b'Geophysics in the surveysNewsBecause we cannot (easily!) account for the absolute travel times of body waves plunging through the deep Earth along entire earthquake-to-station propagation paths, well rely on measurements of relative arrival times between stations rather than absolute travel times to model the local, sub-array 3D velocity structure. The cost of this normalisation is that only relative, rather than absolute, 3D velocity variations can be inferred. However, the higher frequency body waves resolve velocity structure at a length scale that the lower frequency surface waves cannot, so the additional information is extremely valuable. Superimposing the granular, relative velocity variations inferred by teleseismic body-wave travel time analysis upon the longer wavelength absolute velocity structure inferred by teleseismic surface wave and ambient noise analyses leverages both data types.Because seismic and MT data have contrasting sensitivities to temperature and composition, the combination of these complementary data - at a similar resolution - offers the promise of distinguishing between thermal and compositional signals, something neither data set alone can rigorously accomplish. In particular, signatures of metasomatism Figure 1.The white symbols show the planned locations of the AusArray SA broadband seismic stations.and fluid pathways should be more Other seismic stations currently operating in South Australia include those of the Australian Nationalreadily identifiable, and these are of Seismic Network (red), Australian Seismometers in School program (blue) and ANUs Lake Eyre Basin seismiccourse primary indicators of mineral array (yellow). Diamonds signify broadband stations, triangles short period stations. prospectivity. Along with MT model data, the seismic model data will ultimately be waves at periods of 20-120s will bethe Earth between two stations (i.e.,fed into the LitMod modelling formalism used to probe upper mantle structure,the Greens function) emerges fromas part of the existing ARC Linkage grant constraining absolute 3D velocitythe cross-correlation of the noiseIlluminating AusLAMP: Thermodynamics structure at length scales of many 10srecords. In effect, one seismic stationinversion for mineral systems to model to several 100s of km. Shorter periodserves as a local, virtual earthquakethe 3D thermochemical structure of the (typically less than 20s) surface wavessource for the other, and vice versa. InGawler Craton based on probabilistic transiting in the more compositionallythe case of Australia, this circumventsjoint inversion of multiple geophysical heterogeneous crust are subject to morereliance on complex teleseismicobservables.scattering and attenuation; the longersurface waves for probing crustalDeployment of the AusArray SA seismic the propagation path, the greater thestructure. Ambient noise tomographyarray was due to commence in April wavefield complexity accrued - especiallywill be used to constrain absolute2020, but this has been temporarily if terrane or plate boundaries are3D crustal velocity structure beneathpostponed in light of the COVID-19 encountered. This renders teleseismicthe eastern-central Gawler at lengthpandemic. Once deployed, the array surface waves at periods less than 20sscales of several 10s of km (targeted,will record continuously for at least 15 difficult to interpret. dense Large-N arrays of nodalmonths in order to record sufficient Ambient seismic noise refers toseismometers can extend resolution ofearthquake and noise data for modelling the diffuse energy field excited by3D upper crustal velocity structure topurposes. The raw seismic data will be ocean swells and infragravity wavessub-km length scales). housed on the AusPass passive seismic interacting with the seafloor. Itsunlike surface waves, whose peaks anddata server and the derived model spectral power is highest in thetroughs can be readily tracked as theyoutputs will be made available on SARIG.primary (10-20s) and secondaryjourney across a seismic array (thus (5-10s) micro-seismic bands. Ambientfacilitating measurement of absoluteJohn ODonnellnoise correlation exploits the fact thatvelocities), body waves from distantGeological Survey of South Australiathe approximate impulse response ofearthquakes arrive sub-vertically.John.ODonnell2@sa.gov.auJuNE 2020 PREVIEW 18'