b'AEGC 2023Short abstractsNorth Carnarvon basin is a geologically complex basin withmeasurement provides high resolution UCS data with fine the overlap of play types at three discrete stratigraphic levels -spatial sampling, providing a rich, statistically-significant dataset Cretaceous, Jurassic unconformity and intra-Triassic reservoirs.for determination of the distribution of rock strength. This Previous attempts to detect, resolve and quantify lateral andinformation feeds into blasting and comminution optimisation. vertical velocity variations of various wavelengths in theDrilling through the current bench and sampling the next overburden were unsuccessful, and therefore the 2018 legacybench down expands the application of this measurement seismic image showed significant distortion, resulting in non- to drill pattern planning based on the measured mechanical conformance of gas-related amplitudes to structure. properties of the next bench. Survey hardware, deployment and processing is presented along with an example application to a Recent developments in seismic imaging technology andcopper porphyry mine.seismic-based uncertainty analysis were combined to quantify the structural uncertainty associated with mapping the extent of the Pinhoe gas field. The results of this study significantlyHourly 4-D subsurface time-lapse monitoring using aid the understanding of the field structural uncertaintiesseismic ambient noiseand reservoir volumetric estimates and reduce the seismic interpretation risk for field development decisions, includingPeng Guo and Erdinc Sayginwell planning. CSIROTo address the project objectives and challenges, a tailoredWe use seismic ambient noise recorded by the ocean bottom earth model building workflow, utilising full-waveform inversionnodes (OBNs) in the Gorgon gas field, Western Australia to combined with tomographic model updates, was implemented,compute time-lapse seafloor shear-wave velocity models. The and this successfully achieved an accurate velocity model thatextracted hourly cross-correlation (CC) functions of 0.11Hz enhanced seismic image quality and conformance of gas- contain mainly Scholte waves with very high signal to noise related amplitudes to structure. ratio. The conventional time-lapse analysis suggests relative The accuracy of the final earth model was verified using thevelocity variations (dv/v) up to 1% assuming a spatially mis-tie between the key seismic reflector and the key seismichomogeneous dv/v, with a likely 24-hour cycling pattern. With markers in the wells. The level of accuracy was within 1% athigh-resolution baseline models from full waveform inversion reservoir level and confirmed that these imaging products wereof Scholte waves, we propose a double-difference waveform suitable for seismic-based uncertainty analysis. inversion (DD-WI) method using travel time differences for localising the time-lapse dv/v in the heterogeneous subsurface This advanced imaging workflow has provided the mostin depth. The time-lapse velocity models show velocity increase/reliable image and structure map of the Pinhoe field produceddecrease patterns in agreement with that from conventional to date, with a deeper understanding of the field volumetricanalysis, with more notable changes at the shallower depths. uncertainties to assist with the field development planning.We demonstrate the feasibility of using ambient noise for This was achieved through a collaborative partnership betweenquantitative monitoring of subsurface property changes in the the geophysicists understanding of the signal processing andhorizontal and depth domain at an hourly basis.imaging, combined with geologists in-depth knowledge in Pinhoe field. Numerical modelling of the effect of the borehole environment on seismic amplitudes measured by fibre-UCS while drilling - actionable orebody intelligence foroptic distributed acoustic sensors.mining efficiency improvement.Boris Gurevich, Andrej Bona and Roman PevznerRoger GriffithsCurtin UniversityOricaRecent studies use seismic strain amplitudes recorded by Uniaxial Compressive Strength (UCS), also known as unconfineddistributed fibre-optic acoustic sensors (DAS) to estimate compressive strength, is a key rock property which influenceselastic properties of the formation (the stiffer the formation, multiple steps along the mining value chain includingthe smaller the strain). However, the borehole DAS response drilling, blasting and comminution operations. UCS is usuallycan be affected by the borehole environment. We model this measured on drill cores in the laboratory, a process that is botheffect numerically with 1.5D full wave reflectivity method time consuming and expensive, while offering very limitedimplemented in OASES software (3D wave propagation in a sampling of the overall volume of rock to be extracted. We1D model). In these simulations, cement, casing and wellbore have developed a novel accelerometer-based measurementare represented by infinite vertical layers. For a P-wave with acquired while drilling which can determine the UCS of thea dominant frequency of 40 Hz propagating parallel to a rock being drilled from the vibrations induced in the drillstring.10-cm-thick cement layer, the vertical strain amplitude in The fundamental response measured is a time delay, not athe cement (with or without a 1-cm-thick steel casing) differs force or torque, and so does not suffer from the calibrationfrom the amplitude away from the well by less than 5%. The and consistency issues that plague attempted application of(small) effect of the cement layer extends some 200 m into Measurement while drilling (MWD) data to rock parameterthe formation. The vertical strain in a liquid-filled borehole determination. In addition, the measurement is insensitive(modelled by a 10-cm-thick liquid layer) is comparable to that to bit condition and driller actions, resulting in a significantlyin the formation (but can be larger or smaller, depending on more robust evaluation of the subsurface than availablethe source configuration). However, DAS is not measuring through MWD analysis. The measured delay correlates stronglystrain in the fluid; it measures strain in an optical fibre or cable with UCS, so when acquired during blasthole drilling theimmersed in the fluid. Modelling of an optical cable by a 1-cm-101 PREVIEW FEBRUARY 2023'