GEOP 446L ? Exploration Geophysics II ? Spring 2003

Lab 2: Gravity Survey - Data processing and report

Latitude correction

The latitude correction is usually made by using the International Gravity Formula (Lab

1: Introduction to Gravity) but for surveys which cover less than one degree and which are

not tied into an absolute gravity network, a simpler correction can be made. A local base

station is selected at which to determine the horizontal gravity gradient gL

gL = 0.8108 sin2 mGal per km Northwhere is the latitude of the local base station in degrees. Note that this value is

subtracted from the measured value for stations north of the base station and added for

stations south of the base station.

Free-air correction

The free-air correction is to account for variations in the distance to the center of the

Earth (or height above the geoid). The correction has a theoretical value of gFA = 0.3086

mGal/m for meters above sea level (or above a reference station).

Bouguer correction

The Bouguer correction accounts for the rock mass between the observation point and

sea level (or a reference station). The correction is determined by

gB = 2piGh = 0.04193h mGal

where is the density of the rock (in g/cm3) and h is the elevation in meters. To determine

the density to be used for this survey, use the cross section (Figure 8) and the table of

densities handed out with the lab.

Bouguer anomaly

The Bouguer anomaly is what is left when all of the corrections have been applied (gObs

is the value measured with the gravimeter).

gBA = gObs + gL + gFA gB

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Data processing

convert from dial divisions to mGal (0.0839 mGal/div)

determine the instrumental drift and apply the corrections to the observed data (usethe information from Lab 1: Introduction to Gravity)

calculate and apply the corrections to the gravity values measured during the survey

plot the corrected data (which is the Bouguer anomaly) as a function of distance (thisdoes not have to be in meters, longitude is fine)

compare your resuts with the cross section handed out with the lab (Figure 8)

? Make sure the units in all of your caclulations are correct and remember to label all graphs

and plots with the correct units

Lab Report

The written lab report should include:

purpose of the survey

description of the survey location, survey procedure (type of equipment, location de-termination, number of stations, measurements at each station), other relevant infor-

mation

data processing - type corrections applied, etc.

results and analysis - description of the plots, significant features in the data

conclusions - what were the results?

Questions to keep in mind:

Is there a significant anomaly associated with the fault?

How does your plot compare with the gravity anomaly determined from a previoussurvey (Figure 8, Lab 2 handout)?

If there is no observable anomaly, what are the possible explanations for this?

What could have been done to improve the survey?

Lab reports due: Monday February 24th (in class)

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