Do stopes contribute to the seismic source?

Parameters such as source location, seismic moment, energy, source size, and stress drop are routinely calculated from mining-induced seismic data. Seismic moment tensors are inverted less routinely because their calculation is more complex and their accuracy depends on the network geometry, among a number of other factors. The models utilized in the source parameter calculations, the most well-known of which is the Brune model, were developed for the global seismicity problem and assume a solid, homogeneous Earth model. However, the tabular orebodies in South African gold and platinum mines are mined extensively and the excavations (stopes) can extend for many kilometres. The seismic source mechanisms on deep-level gold mines are generally compatible with shear failure (Hoffmann et al., 2013), whereas the source mechanisms of events at intermediate-level bord and pillar mines in the platinum district are more compatible with pillar failure and accompanying stope closure (Spottiswoode, Scheepers, and Ledwaba, 2006; Malovichko, van Aswegen, and Clark, 2012). In this paper we investigate the influence of the stope on seismic inversions for the scalar moment, corner frequency/source radius, stress drop through numerical modelling using WAVE3D. The main objective is to determine whether the source parameters calculated from the recorded waveforms are due to a combination of the stope and shearing sources, rather than being related only to a shear source in the host rock. The modelled source is shear rupture in the footwall of a stope. The results show that the stope appears to have an appreciable effect on the seismic inversions. The seismic moment and source radius of the shear source in the stope are larger for the model with a stope compared to the model with no stope. The stress drop for the case with a stope is less than the applied stress drop, which could be an effect of the apparently larger source. This work provides a possible explanation of the second corner frequency often observed in the spectra of seismograms recorded in South Africa platinum mines. This has implications for the accurate determination of source parameters and the assessment of the intensity of shaking in stopes

Simplified Seismic Modelling of Fractured Rock: How Effective is the Localised Effective Medium Compared to Explicit Representation of Individual Fractures

Seismic waves can be an effective probe to retrieve fracture properties particularly when measurements are coupled with forward and inverse modelling. These seismic models then need an appropriate representation of the fracturing. The fractures can be modelled either explicitly, considering zero thickness frictional slip surfaces, or by considering an effective medium which incorporates the effect of the fractures into the properties of the medium, creating anisotropy in the wave velocities. In this work, we use a third approach which is a hybrid of the previous two. The area surrounding the predefined fracture is treated as an effective medium and the rest of the medium is made homogeneous and isotropic, creating a Localised Effective Medium (LEM). LEM can be as accurate as the explicit but more efficient in run-time. We have shown that the LEM model can closely match an explicit model in reproducing waveforms recorded in a laboratory experiment, for wave propagating parallel and perpendicular to the fractures. The LEM model performs close to the explicit model when the wavelength is much larger than the element size and larger than the fracture spacing. By the definition of the LEM model, we expect that as the LEM layer becomes coarser the model will start approaching the effective medium result. However, what are the limitations of the LEM and is there a balance between the stiffness, the frequency and the thickness, where the LEM performs close to an explicit model or approaches the effective medium model? To define the limits of the LEM we experiment varying fracture stiffness and source frequency. We then compare for each frequency and stiffness the explicit and effective medium with five models of LEM with different thickness. Finally, we conclude that the thick LEM layers with lower resolution perform the same as the thinner and finer resolution LEM layers for lower frequencies and higher fracture stiffness

Modelling P-Wave Propagation in a Medium With Multiple Parallel Fractures and Direct Comparison With Experimental Recordings.

We examine P-wave propagation in a fractured medium using effective medium, explicit fractures, and localized effective medium representations of fracturing, to quantify their effectiveness. We model a published experiment with multiple parallel fractures. Initial models assume uniform fracture stiffness across all fractures. A methodology is presented for inverting a source from the experiment. We find that the waveforms from the three models do not match each other. For propagation parallel to the fractures, the explicit model performs best with excellent agreement with the experimental waveform. The waveform from the localized TI model is reasonably similar, matching arrival, predominant period, frequency content, but not amplitude. The TI model is a poor match with significant differences in period, amplitude and high frequency content. For perpendicular propagation, none of the models properly match the experimental waveform. All models reproduce the significant delay in arrival, but only the explicit and local TI models produce a reduction in period and frequency content mimicking the experiment. All models produce a reduction in amplitude but not to the degree of the experiment. An explicit model accounting for the effect of the non-uniform stress-field better matches the experiment, indicating similar developments are needed for the other two representations

Exploring trends in microcrack properties of sedimentary rocks: An audit of dry and water saturated sandstone core velocity-stress measurements.

