An important aspect of microseismic monitoring is the creation of accurate velocity models for the monitoring region. Sedimentary rock forms through deposition over time. Layers of rock are created from different materials or under varying conditions and naturally each layer of rock will have different properties. A velocity model maps out the layers of rock in a monitoring region and the expected speed that seismic waves (P-waves, S-waves, including anisotropic effects) will travel in each layer.
When considering event location, the proper construction and validation of velocity models is critically important. Information such as P and S-wave arrival time (distance) and hodogram analysis (direction) are used in conjunction with the velocity model to accurately locate events. Deviations from the actual and assumed velocity models will lead to systematic mislocation of events.
Corrections to the velocity model and event locations can be performed using various inversion techniques. This allows the processor to account for velocity variations such as heterogeneities, raypath effects and uncertainties in observation and treatment well positions.
ESG uses a number of different techniques to develop velocity models, but in all cases, the model is a product of information supplied by the client.
Seismic Anisotropy
Anisotropy is defined as the property of being directionally dependent. Seismic anisotropy is a recognized parameter in geophysics that refers to the variation of wave velocities with direction of propagation.
The image to the left provides an example of a waveform travelling through an anisotropic medium. Notice how the S-wave splits into horizontal and vertical components once it enters the anisotropic medium, and therefore the horizontal and vertical component waveforms exit the material at different points in time.
In recent years, seismic anisotropy has become a hot topic in the oil and gas industry. The anisotropic nature of sedimentary basins containing oil and gas reservoirs can lead to higher microseismic event location errors if the velocity models used for the reservoir do not accurately account for anisotropy. Formation velocity information is primarily collected in the field using logging methods such as sonic logs. Unfortunately, these tools measure the velocities along the borehole axis (i.e. vertical), but do not necessarily take into account horizontal velocities.
In particular, a phenomenon known as Shear-wave splitting or S-wave splitting can be observed in anisotropic reservoirs. When a shear wave travels through an anisotropic region, it separates into two orthogonal waves, with one wave travelling faster than the other. The S-wave that travels faster will arrive at the geophones first. An example of shear-wave splitting is provided in the image to the right. Notice how the S-wave in this rotated signal demonstrates two clear arrivals (blue and green waveforms).
ESG’s Approach to Anisotropy
ESG has developed a number of methods to address the effects of seismic anisotropy on microseismic data.
Array Design
If ESG is involved with data acquisition or provides a feasibility analysis for the site prior of data acquisition, ESG can recommend the optimal array positioning and configuration to minimize the effects of seismic anisotropy.
Anisotropic Ray-tracing Location Algorithms
If detailed information about the anisotropic rock parameters or velocities are available, they can be directly incorporated into ESG’s proprietary ray-tracing source location algorithms.
Particle Swarm Optimization
Most often, detailed information about anisotropic velocities are not available; therefore ESG uses its proprietary Particle Swarm Optimization (PSO) routine to adjust the velocity models and improve event locations.
ESG''s PSO algorithm refines locations by perturbing them together with the velocity model, increasing confidence in the results. PSO effectively calibrates velocity models using control events with known locations (such as perforations).
