Dan Quinn

Velocity calibration for microseismic monitoring: A very fast simulated annealing (VFSA) approach for joint-objective optimization

To accurately locate microearthquakes that are genetically related to hydraulic fracture stimulation, a thorough knowledge of the velocity structure between monitoring and fracturing treatment wells is essential. Very fast simulated annealing (VFSA) is implemented to invert for a flat-layered velocity model between wells using perforation or string-shot data. A two-point ray-tracing method is used to find the ray parameter p for a ray traveling from a source to a receiver. The original traveltime-calculation formula is modified to account for the borehole source-receiver geometry. VFSA is used as a tool to optimize P- and S-wave velocities simultaneously. Unlike previous applications of VFSA, two improvements result from a new study: (1) both P- and S-wave arrival-time misfits are considered in a joint-objective function, and (2) P- and S-wave velocities are perturbed simultaneously during annealing. The inverted velocities follow the true values closely with a very small root-mean-square error, indicating the inverted model is close to the global minimum solution whose rms error should be zero for synthetic examples. Data noise contaminates inverted models, but not substantially in synthetic test results. A comparison of models inverted using VFSA and Occams inversion technique indicates that inverted models using VFSA are superior to those using Occams method in terms of velocity accuracy.

Vertical seismic profile S-S imaging with vertical vibrators

The vertical vibratory source is ubiquitous in land P-P seismic acquisition. However, the source may be observed to produce a rich field of other (S-wave-like) modes that can be identified and processed to exploit the characteristics of the S wavefield at no additional acquisition cost. S-waves lack gas sensitivity and have complementary characteristics when compared to P-wave data in response to the same stratigraphy. Sh modes are decoupled and tend to have less interference, enhancing their S/N and facilitating processing. Direct S-wave arrival times can be inverted for velocity, leading to the construction of Vp/Vs and other attributes. This paper presents an example of combined P- and source-generated S-wavefield processing through imaging for zero-offset and offset vertical seismic profiles (VSPs).