b'Minerals geophysics Minerals geophysicsthe beginning and end of each readingwell under 2 g/cc, in outcropping mafic loop and scaling any differences acrossterrain it may be well over 3 g/cc.the readings taken during the loop. Correction for geographic position usesWhere there is no information on the a latitude dependent formula to correctsubsurface materials, an approximate gravity values for effects from the oblateBouguer density value may be deduced spheroidal shape of the Earth. Applyingby plotting Bouguer gravity profiles all these corrections generates observedcalculated for a series of densities gravity values. and comparing them with the corresponding topographic profile. If The elevation dependent free-airthese gravity data arent provided in the correction uses a formula to adjustsurvey results, you will have to calculate observed gravity values for variationsthem from the data you do have. The Terry Harveyin station heights. This formula canresulting Bouguer gravity profiles will Associate Editor forsimplistically be expressed as 0.3086range from topography sympathetic Mineralsgeophysics x station height and is added to each(density too low) to topography email@example.com gravity value. The station height is usuallyantipathetic (density too high). The measured from the Australian Heightgravity profile showing minimum Datum (AHD), but, because we arecorrelation with topography will Gravity corrections dealing with relative and not absoluteidentify the Bouguer density to use. Of gravity values, could be measured fromcourse, this approach does assume that For any gravity survey, a series ofany convenient datum.topographic variations are independent corrections is required to transformof any inherent gravity anomalism, raw field data into the finishedThe elevation and density dependentwhich may not necessarily be the case. product. In exploration-style (i.e.: semi- Bouguer correction uses a formulaSo, caution is needed, and you may have regional, detailed, or very detailed)to adjust free-air gravity values forto select an appropriate area within your vertical component gravity surveys,the presence of material beneathsurvey for this process.taking the infinite slab approacheach station. By assuming a slab of these corrections typically comprise:infinite lateral extent, this formula canThe topography dependent terrain conversion from instrument units tosimplistically be expressed as 0.04191 xcorrection is effectively a modification to milligals, correction for Earth tidal drift,density x station height, and is subtractedthe Bouguer correction to take account correction for instrument drift, correctionfrom each gravity value. The Bouguerof material missing from the slab below for geographic position (latitude),density is an estimate of the averagethe station elevation (valleys) and correction for gravity station height,density of the material constituting thematerial added to the slab above the correction for underlying rock densityslab; the station height is as defined forstation elevation (hills). Both effectively and, where appropriate, correction forthe free-air correction. reduce the magnitude of the Bouguer terrain topography. In most cases theCommon practice for large scale surveysslab effect, so the terrain correction is gravity survey results are also tied to theis to use 2.67 g/cc as the Bougueradded to gravity values to compensate. Australian National Gravity Databasedensity, this value historically beingHere too, the density value(s) used (ANGD) via a tie reading loop betweenconsidered the average density of crustalshould be appropriate to the materials the survey and a known ANGD station. rocks. Such a value may have merit forinvolved. In the past this correction was regional scale gravity surveys, but it iscalculated by subdividing the terrain In the main these are routine corrections,around each station into zones and and for commercial surveys are typicallyclearly inappropriate for limited extentmanually estimating the average terrain applied by the geophysical contractorexploration surveys, where variationselevation difference for each zone; prior to delivery of the final version data.in magnitudes of Bouguer correctionsdigital elevation models now permit this Details of all these corrections should beare controlled by the slab thicknessto be done semi-automatically. Terrain provided in the accompanying contractorsmeasured from each station height tocorrections are not always necessary in survey report. Ill elaborate a little onthe height of the lowest station in themineral exploration surveys, but may be implications for the elevation, rock densitysurvey. The portion of the Bouguer slabappropriate for larger surveys, areas of and terrain corrections, particularly thecorrection for material below the heightsteep topography and for very detailed latter two that require decisions based onof the lowest station will be the same forsurveys on mine sites, for example, where the geological environment. every station in the survey. Thus Bouguerthe ground surface has been disturbed corrections effectively only utilise theby earth works, excavations, etc.With respect to the routine corrections,density of the material above the level of conversion of readings from instrumentthe lowest station. The Bouguer densitySo, when reviewing the results from a units to milligals is achieved bycan be estimated from a knowledgegravity survey, be on the lookout for multiplication with an instrument- of the local near-surface geologypattern elements apparently mimicking specific factor. Correction for Earthof the survey area, or by measuringtopography. They may simply relate to tidal drift is made via existing tables.the density of appropriate samples.the use of an inappropriate Bouguer Correction for instrument drift is madeFor gravity surveys in sand dunes andensity value. Its a distraction you can do by taking local base station readings atappropriate Bouguer density may bewithout.OCTOBER 2020 PREVIEW 32'