Kit Chambers

Moment Tensor Migration Imaging

We develop and apply an imaging procedure for simultaneous location and characterization of seismic source properties called Moment Tensor Migration Imaging. The procedure constructs images for moment tensor components using a weighted diffraction stack migration, and combines ray-theoretical Green’s functions with a reverse time moment tensor imaging methodology. By applying an approximation we term the ‘ray-angles only approximation’, we form an expression for Moment Tensor Migration Imaging where the migration weights depend only on the take-off and arrival angles for rays leaving receiver positions and incident upon the image points. Moment Tensor Migration Imaging retains the benefits of diffraction stack procedures for source location and characterization, namely speed, flexibility, and the potential for incorporating non-linear stacking procedures, whilst also providing the benefits of moment tensor imaging such as: the inclusion of multiple phase and multiple component data; the collapsing of the source radiation pattern; estimation of the moment tensor. We examine variations of the imaging procedure through a synthetic test. We show that although the assumptions required for the imaging and ray-angles only approximation may not be strictly valid for realistic survey geometries, a simple weight adjustment can be used to obtain more accurate and stable results in these situations. In our synthetic example we find that the use of a P-wave only migration without this reweighting structure produces poor results, whereby the resulting images show activity upon incorrect moment tensor components. However, many of these effects are mitigated by use of the reweighting scheme and the results are further improved through the introduction of non-linear stacking operators such as semblance weighted stacks. The highest quality moment tensor images (for the synthetic test examined here) are obtained through the use of both P-wave and S-wave wave fields. This highlights the importance of multicomponent data and multiphase modelling when characterizing seismic sources. We also find that the imaged moment tensor components vary proportionately when the input velocities are perturbed by a scale factor. This suggests, for the geometry investigated here, derived source properties such as fault-plane solutions and shear-tensile components will not be influenced by bulk changes in seismic velocities. Finally, we show the application to a real microseismic event observed using a surface array during hydraulic fracturing. We find that the procedure collapses the seismic radiation pattern into an anomaly with a maximum at the hypocentre and our derived mechanism is consistent with the observed radiation pattern from the source.

Investigation of induced microseismicity at Valhall using the Life of Field Seismic array

Here we use data from the Life of Field Seismic (LoFS) surface array in conjunction with a migration-style approach to locate microseismic events in Valhall Field during 6.5 hours of hydraulic fracturing. We identify an event distribution that extends roughly 300 m vertically and 200 m horizontally with sources both within the chalk layer, containing the reservoir, and the overburden. However, the temporal distribution of the events is not consistent with the induced fracturing extending into the overburden. Instead, the synchronous increase in event activity with downhole pressure could be interpreted as evidence that elastic wavefronts originating from well activity trigger movement on faults where the in-situ stress is very close to that required for failure.

Testing the the Ability of Surface Arrays to Locate Microseismicity

Here we test the ability of surface sensors to locate microseismic activity using signals recorded whilst 18 perforation shots, with a variety of magnitudes, were trigged in 6 different wells beneath an array. As the perforation shots provide signals with known position and origin time the analysis of this dataset provides useful insights into the ability and accuracy of surface arrays for monitoring microseismic activity. We apply a migration style approach to locate the perforation shots. The imaging method is based on stacking all data consistent with an arrival from a particular time and point in the subsurface and is ideally suited for application to large arrays of surface sensors as it does not require any manual analysis of the data, such as picking arrival times. In all but one case the signals from the perforation shots are not visible in the raw or pre-processed data. However, clear source images are produced for 12 of the perforation shots. The results show that arrays of surface sensors are capable of imaging microseismic activity, even when the signals from events are not visible in individual traces. Factors which contribute to the successful imaging of the perforation shots could include the shot’s position and its relationship to the lithology, the presence of other noise sources, and the coupling of the perforation shot with the ground.

Diffractivity — Another attribute for the interpretation of seismic data in hard rock environment, a case study

AbstractThe primary objective of seismic exploration in a hard rock environment is the detection of heterogeneities such as fracture zones, small-scale geobodies, intrusions, and steeply dipping structures that are often associated with mineral deposits. Prospecting in such environments using seismic-reflection methods is more challenging than in sedimentary settings due to lack of continuous reflector beds and predominance of steeply dipping hard rock formations. The heterogeneities and “fractal” aspect of hard rock geologic environment produce considerable scattering of the seismic energy in the form of diffracted waves. These scatterers can be traced back to irregular and often “sharp-shaped” mineral bodies, magmatic intrusions, faults, and complex and heterogeneous shear zones. Due to the natural lack of reflectors and abundant number of diffractors, there are only a few case studies of diffraction imaging in hard rock environments. There are almost no theoretical models or field examples of diffracti...

Imaging induced seismicity with the LoFS permanent sensor surface array

Here we use data from the Life of Field Seismic (LoFS) surface array in conjunction with a migration style approach to locate microseismic events in the Valhall field during 6.5 hours of hydraulic fracturing. The method is fully automated and requires minimal user input. This makes it ideal for application to data from surface arrays such as the LoFS, where signals from individual microseismic events are not visible in recordings from a single geophone. We initially identified 1111 events in the migration results including a major group consisting of sources both within and above the reservoir, forming a planar feature with near vertical dip. Cluster analysis was used to separate the 361 events forming this feature from the parent dataset. Events were recorded throughout the injection period and reveal a large peak in seismicity after about 4.75 hours, which is synchronous with an increase in the downhole pressure. Using a variety of synthetic tests we examined the resolution and robustness of our results. Modest perturbations (±5%) to the 3D velocity model used to locate events primarily cause a static shift in the event depths. However, using a homogenous model resulted in a considerable reduction in the imaging power. We also performed a series of semi-synthetic tests (synthetic data with real noise) to establish the minium SNR requirements for event detection. Events with SNR’s greater than 0.25 are detectable by our imaging approach, this limit is related to the nature of the coherent noise in the data. Experiments with double-couple sources showed that movements on faults consistent with the planar event distribution are unlikely to be imaged by our procedure. Hence we believe that the sources observed do not have mechanisms consistent with their planar distribution and/or they contain a significant nondouble couple component. One possible interpretation of the events is that elastic or pressure wavefronts emanating from the well trigger activity on aligned faults in and around the reservoir where the in situ stress is close to that required for failure.