3D velocity model with complex geology and voids for microseismic location and mechanism

Abstract

A microseismic monitoring system provides a vital window into a rock mass to see where stress induced fracturing is occurring in relation to mining operations. A main factor for the accuracy of the microseismic locations is the velocity model assumed for the rock mass. The majority of mines that use microseismic systems use a single velocity model for location purposes which assumes the same elastic modulus properties throughout the volume. This study shows examples of event locations that were calculated using a velocity model that accounts for multiple complex shaped geological units each with their own properties. The method allows multiple voids to be added that could be air filled, brine filled, or cement paste back filled, thus mimicking mining and geotechnical operations such as stope mining, cave mining, solution mining, or underground cavern storage. Going beyond the dots, microseismic systems provide an important way to understand the failure mechanics of the rock fracturing. With good data quality, each located event can be solved for the source mechanism (moment tensor) and interpreted in terms of whether the event is dominantly tensile opening, closing, or shear slip. The orientation of each event failed zone can be quantified providing useful information about the discrete fracture network (DFN). This paper provides examples of source mechanism solutions using a full 3D velocity model. It is shown that the ray path of each sensor does affect the source mechanism solution when comparing a single velocity model solution and a 3D velocity model solution. Microseismic systems offer important daily information for mine operation, safety and planning. Improvements to the accuracy of seismic results by using enhanced processing methods and regular calibration, allow a mine to more confidently integrate seismic results with numerical models and make decisions. This is especially important as mines move to different excavation methods such as block caving and extend to greater depths and stresses. Recently, microseismics are playing an essential role in applications such as the monitoring of shale gas hydraulic fracturing and underground storage caverns (Baig et al. 2012). Being able to monitor a 3D volume of rock deep under the ground in real time is very important for tracking any rock fracturing and coalescence that may be occurring. https://papers.acg.uwa.edu.au/p/1410_48_Collins/ 3D velocity model with complex geology and voids for microseismic location and mechanism DS Collins et al.

Microseismic monitoring of rockbursts with the ensemble Kalman filter

“…Such system is a dense seismic network used to monitor microseismic events, with local magnitudes down to −4, around the active mining face (Gibowicz and Kijko, 2013). The accuracy of the location of Figure 1 Example of a typical microseismic system set-up, courtesy of ESG Solutions (Collins et al, 2014). seismic events is of the order of 20-50 m or 50-100 m, depending on the number of sensors, the network size and geometry (Li et al, 2018).…”

“…Microseismic monitoring systems in mines have changed considerably over the past 25 years. Modern systems consist of a 3D distribution of sensors using a mixed array of uniaxial and triaxial sensors, and can capture over 100 events per minute (Collins et al ., 2014). Figure 1 shows a schematic outline of a typical mine seismic monitoring system with surface and underground sensors.…”

Relocating Mining Microseismic Earthquakes in a 3-D Velocity Model Using a Windowed Cross-Correlation Technique

“…To the best of our knowledge, only a few studies have applied simple 3-D velocity models for microseismic source location in mines. Collins et al [23] tried to use a 1-D layered velocity model with a low velocity zone for microseismic source location in a mine, and Peng and Wang [24] treated the P wave velocity in the mined-out regions and tunnels as the wave velocity of air and other zones as a homogeneous velocity model. Gharti et al [25] considered velocities of air, rock and ore in their mining applications.…”

“…Some researchers have considered these voids as a simple 3-D velocity model with regular geological shapes. Collins et al [23] showed an improvement in the location accuracy when accounting for mining stopes at a hard rock mine. Gharti et al [25] considered velocities of air, rock and ore in their mining applications.…”

The design, optimisation, and use of the seismic system at the deep and high-stress block cave Deep Mill Level Zone mine

“…To date, two array optimisations have been performed. In June 2016 a single velocity model (termed here 1DVM) was established using ten drawbell blasts with known locations ( Figure 5), and in March 2017 a mesoscale, semi-static velocity model which incorporates gross geology and the cave outline (termed here 3DVM (Collins et al 2014;Pinnock et al 2016) was derived using oriented sensors and known geological domains and excavations ( Figure 6). Blast locations were determined on mine plans to within 5 m of the centre position of a planned blast volume.…”