b'NewsGeophysics in the surveysGeological Survey of South Australia: Legacy ground gravity, to relevel or not to relevel - an informal spielAn ongoing task for geophysicists at theIm not going to present any of theolder surveys here, where survey reports Geological Survey of South Australiaactual data in this short news piece, andare rarelyavailable.(GSSA) is to maintain a databaseI request the readers imagination for the containing gravity observations acquiredfollowing. Imagine these scenarios: Even without a survey report, additional in SA. For a variety of reasons, we areresearch can yield satisfactory results. A gravity survey where the observedIn a second survey we had to examine obliged to house the data as we receivegravity values are noticeably di\x1ferent it. So, for example, if explorer A asksthe data, determine which point was to neighbouring values, the base station (its usually the \x1erst for the public domain 2004 gravityA gravity survey where the elevationpoint) and compare its value with other survey conducted by contractor B fordata are noticeably di\x1ferent to DEM/ mentions of the base station in other companyC, we hand it over as it is.SRTM/surrounding values, reports/surveys. For this second survey, Or, as is more common now, explorer A downloads the data directly o\x1fA gravity survey where both observeda value of 7.096 mGal was subtracted SARIG. The gravity data as we receivegravity and elevations are noticeablyfrom all the observed gravity values. This it is digitally paperclipped to the PDFdi\x1ferent to surrounding values, di\x1ference was the di\x1ference between containing the history of a tenement. A discrete portion of a survey isthe base station value in the data, and noticeably di\x1ferent from the rest ofthe value of a gravity station at the same So, what happens if (say) during a survey athesurvey, spatial coordinates on SARIG. Weve no gravity operator or processer writes downidea why they are di\x1ferentmaybe it a base station value incorrectly and thisA big red or blue dot on a gravitywas a typo againbut as the survey carries over to the gravity reduction andmap,and stood out by approximately 7 mGal this the Bouguer Gravity looks \x1ene? It meansGravity surveys conducted alongwas another simple evidence-based that when explorer A downloads thearoad/s. adjustment. Again, the relevelled survey survey from SARIG, they are downloadingnow blends in with the rest of the state.data that has some issue with it. And fromOften the \x1erst time an issue is noticed is a strict Government auditors perspectivewhen looking at the Bouguer Gravity of aRelevelling surveys is therefore easy thats okay. The data is evidence thatgroup of surveys, such as a state grid. Theenough, but we need to consider how we money was spent on a tenement. We dontBouguer Gravity is calculated using themanage the data on our database and modify someone elses data. In a veryobserved gravity and the elevation (and thewhat our customers receive when they bureaucratic and government-paperworklatitude) of the measurement point. Theserequest data.sense: all is as it should be. are the quantities that Ill analyse in the \x1erstWithout going into a discussion on the instance to quantify any adjustment. pros and cons of handling the data in But theres a problem. Wed much rather distribute data that will help explorersIf the issue is with the observed gravity,di\x1ferent ways, here is the resolution on make the correct exploration decisions.it becomes necessary to also revisitwhich we have landed. Raw gravity data When it comes down to it, our job is toother factors such as the time of the\x1elesthat is gravity data as we receive boost the economy of the State, andmeasurement and the base station usedfrom companies in the \x1erst place were more likely to succeed in doing thatto level the survey. Ive not yet foundare attached to open \x1ele envelopes if the data that people are using to makeissues with measurement times, but Ive(documents containing the entire history economic decisions are correct. identi\x1eed two instances of base stationof a mineral tenement) and are therefore issues in historical surveys.available to stakeholders unedited and The problem becomes apparent whenuntouched. Our database contains both the geophysicists at the GSSA undertakeOne survey we noticed had observedthe unlevelled and the relevelled data. another one of their duties: creating agravity values approximately 10 mGalThere is an entry for the unlevelled data seamless image of the Bouguer gravityhigher than surrounding points. The baseand a separate entry for the relevelled of South Australia. We can load down allstation used is recorded in the report anddata. If the unlevelled data is public the gravity data from our database andhas the value of A mGal. Searching fordomain, we can set it without an Export work our way through Katona (2017)sthe same base station in the databaseQuality tick, meaning the survey doesnt Supervised Variable Density Griddingresulted in another value, B mGal. The\x1end its way to the spatial layers on SARIG. methodology. Were working throughdi\x1ference A minus B is 10 mGal. AfterThe relevelled data has the Export Quality this process again now, and are startingsubtracting 10 mGal from the observedticked on, meaning the data becomes a to revisit some of the older surveys togravity and re-gridding the survey withvisible part of the point dataset on SARIG. produce a more correct image.its neighbours, we observed that the survey blends in seamlessly.In essence, the data is available twice. We create a statewide image and we seeThe original company data attached to that some areas of gravity stand out likeOccams razor states that the simplestthe envelope, and a relevelled dataset plateaus, and the sides of the plateaussolution is probably the correct one, soincorporated into the statewide point coincide with the boundary of the survey.I would suggest that a typo was made,dataset. Does this make it confusing for Coincidence? Generally, no. But we needand \x1exing the typo \x1exed the survey.people using the data? Possibly. If anyone to be careful. For example, does a newerThis illustrates the value of a surveyhas questions about the data that they are survey need relevelling or do the olderreports containing the base stationlooking at remember, if it is paperclipped to surveys around it need attention? information.However, were looking ata pdf then we de\x1enitely havent modi\x1eed it.APRIL 2024PREVIEW 21'