The document discusses the gravity method as a geophysical exploration technique, detailing its principles, applications in various fields like petroleum and mineral prospecting, and methodologies including gravity surveys and data correction. It explores concepts like gravitational anomalies, the significance of rock densities, and advancements in measurement technology, while also addressing limitations and case studies related to the method. Key findings include the importance of density contrasts for feature detection and the passive, non-invasive nature of gravity measurements, making it suitable for diverse settings.

1. GRAVITY METHOD
Balignot, O.R., Bautista, B.R.,Torres, P.

2. REPORT OUTLINE
• INTRODUCTION
• USES
• THEORIES/CONCEPTS
• MATERIALS
• METHODOLOGY
• CASE STUDIES
• ADVANCEMENTS
• LIMITATIONS

3. INTRODUCTION
WHAT IS GRAVITY?

4. Gravity
• The natural force that tends to cause physical
things to move towards each other: the force
that causes things to fall. (Merriam-Webster
Dictionary)
• Differences in rock density produce small
changes in the Earth’s gravity field that can be
measured using portable instruments known
as gravity meters or gravimeters. (Field
Geophysics by John Milsom)

5. USES
• Petroleum and Mineral prospecting
• Seismology
• Geodesy
• Metrology

6. Theories/Concepts
The gravity method is based on two laws derived by Sir Isaac
Newton, which he described in Philosophiæ Naturalis
Principia Mathematica (July 1967):
• Universal Law of Gravitation
• Second Law of Motion
Units
standard gravity ɡ0 or ɡn = 9.80665 m/s² 9.81 m/s²
gravity unit, gu = micrometer per second, μms-2
c.g.s. unit of gravity, milligal =1 mgal = 10-3 gal= 10-3 cms-2
= 10 gu-3

7. Theories/Concepts
Geoid is the equipotential surface that would coincide with
the mean ocean surface of the Earth if the oceans and
atmosphere were in equilibrium, at rest relative to the
rotating Earth and extended through the continents.
Reference ellipsoid is the idealized geometrical
representation of the Earth
Gravity anomaly is the difference between gravity measured
at a point and a model value at that point that is based on
the normal gravity of a reference ellipsoid.
+ gravity anomaly = geoid surface is higher than
reference ellipsoid
- gravity anomaly = geoid surface is lower than
reference ellipsoid

8. 1. Ocean
2. Reference ellipsoid 5. Geoid
4. Continent
3. Local plumb line

9. Theories/Concepts
Densities of Sedimentary Rocks
• Sedimentary rocks exhibit the greatest range of density variation
due to factors such as:
Mineral composition, Cementation, Porosity, Pore fluid type
• Typically the contrast between adjacent sedimentary layers is
less than 0.25 Mg m-3.
• Density is increased by depth of burial:
Sandstones and Limestones: density is increased by infilling
of the pore space, not by volume change.
Shales: density increased by compaction, and ultimately
recrystallization into minerals with higher densities.

10. Densities of Igneous Rocks
• Igneous rocks tend to be denser than sedimentary rocks,
with the density controlled primarily by silica content:
Mafic rocks are thus more dense than felsic.
Ultramafic rocks are most dense.
• The range of density variation tends to be less than in
sediments as porosities are typically lower.
Densities of Metamorphic Rocks
• The densities of metamorphic rocks tends to increase with
decreasing acidity and with increasing grade of metamorphism.
• However, variations in density within metamorphic rocks are far
more erratic and can vary considerably over short distances.

12. Concepts
Gravity Survey - Measurements of the gravitational field at a
series of different locations over an area of interest. The
objective in exploration work is to associate variations
with differences in the distribution of densities and hence
rock types.
The primary goal of studying detailed gravity data is to
provide a better understanding of the subsurface geology.
The gravity method is a relatively cheap, non-invasive, non-
destructive method
It is also passive – that is, no energy need be put into the
ground in order to acquire data; thus, the method is well
suited to a populated setting such as urban areas and a
remote setting such as Mars and the Moon.

13. Materials
Gravity-measuring equipment
Falling bodies - directly computing the acceleration of a body
undergoing free-fall drop by carefully measuring distance and
time as the body falls; absolute
Pendulum - the gravitational acceleration is estimated by
measuring the period oscillation of a pendulum; absolute,
relative
Gravimeters - are basically spring balances carrying a constant
mass. Variations in the weight of the mass caused by variations
in gravity cause the length of the spring to vary and give a
measure of the change in gravity; absolute, relative
Positioning equipment – theodolites, GPS receivers

14. Materials
- a gravimeter that expresses measured g as the difference in
g between two sites
•Survey: Measurement of relative gravity as a function
of position on, or under, the surface of the earth or on
the sea floor.
•Stationary: Continuous measurement of changes in
gravity or low-frequency earth motion as a function
of time at a fixed location on, or under, the surface of
the earth or on the sea floor
•Dynamic: Measurement of relative gravity as
a function of position from a moving platform such
as an aircraft or a ship.
Relative gravimeters

15. Materials
Sensor Technology
1. Fused quartz - has high strength, remains almost perfectly
elastic up to its breaking stress, can be welded into compact
structures, free of tares when transported, accurate and quick
and easy to operate
2. Metal - very low long-term drift and low temperature
coefficient, high accuracy, more prone to tares in response to
transportation
3. Superconducting – uses magnetic levitation, extremely low drift
4. Inertial-grade accelerometer – excellent dynamic performance
on moving platforms, very compact
Relative gravimeters

