8/12/2019 In Situ Density Assignment

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http://serc.carleton.edu/NAGTWorkshops/geophysics/activities/18922.html

cott ". White

#ept. Geological ciences

$niversity o% outh &arolina

&olum'ia( & 292)8

s*hite+geol.sc.edu

,n itu density determination using Nettleton-s method

Knowing the bulk density is obviously important to interpreting gravity data, but

representative bulk density data is notoriously difficult to obtain. Samples collected in the

field tend to have a bias toward what can be collected from the surface, and thus may be more

weathered, less fluid-saturated, or otherwise unrepresentative of the bulk. Nettletons method

uses a profile measured across a topographic feature to find the best estimate of bulk density

for a region. This method is named after . . Nettleton, one of the pioneers of applied

gravimetry, who reali!ed that no gravity anomalies should remain after the "ouguer

correction in an area of constant density. The presence of topography is necessary to in order

create enough variability so that value which minimi!es the correlation of the "ouguer

anomaly with topography can be found, but the topographic slopes should be gentle enough

to minimi!e precision errors in the terrain correction. #eduction of the data is carried out for

several different assumed density values across the entire profile. The "ouguer anomaly is

plotted across the profile to see how flat the resulting curve is. The density that gives the

flattest profile, hence least correlation between elevation and "ouguer anomaly, is considered

to be the closest value to the true bulk density of the rock.

ackground/Theory

$n gravity surveys the gravitational acceleration at each survey point is found by measuring

the difference in acceleration between the survey point and a known reference point. $n our

case the reference point is the local gravity base station %in the lobby of the &oker ife

Sciences "ldg.' (rom the known value at base station and the difference we calculate gobs.

The "ouguer anomaly at each survey point is obtained from the following e)uation*

g"ou+ gobs- g ah - 2h T f %'

g+ /0123.14% 2.2245011/4 sin5- 2.222253675 sin6' mal

+ 7.7032-N m5kg-5+ 7.7032-7N m5kg-5%mal8%m s-5''

* atitude of survey point

h* elevation of survey point %height above sea level'

* rock density

T* terrain correction %linearly dependent on 9'

f* correction due to drift

This e)uation may be reduced and rewritten as*

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8/12/2019 In Situ Density Assignment

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g"ou+ gobs- ah - %2 - T:' f %5'

where a + 2.3217 mal8m, and T: is the geometrical part of the terrain correction*

2

:

TT =

%3'

;ere 2is the density used in the terrain correction tables. T: varies from one survey point to

the ne