Abstract

To reduce the field effort required for 2-D and 3-D shallow seismic surveying, we have developed a towed land-streamer system. It was constructed with self-orienting gimbal-mounted geophones housed in heavy (1 kg) cylindrical casings, sturdy seismic cables with reinforced kevlar sheathing, robust waterproof connectors, and a reinforced rubber sheet that helped prevent cable snagging, maintained geophone alignment, and provided additional hold-down weight for the geophones. Each cable had takeouts for 12 geophones at 1 m or 2 m intervals. By eliminating the need for manual geophone planting and cable laying, acquisition of 2-D profiles with this system proved to be 50–100% faster with 30–40% fewer personnel than conventional procedures. Costs of the land-streamer system and total weight to be pulled could be minimized by employing nonuniform receiver configurations. Short receiver intervals (e.g., 1 m) at near offsets were necessary for identifying and mapping shallow (< 50 m) reflections, whereas larger receiver intervals (e.g., 2 m) at far offsets were sufficient for imaging deeper (> 50 m) reflections and estimating velocity-depth functions. Our land-streamer system has been tested successfully on a variety of recording surfaces (e.g., meadow, asphalt road, and compact gravel track). The heavy weight of the geophone casings and rubber sheet ensured good geophone-to-ground coupling. On the asphalt surface, a greater proportion of high-frequency (above 300-350 Hz) energy was recorded by the land streamer than by standard baseplate-mounted geophones. The land-streamer system is a practical and efficient means for surveying in urbanized areas.

Acquisition and processing of 3-D shallow seismic data with the land-streamer system was simulated by appropriately decimating and reprocessing an existing 3-D shallow seismic data set. Average subsurface coverage of the original data was ∼50 fold, whereas that of the simulated data was ∼5 fold. The effort required to collect the simulated pseudo–3-D data set would have been approximately 7% of that needed for the original field campaign. Application of important data-dependent processing procedures (e.g., refraction static corrections and velocity analyses) to the simulated data set produced surprisingly good results. Because receiver spacing along simulated cross-lines (6 m) was double that along in-lines (3 m), a pattern of high and low amplitudes was observed on cross-sections and time slices at early traveltimes (≤ 50 ms). At greater traveltimes, all major reflections could be identified and mapped on the land-streamer data set. With this cost-effective approach to pseudo–3-D seismic data acquisition, it is expected that shallow 3-D seismic reflection surveying will become attractive for a broader range of engineering and environmental applications.

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