Zero-offset vertical seismic profiles of coalbed methane strata: a comparison of three vibrating sources
Sarah Elizabeth Richardson, Donald C. Lawton
Zero-offset vertical seismic profiles (VSPs) were acquired at a coalbed methane test site using three different sources. A 44,000 lb. P-wave source ("big-P") sweeping 8-150 Hz, a smaller vibratory P-wave source ("mini-P") sweeping 8-250 Hz, and a shear-wave source ("mini-S") sweeping 8-150 Hz all effectively imaged the target coal zone. Bandwidth comparisons show useable frequencies of 8-150 Hz in the big-P data, whereas the mini-P data contains frequencies ranging 8-220 Hz. Shear wave attenuation was considerably higher than that of P-waves, with the mini-S source yielding useable bandwidth of 8-80 Hz. Such low attenuation of the mini-P source suggests that high-bandwidth converted-wave data may be obtained using the mini-P source. Although Ardley coal zone contacts at the Red Deer site may be effectively imaged using any of the three sources tested, lithological changes within the coal may only be detected using the high-frequency mini-P source.