Reflection imaging for crosswell seismic data: Friendswood, Texas
Guoping Li, Robert R. Stewart
This paper outlines a processing flow for extracting reflections from crosswell seismic data and using them to construct an image. This flow is applied to a field data set acquired by Exxon Production Research Company in two 1000-ft boreholes separated by 600 ft near Friendswood, Texas. The crosswell seismic data consisted of 98 shot gathers each having 96 traces. The data are part of a combined seismic experiment which also collected reversed vertical seismic profiling (RVSP) and surface seismic CDP data at the same site. The source was shot from the source well between 30 ft and 1000 ft (spacing at 10 ft) into the receiver well where hydrophones, placed from 10 ft to 960 ft in depth at 10 ft intervals, recorded the data. The seismic traces were sampled at 1/4 ms with a record length of 1.0 second. The Friendswood crosswell data are dominated by strong low frequency tube waves, which obscure the reflections. Because these tube waves have a linear nature in common source gathers, a 9-trace median filter is used to eliminate them. A 70-620 Hz bandpass filter is applied to suppress low-frequency and high-frequency noise. Strong direct arrivals are removed in common interval gathers by running a 9-trace median filter. An f-k filter is applied in common source gathers to separate up- and down-going reflected waves. Velocity information, used for moveout corrections, is derived from velocity scanning in a zero-interval gather, which collects traces with zero interval between the source and receiver depths. Reflection data are sorted into common reflection point (CRP) gathers where both horizontal and vertical moveout are corrected. Stacked sections are generated by summing the moveout-corrected CRP gathers. Following that, an up and down-going wavefield combination and a time-depth conversion are performed to produce a depth section. The stacked sections obtained show high-resolution imaging of subsurface layers, with a resolution of about 10 ft. A comparison of the crosswell reflection imaging results with the RVSP and CDP stacked sections processed by Chen et al. (1990) is made. This case study shows that the crosswell reflection imaging procedure presented here is an effective means of imaging.