What is Gravity Anomaly?
A gravity anomaly can be defined as the differences between the corrected measured gravity and the theoretical (normal) gravity.
In most cases gravity anomaly reading is given in milliGal (mGal).
As the definition above describes, there are two terms you must understand, normal gravity and the observed gravity. We have to know these terms as described below in order to get basic insight on this topic
- 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: Is the gravity readings observed at each gravity station after correction have been made for Drift and Tides.
factors affecting gravity anomaly
- dimensions: When we are talking about the dimension of a body we simply mean the size. As the size of the body increases, the probability of producing great gravity attraction also increases, and vice versa is true provided that other factors such as depth of burial, and density contrast are kept constant. We are not talking about shape here, because the 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. Look the figure 1, below to get the basic insight, that the body A, causes high gravity anomaly compared to body B
Figure 1: Showing how dimensions affect gravity anomaly
- density contrast: Since the density contrast is the difference between the density of the body of interest and the host material that surrounds it. If the density contrast is great then the gravity anomaly is also increasing and vice versa is true.
- depth of anomalous body: There is a proof 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. However when we describe the effect of depth of an anomalous body we have to keep in mind that other factors such as dimension should be constant.
Causes of gravity anomaly
- Heterogenous (Non-uniform) density of Earth's interior. This is because the Earth is made up of different types of rocks and other materials that show significant variation in their mass and shapes hence density will not be uniform. The difference between densities of what we expect to look and their environment gives the rise of gravity anomaly.
common types of anomaly
We can classify gravity anomaly based on the following criteria,
Gravity anomaly related to Spatial variation
There are two (2) common types of gravity anomaly of this type
i/ bouguer anomaly
BA = go + (FAC - BC + Tide C) - gn
Where, BA - Bouguer Anomaly, go - Observed gravity, gn - Normal gravity, FAC - Free Air correction, BC - Bouguer correction, Tide C - Tide correction.
ii/ free air anomaly
It is defined by free air, terrain, and tidal corrections
FAA = go + (FAC + TC +Tide C) - gn
Where, FAA - Free Air Anomaly, go - observed gravity, FAC - Free Air correction, TC - Terrain correction, Tide C - Tide correction, gn - Normal gravity
Gravity anomaly related to scale
It related to the scale at which the effect of density contrast can be experienced
- residual anomaly: Are those gravity anomalies associated with short wavelength.
They are mostly used in geological knowledges
- Local Anomalies : Are the ones that relate directly to the structure or feature of interest.
Any wavelength that is not related by feature of interest is considered as Noise.
- regional anomaly: They have long wavelengths due to deep density contrast.
They are used in Large scale studies in Earth's crust such as Mountain Ranges, Oceanic Ridges and Subduction zones
N.B Wavelength : Is the measure of depth of anomalous mass.
Gravity anomaly is said to be a negative gravity anomaly if the density of the body of interest is lower than the density of its surrounding materials (host rocks). For example a low-density salt dome (2150 kgm-3) intruding higher-density carbonate rocks (2500kgm-3) has a density contrast (-350 kgm-3).
Also a gravity anomaly is said to be a positive gravity anomaly, when the density of the body of interest is higher than the density of its surrounding rocks. For example a volcanic plug with density 2800kgm-3 intruding a granite body having a density of 2600kgm-3 has a density contrast (+200kgm-3).
INTERPRETATION OF GRAVITY ANOMALY
How do gravity anomalies be interpreted?
Gravity anomalies can be interpreted either using visual (pattern) analysis or by inverse modeling.
To start with Pattern analysis, which involves the visual inspection of the gravity anomaly trends along the profile. However this should be done using a residual anomaly at which the separation of the regional trend from Bouguer anomaly has been done. The lateral changes in gravity anomaly amplitude may indicate structural discontinuity. However there must be a change in density (density contrast) for this technique to work, otherwise the body of interest (target) will not be identified.
Figure: Gravity anomaly pattern along profile indicates subsurface density distribution.
Figure: Lateral changes in gravity anomaly indicate a Fault.
Another interpretation technique for gravity anomalies is using Inverse Modeling. This can be used to obtain the possible subsurface interpretation of an observed gravity anomaly which has a good fit to the standard geometry shapes. The primary objective of gravity modeling is to approximate the density, depth and geometry of one or more subsurface bodies.
And the fact that before the advancement of computers, solutions to simple geometries such as spheres, cylinders, prisms, thin sheets were used to approximate subsurface mass distributions using residual gravity anomalies (Telford et al., 1990).
If the calculated values do not match the observed anomalies, the model is changed and performed again and again until the match between the calculated values and the observed anomalies shows an accepted pre-defined statistics.
Inverse modeling can be done by using 2-D , 2.5-D and 3-D irregularly shaped bodies, however the solution resulted by using an inverse modeling as gravity interpretation tool is not unique as there is an infinity number of the subsurface density distribution.
Another factor regarding the non - uniqueness interpretation of gravity anomaly using inverse modeling is that the measured anomaly relies on the density contrast (difference) between body of interest and its surrounding materials (host rocks).
Example a sphere with the density of 3.1 g/cm3 surrounded by the rocks with a density 3.3 g/cm3 will produce exactly the same gravity anomaly as a sphere with the density of 2.5 g/cm3 surrounded by the rocks with a density 2.7 g/cm3.
The two (2) figures below show two different geology models with the same gravity anomaly, one with limestone having 2.55Mg/m3 while another using granite (2.6 Mg/m3).
So this technique needs data from other methods such as geology, boreholes, remote sensing to act as a base line (reference point) so as to narrow an infinity density distribution into a finite possible number of density distributions.
Variation of gravity amplitude, such as always amplitude with short (narrow) wavelength signifies a shallower geology structure, while deep seated structures produce a long (broad) wavelength gravity anomaly. See the figure below.
Figure: A sphere having different depths with their gravity anomalies (Lowrie, 2007)
PRESENTATION OF GRAVITY ANOMALY
How do gravity anomalies be presented?
Like any other geophysical data, gravity anomalies can also be presented either in 2-D as profiles or in 3-D as maps. Regarding 2-D gravity anomaly profiles are presented in the x - y plane with the y - axis used to represent the gravity anomaly amplitude while the x - axis stands for space (horizontal distance). Example 2-D gravity anomaly profiles are bouguer anomaly profile and residual anomaly profile.
Figure: Residual anomaly profile.
In 3-D gravity mapping, the gravity anomalies are presented into grid-like map patterns such as contour maps. The bouguer gravity map is always presented as a contour map with the gravity values used to create contour lines.
Another presentation of gravity anomaly as a map is by using a 2 5-D which usually involves extrapolating between a number of parallel profiles while joining all points of equal values with isolines which is equivalent to contours map.
Figure: Bouguer contour map using 50g.u as contour interval.
To understand the concept behind gravity anomaly is important when dealing with gravity method. And this article provide basic theory regarding gravity anomaly ranging from its meaning, causes, factors affecting it, types, interpretation and presentation.
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