Design and application of a towed land-streamer system for cost-effective 2-D and pseudo-3D shallow seismic data acquisition

To reduce the field effort required for 2-D and 3-D shallow seismic surveyi ng, we have developed a towed land-streamer system. It was constructed with self-orienting gimbal-mounted geophones housed in heavy (1kg) cylindrical casings, sturdy seismic cables with reinforced kevlar sheathing: robust wat erproof 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. Casts of the land-streamer system and total weight to be pulled could be minimiz ed by employing nonuniform receiver configurations. Short receiver interval s (e.g., 1 m) at near offsets were necessary for identifying and mapping sh allow(< 50 m) reflections, whereas larger receiver intervals (e.g., 2 m) at far offsets were sufficient for imaging deeper (> 50 m) reflections and es timating velocity-depth functions. Our land-streamer system has been tested successfully on a variety of recording surfaces (e.g., meadow, asphalt roa d, and compact gravel track). The heavy weight of the geophone casings and rubber sheet ensured good geophone-to-ground coupling. On the asphalt surfa ce, a greater proportion of high-frequency (above 300-350 Hz) energy was re corded by the land streamer than by standard baseplate-mounted geophones. T he 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-stream er system was simulated by appropriately decimating and reprocessing an exi sting 3-D shallow seismic data set. Average subsurface coverage of the orig inal data was similar to 50 fold, whereas that of the simulated data was si milar to5 fold. The effort required to collect the simulated pseudo-3-D dat a set would have been approximately 7% of that needed for the original fiel d 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 alon g simulated cross-lines (6 m) was double that along in-lines (3 m), a patte rn of high and low amplitudes was observed on cross-sections and time slice s at early traveltimes (less than or equal to 50 ms). At greater traveltime s, all major reflections could be identified and mapped on the land-streame r data set. With this cost-effective approach to pseudo-3-D seismic data ac quisition, it is expected that shallow 3-D seismic reflection surveying wil l become attractive for a broader range of engineering and environmental ap plications.