b'CommentaryEnvironmental geophysicsfirst injection and the third after the Environmental geophysicssecond injection. These results are summarised in Yurikov et al. (2022). The second data collection mode (slightly less conventional) was based on the use Mike Hatchof nine surface orbital vibrators (SOV) at Associate Editor for Environmentalgeophysics discrete locations around the test site as Emichael.hatch@adelaide.edu.au sources (Isaenkov et al. 2022). And then finally, passive seismic data sets were collected continuously as well (Pevzner Fibre-optic cable based systemsi.e. both naturally sourced, as well asetal. 2023). for datacollection induced. In this case the cable acts muchNeedless to say, imaging these gas like a high-resolution seismic string.injections in a briny aquifer at 1500 m Welcome readers to this issuesTo measure temperature along the FO,depth is a tough ask. The best results column on geophysics applied tomore complex interaction between theare when the full 4D VSP surveys were the environment. For this issue, I amtransmitted light and the fibre itself areperformed, as vibroseis trucks were learning about fibre-optic cable basedmeasured, again producing backscatter.usedas sources at so many sites. This systems often set up to collect seismic,Positions of scattering events (whetherproduced a good ground coverage temperature or even static strain dataacoustic information or temperature)with pretty good multi-fold seismic sometimes called DAS, DTS and DSSalong the FO line are measured baseddata sets, so SN ratios were relatively systems. I got interested in this subjecton time-of-flight calculations for thelow. But this survey takes by far the when Professor Roman Pevzner frompulse and the returned scatter. Eventgreatest amount of effort and cost as Curtin University came to Adelaidetypes are separated by frequency ofrunning the vibroseissystems required to give his SEG Honorary Lecturethe return signal. Higher frequencya lot of time, effort and personnel. titled Surface and Borehole Seismicinformation is related more to seismicFigure 1 shows some of the best results Monitoring of CO 2Geological Storage.events, while the phenomena associatedtaken from the Yurikov paper, showing My original idea when I saw Romanswith temperaturevariation are lowerdifferences between survey results for presentation was to show some examplesfrequency. To give us an idea ofdata collectedduring the pre-injection of work done by the group at Curtin thatresolution, one paper (Banks etal. 2022)survey(M6), post4 kt injection (M7), and Romans is part of, along with some otherstated that temperatures along the FO12 kt injection (M8). work that Eddie Banks is doing at Flinderswere generally accurate to within 0.1oC University and be done with it. But the(calibrating to known temperatures atThe results using the SOVs as transmitters deeper I go the more Ifind, and thethe time that the readings were taken),were not quite as good as when using harder it is for me to find my way out. Soand approximately 0.5m positionalthe vibroseis trucks. Relatively, obviously, back to my originalplan resolution along a 110m long cable. Theboth the fold and the lateral coverage of In his talk, Roman opened a window ontoseismic events measured by the groupthe data are muchlower (nine transmitter an area of research/instrumentation thatat Curtin are in boreholes more thansites vs. ~3400). It is important to note is interesting to me (I guess I had heard1500m deep.here though that while coverage is lower, of it before but hadnt appreciatedit);For the work that the Curtin group havethe surveys using the OVs as sources i.e.Distributed Acoustic Sensors (DAS)been doing in the CO 2sequestrationcould, once procedures were refined, and Distributed Temperature Sensorsspace, they have been using DAS setupsbe run remotely (including much of the (DTS) using fibre-optic (FO) cable asto collect vertical seismic profile datadata processing). Figure 2 shows the both the sensor, as well as the wire(VSP) in five instrumented wells overevolution of the plume as measured (the signal medium). A very briefan approximately 7.3 km2 area in theusing the nine SOVsthe data are limited introduction to the concept follows; forCO2CRCs Otway International Testto the data tracks as determined by the more you should start with the SilixaCentre(Yurikov et al. 2022, Isaenkovsensors in each well and the location of website (https://silixa.com/resources/ etal. 2022, Pevzner et al. 2023). The wellseach SOV. Note that while these data downloads), and then the referenceswereall instrumented to at least 1600msets are limited spatially, they were in the other papers that I mention here.depth, with two stages of liquid CO 2 collected nearly continuously so the Briefly, fibre-optic cable is laid out toinjection into one of these wells, thedevelopment of the injection bubble measure temperature distribution orfirst at ~4kt in December 2020 and thecan be observed before during and after acoustic properties of a mediumoftensecond at ~12kt in early 2021. The DTSinjection (along with reactivation of an the FO is placed in a borehole, and also,instrumentation ran nearly continuouslyold injectionevent). often, is cemented into place so as tofor the duration of the study, so dataThe natural source results used ocean-be part of a permanent monitoringwere collected in at least three modes.generated Rayleigh waves as the system. Laser pulses are transmittedThe most conventional mode was runsource for the study (Pevzner et al. along the FO cable, with some of theas a 4D VSP survey, where the source2023) was also tested. These waves energy backscattered to the sourcewas a vibroseis truck, shaking the earthare present overmuch of the Earth, when various properties of the FO vary.at ~3400 sites over the course of threebut are characterised by wavelengths For example, the FO may be set up todistinct surveys one immediatelyof many kilometres. The results of the sense changes related to seismic events,before injection, the second after theuse of natural source information to 34 PREVIEWFEBRUARY 2024'