Shallow, high-resolution, shear-wave reflection imaging: extended potential in geotechnical surveys

Models of the underground derived by geotechnical investigations in boreholes and used as a primary dataset in many engineering designing and environmental characterization suffer intrinsically from limited lateral sampling relative to the scale of the underground structures of interest. Sensitivity of the change of geotechnical parameters at the soil boundaries to the seismic reflection coefficients, particularly of the shear waves, and the lateral continuity obtained in reflection images, make the seismic reflection method an attractive supplement to the conventional geotechnical point sampling, provided the reflection image offers sufficient resolution and the reflection survey is realizable in engineering scale of space, time and resources. In soft soil formations commonly encountered in engineering sites, a relatively high Vp/Vs ratio results in a greater resolution for the shear-wave surveys than for the P-wave surveys if we can observe shear waves with a frequency comparable to that of the P wave. High-frequency shear waves, therefore, hold the unique potential for offering a technology to map in details the depth region of 5-30 m in soft soil - a technology that is not available now. Further, the water table reflects back much of the P-wave energy, while it has little effect on S wave propagation. The use of shear waves has, therefore, some definite advantages over P waves for shallow investigations. We have developed a small, electromagnetic, horizontal vibrator source for efficient generation of high-frequency shear waves (Ghose et al., 1996; Brouwer et al., 1997). The source is characterized by its easy portability, strength, and resolving power. The total weight of the vibrator is about 100 kg. The vibrator can be transported in the field by one person using a small cart. The vibrator can generate a maximum shear force of 500N on the ground; the effective frequency bandwidth is 10-450 Hz. The software specially designed for driving the horizontal vibrator runs on a personal computer. Recently the horizontal vibrator has been subjected to rigorous feasibility tests in diverse field conditions to evaluate the applicability, potential, and limitations. In this paper, we present the result of both borehole measurements, as weIl as a number of regular, seismic reflection field surveys using the horizontal vibrator.