b'Education matters actively used in geophysical monitoringcasing, which may indicate imperfectionsa signi\x1dcant in\x1buence on the long-applications. The technology is based onof the cement job. The results demonstrateterm stability of Antarctic glaciers and continuous measurements along a \x1dbre- that downhole \x1dbre-optic array installedconsequently sea-level. Because it is optic cable. Distributed temperaturein an abandoned well represents anvery di\x1ccult in most cases to access the sensing (DTS) is used for measuringopportunity to establish a permanentbedrock to determine solid Earth heat and monitoring temperature whilefacility for continuous recording ofloss, we must rely on proxies for the distributed acoustic sensing (DAS) canpassive and active geophysical data andthermal state of the lithosphere such record seismic waves/signals that inducefor exploring various applications. as mantle seismic velocities and Curie axial strain in the cable. depth estimates derived frommagnetics. DAS measurements are also sensitive toHowever, these proxies often relate to Compared to 4D surface seismictemperature changes. Laboratory andpoints deep in the crust/upper mantle monitoring, repeated Vertical Seismic\x1deld tests of DAS sensitivity to changingand thus miss key aspects of heat Pro\x1dling (VSP) surveys with DAS receiverstemperature demonstrate that DAS istransfer in the shallower crust, namely, reduce the cost and invasiveness ofsensitive to long-period temperatureradioactive heat production and thermal timelapse CO 2monitoring considerablychanges and its response is proportionalconductivity. My work focused on the but have limited spatial coverage aroundto the time derivative of temperature.latter by collecting some of, if not the the borehole. This coverage can beInduced \x1dbre strain is linearly related\x1drst thermal conductivity measurements extended by interferometric imagingto slow temperature change and thison Antarctic bedrock and modelling the that utilises free-surface multiples.dependency can be estimated for ae\x1eects of thermal conductivity both the Synthetic and \x1deld studies demonstrateparticular cable. The results can helplateral and vertical transfer of heat. For that interferometric imaging is a viablecompensate for the e\x1eect of temperatureexample, we \x1dnd that in a subglacial method to extend the subsurface imageon low-frequency DAS signal and showvalley, or buried bedrock high, most heat beyond the coverage of standard VSPthat DAS can be used as a distributedwill move through the more conductive imaging. Comparison of the standard andtemperature sensor if direct temperaturebedrock, resulting in heat being moved engineered \x1dbres shows that both \x1dbresmeasurements are not available. away from subglacial valleys and into are sensitive to free-surface multiples,bedrock in regions of geological contacts but the engineered \x1dbre provides muchMost DAS systems are designed towhereby heat will move into the more higher signal to noise ratio, and thus ismeasure signals higher than 1 Hz;conductive of the two mediums. The preferable for interferometric imaginghowever, some DAS systems are sensitiveresult is the creation of localised regions with multiples. The results obtainedto low-frequency ( 1 Hz) signals suchwhere heat \x1bux at the base of the ice with the engineered DAS cable showas reservoir pressure variations. Duringsheet can be 80 to 120% of the regional that in the depth range suitable for bothCO 2injection within the CO2CRC Otwayheat \x1bux creating localised regions of methods, the VSP interferometric imageProject, pressure related strain-rate DASelevated/reduced temperature. Using of re\x1bectors is comparable to the surfacesignals were observed in two monitoringnew thermal conductivity measurements seismic image. wells. These signals are highly correlatedcombined with a larger global database, with the pressure signals measured byI developed a method to predict thermal Borehole-based DTS and DAS utilisedborehole pressure gauges above theconductivity from igneous compositions for continuous monitoring of boreholeperforations in monitoring wells. Analysisand an empirical relationship with decommissioning operations reveal anof the data shows that DAS is able toseismic velocity. We used the later to abundance of valuable information aboutdetect reservoir pressure variationsproduce a continent-wide 3D model of the course of the decommissioninghigher than 104 psi/s. Analysis ofthermal conductivity of the Antarctic process and the quality of the cementpressure variations and strain calculatedlithosphere that can be used to improve job. DAS has detected vibrationalfrom DAS strain rate values allowsgeothermal heat \x1bux.disturbances during the cements settingestimation of the elastic modulus of the up, while DTS was used to assess settingreservoir formation. Obtained resultsA second aspect of my thesis involved up of the cement and curing times asshow that DAS systems can be utilisedusing constraints provided by subglacial well as uniformity of cementation fromnot only as seismic sensors, but alsolakes to place reasonable bounds on the distribution of temperature along theas continuous pressure sensors thatsubglacial heat \x1bux. The existence of borehole. Passive DAS data recorded acan help track possible CO 2leakagessubglacial lakes results from two types year later with the same array shows aninto the overburden. In contrast toof processes: the balance of energy at abundance of seismic events in a widetraditional pressure gauges, DAS isthe rock-ice interface and the ponding frequency range from below 1 mHz tocapable of tracking the pressure pro\x1dleof \x1buids transported beneath the over 200 Hz and includes earthquakes,along the entire well. DAS pressureice sheet, often related to seasonal mine blasts, ocean microseisms, andsensing capabilities open up many newprocesses. I developed unsupervised local human activity. The amplitudes ofapplications to complement subsurfaceand supervised machine learning waves from distant seismic events can bereservoir pressure monitoring, CCUS andmethods to predict the geographic used to estimate and monitor physicalhydrogeological studies. distribution of lake types using a set of properties of the media along the entiresimple observables including crustal extent of the well. Spectral analysis of lowSimon Willcocks, University of Adelaide:thickness, bedrock elevation, ice frequency microseisms shows a strongConstraining subglacial heat \x1bux inthickness, ice velocity, average surface correlation between passively recordedAntarctica from thermal conductivity andtemperature and bedrock slope. The DAS and local weather observations.subglacial lakes. Principal Component Analysis, while Detected peculiar in-hole reverberationsshown not to be a good predictive map, are likely caused by cross\x1bows ofThe rate of heat transfer from the solidis excellent at identifying regions of groundwater behind the intermediateEarth to the base of ice sheets hasAntarctica as either containing active 29 PREVIEW DECEMBER 2023'