Stress dependent rock physics models are being used more routinely to link mechanical deformation and stress perturbations to changes in seismic velocities and seismic anisotropy. In this paper, we invert for the effective non-linear microstructural parameters of 69 dry and saturated sandstone core samples. We evaluate the results in terms of the model input parameters of two non-linear rock physics models: A discrete and an analytic microstructural stress-dependent formulation. The results for the analytic model suggest that the global trend of the initial crack density is lower and initial aspect ratio is larger for the saturated samples compared to the corresponding dry samples. The initial aspect ratios for both the dry and saturated samples are tightly clustered between 0.0002 and 0.001, whereas the initial crack densities show more scatter. The results for the discrete model show higher crack densities for the saturated samples when compared to the corresponding dry samples. With increasing confining stress the crack densities decreases to almost identical values for both the dry and saturated samples. A key result of this paper is that there appears to be a stress dependence of the compliance ratio BN/BT within many of the samples, possibly related to changing microcrack geometry with increasing confining stress. Furthermore, although the compliance ratio BN/BT for dry samples shows a diffuse distribution between 0.4 and 2.0, for saturated samples the distribution is very tightly clustered around 0.5. As confining stresses increase the compliance ratio distributions for the dry and saturated samples become more diffuse but still noticeably different. This result is significant because it reaffirms previous observations that the compliance ratio can be used as an indicator of fluid content within cracks and fractures. From a practical perspective, an overarching purpose of this paper is to investigate the range of input parameters of the microstructural models under both dry and saturated conditions to improve prediction of stress dependent seismic velocity and anisotropy observed in time-lapse seismic data due to hydro-mechanical effects related to fluid production and injection

Full waveform model validation of microseismic shear-wave splitting fracture parameter inversion

Although the relationship between reservoir formation permeability and fractures is complex, it is recognized that fractures play an important role in reservoir fluid flow. Recent research has been using the integrated geomechanics and seismic modelling to characterize tight-gas reservoirs, which requires knowledge of the joint or fracture compliances both on the geo-mechanic and seismic scale. However, populating the geomechanical and/or seismic model with joint and fracture properties is based primarily on laboratory core data, which are on many times smaller length scales than observed in fractured reservoirs. As such, it would be ideal to measure and calibrate fracture compliance from field-scale measurements. The aim of this project is to explore whether observations of seismic anisotropy from P- and S-waves can constrain fracture compliance (normal and shear). In this study, we investigate the feasibility of using microseismic data to invert for fracture density, fracture strike and fracture compliance ratio from shear splitting results

Time-lapse seismic interpretation in tau-p space using pre-stack gather data

We present a new algorithm to measure time-lapse vertical travel-time shifts in seismic pre-stack shot and CMP gathers by tracking traces having constant horizontal slowness in τ-p space. Unlike other methods for measuring these attributes from stacked volumes, our use of pre-stack data avoids errors and uncertainties inevitably introduced in conventional time-lapse processing, such as choosing a suitable migration velocity model and cross-correlation time-window size. Results are localised to a given interval and thus free from overburden effects. This approach is used to estimate layer vertical travel-time shifts, a reservoir compaction-dilation coefficient, and hence calculate both velocity and thickness changes within a reservoir and the overburden. We demonstrate the method using synthetic reflection data generated using both a ray-based and a finite-difference full-waveform algorithms on two suites of models: a simple four-layer reservoir model; and two hydro-mechanical simulation models. We compare our estimates of layer interval vertical time-lapse travel-time shifts and velocity and thickness changes with those of the input model. The results indicate that the new τ-p time-lapse method produces sufficiently accurate results compared to conventional methods

Microseismicity within a karstified rock mass due to cracks and collapses as a tool for risk management

Seismometer arrays have been widely applied to record collapse by controlled explosion in mines and caves. However, most underground failures are natural events, and because they can occur abruptly, underground failures represent a serious geological hazard. An accelerometric array installed on 4 September 2008 has been used to manage the geological risk of the Peschiera Springs drainage plant of Rome's aqueduct, which is located in the Central Apennines approximately 80 km from Rome, Italy. The plant occupies a karstified carbonatic slope that is extensively involved in gravitational deformations, which are responsible for underground failures such as cracks and collapses. To distinguish among different types of recorded events, an automated procedure was implemented based on the duration, peak of ground acceleration (PGA) and PGA variation in the recordings of the plant's accelerometric stations. The frequencies of earthquakes and micro-earthquakes due to underground failures are, in general, well correlated. Nevertheless, many underground failure sequences can be directly associated with the continuous deformations that affect the slope. The cumulative Arias intensity trend derived for the underground failures combined with the failure and earthquake frequencies enabled the definition of a control index (CI) that identifies alarming or emergency conditions. The CI can be used as a tool for managing the geological risk associated with the deformational processes that affect the drainage plant