When conducting gravity surveying effects caused by predictable features that are not part of the "geophysical target." should be removed while the remaining anomaly is then interpreted in terms of sub-surface variations in density. Each known effect is removed from observed data. This is commonly known as Gravity Data Reduction, which I want you to understand its basic concepts. First the various "corrections" are described, and then the presentation options are listed. Keep enjoying your reading!
Methods of gravity data reduction
i/ Tidal and Drift corrections
When Drift and tidal correction is made, it is given as
∆g = gs - gb
Where drift corrected gravity anomaly (∆g) is given as the difference in gravity measured at survey point and at base station at the same time.
Procedures
- Establish the location of one or more gravity base stations
- Establish the locations of gravity stations.
- Reading relative gravity at the base station and time, then proceed to other stations
- At each station records relative gravity and the time taken to take reading.
- After some periods, often 1 hr. Remeasure the relative gravity at the base station.
- Repeat procedures again and again
The general procedures is called LOOPING PROCEDURE. See the figure below
The loop of a survey may be bound by one or multiple gravity base stations. i.e large aerial extent requires the use of multiple base stations.
ii/ Latitude Correction
Geodetic Reference System (GRS) 1967, Formula.
gn=9.780318(1+0.0053024sin2Q- 0.0000059sin22Q) m/s2
For smaller scale
CQ = 0.811Sin2Q mGal/Km N- S
Where Q - Latitude.
gn - Normal gravity /Theoretical gravity.
N.B : At the poles add but equator minus
∆g = gobserved - gn
∆g = gravity Anomaly
iii/ Eötvös correction
This is the correction necessary if the instrument is on a moving platform, such as a ship or aircraft. It accounts for centrifugal acceleration due to motion on the rotating earth. The relation is
Equation: ∆g = 4.040vsinAcosB + 0.001211v2 mGal
Where V is speed in knots, A is heading (direction of travel), and B is latitude. At mid-latitudes, it is about 7.5 mGal for 1 knot of E-W motion.
iv/ Free - Air correction.
FAC = 0.3086h mGal
Where h - elevation in (m)
For above sea level , + ve
For below sea level , -ve.
For 0.1g.u accuracy stations elevation is needed to 4 cm
- Free Air Anomaly (FAA)
gfa = go - gn + 0.3086h
v/ Bouguer Correction
BC = 0.04191h x d mGal
where h in (m) and d is density in g/cc
For crustal value , d - 2.67g/cc
Bouguer correction is always subtracted.
- BOUGUER ANOMALY
Partial anomaly equation for Bouguer anomaly = go - gn + FAC - BC
or BA = FAA - BC
Complete anomaly equation for Bouguer Anomaly = FAA - BC + Terrain correction
vi/ Terrain correction
It is used if the bouguer is inadequate. Its correction is always added (+ve)
Terrain Anomaly = go - gn + FC- BC + TC
where Tc - Value of computed terrain correction
Terminologies
- Gravity Anomaly: Is the discrepancy between the corrected measured gravity and the Theoretical (normal) gravity.
- Normal (Theoretical) Gravity: Is the value of gravity computed without any geological complication.
It is computed by the Normal gravity formula of the Geodetic Reference System.
- Observed Gravity: Gravity readings observed at each gravity station after correction have been made for Drift and Tides.
Factors affecting gravity anomaly
- Dimension (Size): The shape of the body is not a big deal that contributes to changes in gravity value rather the size of that body is what matters. However when considering the size we have to relate it with the depth from the surface. Such that, we can expect the small body near the surface, may produce approximately the smaller gravity attraction than the Large body located at some depth below the surface. Since shape can be modeled by different related figures, for example a buried tunnel can be modeled by a horizontal circular cylinder, a horizontal cylinder can be modeled by horizontal lines, a spherical body can be modeled by a disk and such others,
- Density Contrast: The great the density contrast, results in an increase of gravity anomaly value.
- Depth of anomalous body: There is evidence that as the depth of burial of an anomalous body increases, the gravity anomaly value decreases. Hence these two factors vary inversely to each other.
Causes of gravity anomaly
- Inhomogeneous (Heterogenous) density of Earth's interior.
Common types of Anomaly
There are two (2) types of gravity anomaly
i/ Bouguer anomaly
BA = go + (FAC - BC + Tide C) - gn
ii/ Free Air Anomaly
It defined by free air, terrain, and tidal corrections
FAA = go + (FAC + TC +Tide C) - gn
The flow diagram below shows how these corrections are stepped wise.
Data acquisition ---> Data reduction ---> Sensor velocity correction ---> Tidal correction ---> Drift correction ---> Latitude correction ---> Free Air Correction ---> Bouguer and Terrain Correction
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