Shallow VSP with a vibrator CREWES Research Report — Volume 20 (2008) 1 Shallow VSP survey using a small vibrator source Soo K. Miong, Joe Wong, Eric V. Gallant, Robert R. Stewart, and Kevin W. Hall ABSTRACT A shallow VSP survey was conducted in the Rothney Test Well using the U of C EnviroVibe as a source and a downhole clamping 3-C geophone in the well. The receiver was placed at depths ranging from 5 m to 90 m at half-metre intervals. Two VSPs were recorded with the vibrator located 15 m and 30 m south of the well head. The three- component data were subjected to wavefield separation and vector rotation to separate P and S waves. Down-doing wavefields, as well as up-going and down-going tube waves, were removed to isolate the P-wave reflections. The residual up-going P-wave reflections were mapped using an approximate VSP/CDP procedure. The reflectivity maps showed good correlation with resistivity, gamma ray, and sonic velocity logs in the well. The experiment confirmed the belief that the EnviroVibe vibrator, when used as a source in near-surface VSP surveys, produces enough high-frequency energy to be effective for imaging relatively thin stratigraphy in the upper 100 m. INTRODUCTION The Rothney Test Well is an important part of the Department of Geoscience’s teaching and research infrastructure (Wong et al., 2007). It is drilled into the Paskapoo formation on University of Calgary property near the hamlet of Priddis, Alberta. The well is about 125 m deep, and is protected from collapse by 100-mm-ID PVC casing. It has been used for demonstrating well-logging and VSP surveys using a sledge hammer source (Miong, 2008; Miong et al., 2007). This report presents data from two VSPs obtained by using the U of C EnviroVibe vibrator as a source and a clamping 3-C geophone in the Rothney well. The data were processed to isolate up-going P-wave reflections that then were mapped using an approximate VSP/CDP method. The project was conducted in order to evaluate the effectiveness of using shallow VSPs for imaging near-surface stratigraphy. In this case, we were successful in correlating VSP reflection maps to the layered structure of the Paskapoo formation as identified by sonic velocity, resistivity, and natural gamma-ray logs. ACQUISITION Figure 1 is a photograph of the EnviroVibe source used in the survey. Figure 2 shows the downhole 3-C geophone that clamps to the well case by means of a motor-driven bow spring. The first VSP was recorded with the vibrator offset 15 m south of the wellhead. The second VSP was recorded with the vibrator offset 30 m south of the wellhead. In both cases, the downhole 3-C geophone was placed at depths ranging from 5 m to 90 m with half-metre intervals. For both VSPs, the vibrator was driven with frequencies swept from 20 Hz to 200 Hz. The sweep length was 10 seconds, and the listen time was one second. A-to-D conversion, correlation, and recording were done with a single 24-channel Geometrics Geode connected to a Panasonic CF-30 Toughbook computer. The sampling time was 0.5 msec.
