Deep downhole testing: procedures and analysis for high-resolution vertical seismic profiling
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A study was undertaken to improve the signal quality and the resolution of the velocity profile for deep downhole seismic testing. Deep downhole testing is defined in this research as measurements below 225 m (750 ft). The study demonstrated that current testing procedures can be improved to result in higher signal quality by customizing the excitation frequency of the vibrator to local site conditions of the vibrator-earth system. The earth condition beneath the base plate can be an important factor in the signal quality subject to variations with time when tests are repetitive. This work proposes a convenient method to measure the site localized natural frequency and damping ratio, and recommends using different excitation frequencies for P- and S-wave generation. Properly increasing the excitation duration of the source signal also contributes to the quality of the receiver signal. The source signature of sinusoidal vibratory source is identified. Conventional travel time analysis using vibratory source generally focuses on chirp sweeps. After testing with impulsive sources and chirp sweeps and comparing the results with the durational sinusoidal source, the sinusoidal source was then chosen. This work develops an approach to identifying the source signature of the sinusoidal source and concludes that the normalized source signature is relevant only to four parameters: the fixed-sine excitation frequency, the duration of excitation, the damping ratio of the vibrator-earth system, and the damped natural frequency of the vibrator-earth system. Two of the parameters are designated input to the vibrator and the other two parameters are measured in the field test using the proposed method in this work. A new wavelet-response technique based on deconvolution and consideration of velocity dispersion is explored in travel-time analyses. The wavelet-response technique is also used for development of a new approach to correcting disorientation of receiver tool. The improved downhole procedures and analyses are then used in the analysis of deep downhole test data obtained at Hanford, WA. Downhole testing was performed to a depth of about 420 m (1400 ft) at Hanford site. Improvements in resolving the wave velocity profiles to depths below 300 m (1000) ft are clearly shown.