b'AEGC 2021Short abstractsboreholes. Conventionally, approaches for obtaining zerowith tracer interpretation characterised a hydrodynamic flow reference time are by mounting a seismometer at the top of asystem with short flow paths (2.5 km) and residence times drill-string or plant geophones near a drilling rig and then by(~15 years). A mean annual groundwater mass balance for cross-correlating seismometer/geophone data with recordeda revised extent of the freshwater system, confirmed that drill-bit signals. In our research, we propose a new method tocommunity water demand can be met with the existing address the timing issue. After the newly drilled well installedresource, though additional water infrastructure and a more with DAS fibre optic cable, we use a handful of surface VSPstrategic management approach was required.shots that lie in the same plane with the two wells to obtainThe integrated hydrogeophysical and hydrogeological the zero reference time for different drill-bit positions. Theapproach was fundamental for underpinning long-term water procedure is explained below: security for the community. Specifically, it helped: (i) identify options for siting new production and monitoring bores and (ii) 1. At a particular drill-bit depth, we first use shift and stack todesigning an adaptive groundwater extraction strategy and an increase signal strength and then pick the relative first arrivalimproved monitoring programme.times.2. Cross-correlate the DAS recording from surface VSP shots at the drill-bit position with recordings in another well and find192: Inverting the head wave coefficient with the out the time lags with each channel Werth equation3. By comparing the picked first arrival times and cross-correlated time lags we can predict the zero reference time. Dr Derecke Palmer 1We have carried out computer simulations and tested this1 ASEGmethodology on field dataset. The results achieved are promising. This processing technique further empowers theThe head wave coefficient, the refraction analogue of the SWD dataset to unleash its full potential in exploring thereflection coefficient, is a complex function of the densities subsurface, such as cross-hole tomography construction, cross- and the P- and S-wave velocities in both the weathered and hole imaging and anisotropy study. sub-weathered regions. In general, the head wave coefficient increases with increasing P- and S-wave velocities in the 191: An integrated hydrogeophysical andweathered layer, but it decreases with increasing P- and S-wave hydrogeological approach, to underpin the long-termseismic velocities in the sub-weathered layer.water security of a remote tropical islandUnscaled S-wave velocities in the weathering and sub-weathering can be computed with a new approximation Mr Andrew Taylor 1, Dr Tim Munday1, Mr Chris Turnadge1, Drof the head wave coefficient and the detailed P-wave Joanne Vanderzalm1, Mrs Tania Ibrahimi1, Mr Shane Mule2, Drseismic velocities in each layer. In general, there is excellent Axel Suckow1 and Dr Sebastien Lamontagne3 agreement between the measured and computed values after ten iterations. However, a traveltime-based estimate of 1 CSIRO the S-wave velocities is required to calibrate the amplitude-2 Mineral Resources, CSIRO based estimates.3 CSIRO Land and WaterGroundwater resources that sustain small Indigenous193: Full waveform refraction imaging of the regolithcommunities in remote parts of northern Australia are often poorly characterised. This is mostly due to their remotenessDr Derecke Palmer 1and the practicalities and economics of undertaking field investigations. The Warruwi community on South Goulburn1 ASEGIsland in the Northern Territory is completely reliant onFull waveform refraction imaging with the common intercept groundwater for its livelihood. Recent consecutive poor wetgather (CIG) is a simple application of the stacking procedures seasons highlighted both: (i) the sensitivity of the islandsroutinely employed with seismic reflection data processing to water resources to short-term rainfall variability, and (ii) thethe standard intercept time method. Accordingly, the stacked inadequacy of the islands water infrastructure for meetingfull waveform CIG is the refraction equivalent of the CMP stack water demand during dry periods. Although hydrogeologicalwith reflection methods. The CIG recasts the processing of assessments have been undertaken across discrete parts ofrefraction data into a format which is analogous to standard the island in the past, an integrated approach was requiredreflection data processing methodology.to provide long-term water security for the community.To better characterise and quantify the islands waterStacking with the CIG addresses the two most important resources and water infrastructure requirements, targetedchallenges with near surface refraction seismology. It field investigations were combined with desktop analysessignificantly improves the signal-to-noise ratios of both P- and including: (i) an airborne electromagnetic (AEM) surveyS-wave images (and therefore, the time models) of the base of supported by ground-based geophysical measurements,the weathering, and it enables the convenient investigation of (ii) environmental tracer sampling of groundwater andthe head wave coefficient.(iii) desktop analyses including an evaluation of theThe CIG can be stacked on either common receiver gathers or annual groundwater mass balance. The inverted AEM datacommon shot gathers. As a result, the CIG exhibits the unique constrained using spatial analyses of lithology, groundwaterability to separate the refraction signal into the up-going levels and salinity identified a thin (~20 m) storage-limitedand down-going components. The stacked CIG waveforms unconfined aquifer hosting a freshwater lens system overlyingapproximate time model shifted versions of the source and a regional aquitard. Groundwater level analyses combinedreceiver functions.95 PREVIEW AUGUST 2021'