The Blackfoot III buried geophone experiment
Dan Cieslewicz, Donald C. Lawton
The Blackfoot III buried geophone experiment was composed of three 3-component receiver lines buried to depths of 6, 12 and 18 meters, and one at the surface, occupying the central kilometer of a three kilometer shot line. The vertical channel data quality was found to be similar for all geophone depths, while the radial channel data was found to have highest quality at the surface and poorest for the buried phones. Data from buried geophones were affected by mode leakage, which adversely affected the data quality. The bandwidths of the P-P and P-S reflections were not consistent between the different geophone depths and between stations, indicating that geophone coupling was likely the dominant factor in recorded frequency. Because of this, the frequency analysis indicated no systematic compressional- or shear-wave attenuation through the overburden layer. Higher P-wave interval velocities suggested a thinning of the overburden layer at the eastern third of the line, which corresponds to an increase in surface elevation. The S-wave velocities were not similarly affected. Quantitative calculations of similarity between the processed sections revealed that random noise was the dominant factor controlling the absolute similarities between sections, as any factor that reduced random noise tended to make the sections more similar. Apart from this, sections that were closer in relative depth tended to be more alike, perhaps due to similar receiver notch patterns. Sections from shallower depths also tended to be more similar than sections from deeper depths, perhaps as a result of increased reflection amplitude in shallower phones causing better signal-to-noise ratios. The effect was not as clear for the converted-wave data. On the vertical channel data, the Glauconite valley-fill sandstone appeared as a high-amplitude trough that broadened and decreased in amplitude toward the off-channel lithologies. The overlying coal markers changed from a single peak to a doublet, possibly a compaction-related effect. The radial channel seismic expression of the producing unit was similar, though not as pronounced. Overall, burying geophones does not appear to be a viable way of improving converted-wave bandwidth or determining attenuation constants of the near surface.