In shallow engineering-geophysical applications, there is a lack of controlled, nondestructive, high-resolution mapping tools, particularly for the target depth that ground-penetrating radar cannot reach but which is too shallow for other conventional geophysical methods. For soft soil, this corresponds to a depth of 2 to 30 m. We have developed a portable, high-frequency P-wave vibrator system that is capable of bridging this gap. As far as the important contribution of the seismic source is concerned, penetration and resolution can be individually controlled through easy modulation of the sweep signal generated by this electromagnetic vibrator. The feasibility of this system has been tested in shallow (10-50 m) to very shallow (0-10 m) applications. Seven field data sets representing varying geology, site conditions, and exploration targets are presented to illustrate the applicability. The first three examples show the potential of this portable vibrator source in shallow applications. Under favorable situations, a maximum resolution of about 20 cm for events located at 15-30 m depth could be achieved. Because high-frequency seismic waves suffer from severe attenuation in the dry, unsaturated weathered zone, the penetration is relatively limited when the water table is deeper than 4-5 m. The fourth to seventh field examples illustrate very shallow applications at noisy, asphalt-paved urban sites that are often encountered in civil, geotechnical, and environmental engineering projects. The prospecting targets were thin soil layers or small buried objects. On asphalt, the vibrator can produce high-frequency energy easily. The fourth example shows high-resolution delineation of very shallow soil structures. The last three examples present successful location of buried bodies--often small and closely spaced--in soft soil at depths of 0.5 to 5 m. We observe well-defined reflection events of frequency exceeding 200 Hz. These results suggest that high-frequency seismic reflection imaging using the portable vibrator system can indeed serve as a powerful, nondestructive technique for shallow to very shallow underground prospecting.

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