In-situ, high-frequency P-wave velocity measurements within 1 m of the earth's surface

Seismic P-wave velocities in near-surface materials can be much slower than the speed of sound, waves in air (normally 335 mis or 1100 ft/s). Difficul ties often arise when measuring these low-velocity P-waves because of inter ference by the air wave and the air-coupled waves near the seismic source, at least when gathering data with the more commonly used shallow P-wave sou rces. Additional problems in separating the direct and refracted arrivals w ithin similar to 2m of the source arise from source-generated nonlinear dis placement, even when small energy sources such as sledgehammers, small-cali ber rifles, and seismic blasting caps are used. Using an automotive spark p lug as an energy source allowed us to measure seismic P-wave velocities acc urately, in situ, from a few decimeters to a few meters from the shotpoint. We were able to observe three distinct P-wave velocities at our test site: similar to 130m/s, 180m/s, and 300m/s. Even the third layer, which would n ormally constitute the first detected layer in a shallow-seismic-refraction survey, had a P-wave velocity lower than the speed of sound in air.