High-Resolution Data: A Simple Software Switch

One of the terrific things about the Langseth is the 36-gun, four-string linear gun array, which produces a source pulse that can only be described as beautiful. This source produces useful energy out to frequencies that reflection seismologists generally consider "high" (at least 800 Hz). The only thing that keeps us from recording these frequencies ordinarily is the sample rate, which is usually set to 2 ms (sometimes even 4 ms). Fortunately, the Langseth recording system can be set to different sample rates. On MGL0804 we tested the 0.5 ms sample rate, which gives a Nyquist frequency of 1000 Hz (~800 Hz of practical frequency range when the anti-alias filter is considered).

The results surprised us. The figure below shows a comparison of a "conventional" reflection image, produced at frequencies of 20-200 Hz, and the "high-res" image of the very same data, filtered at 200-600 Hz. The difference is clear. The high-frequency data show much higher resolution of strata and faults. There are several locations where faults are seen in the high-res image (e.g., on the left side of the image) that are invisible in the conventional image.

This result implies that the Langseth can be switched from "crustal" model to "high-res" mode with the simple flick of a switch -- literally during a line change. We believe this opens up opportunities for a new class of targets to be imaged, and for innovative "hybrid" cruises that combine basin-scale, crustal, and high-resolution goals. (The switch to a higher sample rate comes at a cost -- only 3 km of streamer data can be recorded at 0.5 ms, and 6 km at 1 ms, but for certain objectives, this will be a sacrifice worth making. And since it's a simple on-the-fly software change, involving no alterations to the physical streamer, it's easy to change the recording parameters on a line-by-line basis.)

A zoom on the same comparison shows the contrast in detail even more clearly. Here you can see that the faults in the center of the image displace the seafloor slightly, which would be difficult to discern in the conventional image.

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