P-wave and S-wave near-surface characterization in Northeast British Columbia (NEBC)

Liliana M. Zuleta Tobon, Donald C. Lawton

A seismic survey in NEBC was acquired, using shear (SH) and compressional (P) sources and 3-component geophones. First break arrivals were picked in both data sets and the plus-minus time analysis method was used to calculate the depth and velocity of the layers that were detected with the offset of 1500 m used during the acquisition. The intent of this experiment was to gather all the required information in order to elucidate the near-surface P and S-wave velocity-depth structure. This model will help in the processing of a 3D/3C seismic survey that will be acquired in this area, and to provide constraints on registration of PP and PS volumes.

From the P-V data, one refractor was detected and the presence of a channel was confirmed to the east end of the line. The depth of this refractor ranges from 140 m to ~230 m at the channel. The average velocity for the first layer is 1950 m/s and for the second layer is 2800 m/s. From the S-wave data a different model was determined, with two refractors detected to the west and only one refractor to the east. The depth of the first refractor is ~70 m and the second ~140 m; to the east the refractor detected is at ~180 m. The velocity for the first layer is 350 m/s to 420 m/s, and 500 m/s to 680 m/s for the second layer to the west and 1400 m/s for the third refractor to the west. At the east end the velocity of the second refractor is 1100 m/s. Finally the static correction times were computed. For SH data the static times are from -160 ms to -330 ms and for the P data, values ranging from -10 ms to -28 ms are obtained. A comparison with the generalized linear inversion (GLI) method was also made. Results of processing these datasets are shown in Figure 1.