Shear-wave Reflection Signal Recovery through Horizontal-geophone Azimuth Estimation

Nine-component (9-C) 3-D surveys have been acquired to target the unconventional Niobrara reservoir in the Wattenberg Field (Greeley, Colorado) after drilling 11 horizontal wells (Baseline), immediately after completion and fracking (Monitor 1), and then again after two years of production (Monitor 2). Monitor 1 shear-wave limited-azimuth stacks showed anomalous energy during commercial data processing. This anomalous energy was described as signal leakage, attributed to wet surface conditions that existed during data acquisition, and resulted in the converted-wave and shear-wave data being deemed unusable.

We expose the signal leakage on Monitor 1 prestack shear-wave and converted-wave Common-Offset-Common-Azimuth (COCA) gathers, and recognize the cause as a global mis-orientation of the nominal horizontal-receiver azimuth during data acquisition. We generate prestack synthetic COCA gathers, and qualitatively reproduce the signal leakage by assuming a horizontal-geophone mis-orientation of 10 degrees.

Horizontal-geophone azimuth estimates for all three surveys confirms the global misalignment of Monitor 1, while Baseline and Monitor estimates are centered around the desired nominal azimuth. We then apply a global correction to the Monitor 1 data to remove the signal leakage producing useable converted-wave and shear-wave datasets.