From lab to field (2)

Early detection of rockburst risk areas is a prerequisite for ensuring safe production in coal mines. A reliable and efficient detection method is essential for identifying stress distribution and disturbances in coal and rock masses during mining. In this study, we employed seismic traveltime tomography to study the three-dimensional velocity structure of the continuously mining and excavating area of a coal mine by inverting a number of P-wave arrival times from the induced micro-earthquakes (M < 2.0). The response characteristics and stress evolution of inversion areas at different mining stages were investigated by examining the obtained time-lapse velocity structure. The microseismic (MS) dataset was partitioned into two-lapse subsets in a temporal sequence, with each subset containing 367 relocated events. A series of checkerboard and restoring resolution tests, as well as field investigations, confirmed the resolution, robustness, and reliability of the tomographic results. In addition, absolute velocity change patterns were proposed to assess and predict rockburst risk areas caused by mining activities. The results showed that high-velocity zones could be attributed to roof subsidence, stress transfer after coal and rock mass rupture, and areas with special geological conditions in isolated working face situations. The coupled effect of stress evolution and geological conditions may lead to more pronounced stress concentrations during the mining process. Induced events can occur not only in areas between high and low stress but also near the isoline edges of pattern changes. Our established velocity patterns have significant potential for predicting hazardous areas, with a timeliness of up to 10 days.

Chile is host to some of the most extensive natural resources in the world. However, these resources also pose some key challenges: the complex geology present - faults, rock heterogeneities and discontinuities. The aim of the GeoSafe project is to twofold: to develop a new collaboration between the Rock Mechanics Laboratory (RML), University of Portsmouth (UoP) and the School of Mining Engineering and Structural & Geotechnical Engineering, Pontificia Universidad Católica de Chile (PUC), and to measure and link underpinning rock-physics parameters to the development of fracture damage and heterogeneity in mines. New data is needed in order to understand and manage physical safety (e.g. bench collapse or rock-bursts) and environmental safety (e.g. mapping and simulating likely fluid/contaminant flow) in stressed rock masses.
Here we present data from two mining districts. The Tiltil artisanal mine, and the El Teniente large scale facility in the O'Higgins region, ~2 hours South-East from Santiago. Both are dominated by complex series of mineralized bodies hosting porphyry-copper type deposits within successions of volcanic and plutonic rocks. The Tiltil district also hosts porphyry coppers as strata bound deposits. As with many areas of the Andes, both districts are heavily faulted with abundant mineralization and alteration which combine to generate significantly different rock mechanical properties even within the same rock unit. Over the course of 18 months the team conducted 4 bilateral rock collection visits to generate an extensive database on these units, and conducted field surveys of the rock masses insitu (within tunnels).
Rock Physics data shows that the inherent anisotropy has a huge effect in both areas, with Unconfined Compressive Strength (UCS) ranging from 70 to 114 MPa for a dacite lithology and 90 to 140 MPa for an andesite (designated CMET) unit. Tensile strength reveals tensile strength between 6.2 and 10.9 MPa (dacite) and 10.7 and 13.2 MPa (CMET). Triaxial experiments also show significant variability with strength ranging from approximately 50MPa at 5MPa confining pressure to approximately 175MPa at 25MPa confining pressure, and with complex strain paths. We interpret this to be as a result of the orientated veining in the rock mass (and thus samples) which controls the deformation and influences the eventual strength at failure. This is supported by cyclical experiments using Acoustic Emission (both USC and triaxial) suggesting that damage preferentially accumulates along the veining. Future work will include these data into new COMSOL-based models of the larger rock mass.

We performed ultrasonic reflection measurements on a basalt sample from the Hellisheidi geothermal field, Iceland, as part of the Synergetic Utilisation of CO2 storage Coupled with geothermal EnErgy Deployment (SUCCEED) project. We performed the measurements on a sample, representing the geothermal-reservoir rocks, placed in a cylindrical steel borehole simulator. The cylindrical basalt sample had a diameter of 40 cm and height of 48 cm. It was subjected to 23.3 MPa axial pressure and 14.7 MPa radial pressure. We performed measurements before CO2 injection and at different levels of injected CO2, where the injection was performed in the sample at an artificially constructed horizontal fracture. During CO2 injection, the temperature was 40°C. We performed the reflection measurements using Panasonic 1-MHz ultrasonic transducers. We used two fixed points distanced at 20 mm for source transducers. We deployed a construct with four receiver transducers with fixed distance between them of 25 mm. The construct was placed in-line with the two sources, and measurements were taken from each source. The construct was then slid 5 mm away from the closest source, and new measurements were taken. The construct was moved five times to obtain 21 separate receiver points spaced at 5 mm. In this way, we measured two common-source gathers each with 21 receivers. The transducers were placed on one of the plane sides of the borehole simulator.
To reach the target depth level of the artificial fracture and return to the receivers, the ultrasonic waves pass two times through the steel cap of the simulator (215 mm thickness) and an underlying aluminium plate (30 mm thickness), while the fracture is 67.5 mm below the top of the basalt sample. To eliminate the kinematic influence of the steel and aluminium, we apply seismic interferometry for retrieval of non-physical reflections. We do this by correlating at each receiver the reflection arrival from the basalt/aluminium interface with the reflection arrival from the basalt fracture, and them sum the 21 correlation results over the receivers. This retrieves a non-physical reflection arrival between the two sources (turning one of them into a virtual receiver), which in a kinematic way corresponds to a reflected wave that has propagated only between the basalt/aluminium interface and the fracture. In this way, we monitor directly for seismic-velocity changes inside the basalt. We show comparisons of the thus-estimated dry-measurement velocities and the velocities at the different CO2 injection levels.

Acknowledgements
The SUCCEED project is funded through the ACT – Accelerating CCS Technologies (Project No 299670) programme. Financial contributions by the Department for Business, Energy & Industrial Strategy UK (BEIS), the Ministry of Economic Affairs and Climate Policy, the Netherlands, the Scientific and Technological Research Council of Turkey (TUBITAK), Orkuveita Reykjavíkur/Reykjavik Energy Iceland (OR) and Istituto Nazionale di Oceanografia e di Geofisica Sperimentale Italy (OGS) are gratefully acknowledged.