while decreasing the cost. Obviously, longer battery life would be a plus as well.

We can also expect to see WAZ and full-azimuth surveys shot with broadband technologies like BroadSeis, GeoStreamer, and others. Couple that with simultaneous shooting, and WAZ techniques will be able to cost-effectively move from exploration tools to routine appraisal and development tools.

Also expect to see improved sources down the road. With the push to see broadband seismic and pressure from environmental protection groups, the industry needs to find better marine sources. Whether this is the marine vibrator or some other new technology remains to be seen, but lower frequencies and limits on higher frequencies will definitely be in order. FIGURE 1. Amplitude-frequency reponses differ for receivers at different depths. (Image courtesy of Apache Corp.)