A three-component field study of the effect of the low-velocity layer on converted-wave seismic data

Dan Cieslewicz and Don C. Lawton

ABSTRACT

A field experiment was devised to study shear-wave attenuation in the low-velocity surface layer by burying three geophones at approximately 6m, 12m, and 18m depths, and placing one on the surface. The geophones collected data during the shooting of aregular dynamite-source three-component line. A frequency analysis of the regular and converted-wave reflections showed that P-S reflections were already substantially attenuated compared to P-P reflections even at a depth of 18m. No difference in attenuation was observed between the different geophone depths. Uphole interval velocity calculations, based on reflected arrivals, showed the presence of the water table between 12 and 18m depths. Above the water table, Vp/Vs was 2 to 3, while belowthe water table, Vp/Vs had a very large value of 10.7. The experiment should be repeated with geophones that preferably reach the top of the bedrock.

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