16. LaCoste ‘G’; steel
Worden ‘Student’; quartz
Sodin; quartz
General internal mechanism
LaCoste ‘G’
Scintrex CG-5; quartz

17. Materials
- an instrument used to measure gravity absolutely,
traceable to time and length standards
Absolute gravimeters
LaCoste FG5

18. Methodology
1. identification of target
2. choosing survey parameters for the target
- base stations
- general orientation
- station spacing-density
3. selecting placement of stations
4. station setup (elevation and position measurements and
instrument calibration)
5. instrument reading
6. data processing (gravity reduction/correction,
interpretation)

19. Methodology
1. Measure a base station
2. Measure more stations
3. Remeasure the base station approximately every two
hours
4. Record data
Measure: base 1 new base station base 1 new
base station base 1
Station Measuring

20. Methodology
Calibration of gravimeters
Calibration is usually done by the manufacturer. Two
methods are used:
1. Take a reading at two stations of known g and determine
the difference in g per scale division, or
2. Use a tilt table.
Station Setup

21. General rules of gravity interpretation are:
• Higher than average density bodies will cause a positive
gravity anomaly with the amplitude being in proportion to the
density excess.
• Lower than average density bodies will cause a negative
gravity anomaly.
• The aerial extent of the anomaly will reflect the dimensions
of the body causing it
• A sharp high frequency anomaly will generally indicate a
shallow body
• A broad low frequency anomaly will generally indicate a deep
body
• The edges of a body will tend to lie under inflection points on
the gravity profile
• The depth of a body can be estimated by half the width of
the straight slope (between the points of maximum curvature)
of the anomaly in its profile.

22. Methodology
1. Latitude - correction for N-S distance
2. Free-Air - correction for elevation above the data plane
3. Bouguer - correction for excess mass above the data
plane
4. Terrain - correction for variations in topography
5. Tides - attraction of Sun and Moon
6. Eötvös - correction for moving vehicle
7. Isostacy - variations in crustal thickness
Data Correction/Reduction

23. Methodology
LATITUDE CORRECTIONS
•usually made by subtracting the normal gravity, calculated
from the International Gravity Formula, from the observed
or absolute gravity.
•Gravity increases towards poles, so latitude correction
is more negative towards poles (as subtracted).
FREE-AIR CORRECTION
•Corrects for reduction in gravity with height above geoid,
irrespective of nature of rock below.
Data Correction/Reduction

24. Methodology
BOUGUER CORRECTION
•dgB, accounts for effect of rock mass by calculating extra
gravitational pull exerted by rock slab.
•Assumes flat topography. In rough areas terrain
corrections required.
TERRAIN CORRECTIONS
•Bouguer correction assumes subdued topography.
Additional terrain corrections must be applied where
measurements near to mountains or valleys.
•If station next to mountain, there is an upward force on
gravimeter from mountain that reduces reading.
Data Correction/Reduction

25. Methodology
•If station is next to valley, there is an absence of the
downward force on gravimeter assumed in Bouguer
correction, which reduces free-air anomaly too much.
TIDAL CORRECTIONS
•Pull of Sun and Moon large enough to affect gravity
reading. Changes gobs with period of 12 hours or so.
•Earth tide corrections can be corrected by repeated
readings at same station.
Data Correction/Reduction

26. Methodology
EÖTVÖS CORRECTION
•If gravimeter is in moving vehicle such as ship or plane, it is
affected by vertical component of Coriolis acceleration,
which depends on speed and travel direction of vehicle.
Two components:
•Outward acting centrifugal acceleration due to movement
of vehicle over curved surface of Earth.
•Change in centrifugal acceleration due to movement
relative to Earth’s rotational axis. If vehicle moves east, it’s
rotational speed is increased; if west, its speed is reduced.
Data Correction/Reduction

27. Methodology
ISOSTATIC CORRECTION
•If no lateral density variations in Earth’s crust, Bouguer
Anomaly would be the same, i.e. Earth’s gravity at the
equator at geoid.
Data Correction/Reduction

28. Methodology
Airy Isostasy
•Airy proposed that crust is thicker beneath mountains and
thinner beneath the oceans.
•topographic highs are supported by deep crustal roots,
while topographic lows are found above thinned crust
Pratt Isostasy
•Pratt proposed that observation could be explained by
lateral changes in density within a uniform thickness crust.
•mountains occur where there's less-dense crust, and basins
where there's more-dense crust
Data Correction/Reduction

29. Methodology
FREE-AIR ANOMALY,
- is the measured gravity anomaly after a free-air correction
is applied to correct for the elevation at which a
measurement is made
- the free-air correction does so by adjusting these
measurements of gravity to what would have been
measured at a reference level
Data Correction/Reduction

30. Methodology
BOUGUER ANOMALY
- the main end-product of gravity data reduction which
correlates with density variation of the upper crust.
- Is is the difference between the observed gravity value,
adjusted by the algebraic sum of all necessary corrections
and of the gravity at the base station.
Data Correction/Reduction

32. Case Studies
• A gravity survey was conducted in Guinsaugon, St. Bernard,
Southern Leyte, Philippines in April 2006, to determine the
subsurface structure of the Leyte segment of the Philippine
Fault Zone (PFZ), where a massive landslide killed 1119 villagers
on 17 February 2006.
• In May 1981, precise gravity measurements on surveyed
benchmarks were conducted in the Tongonan area to establish
baseline gravity data for future repeat measurements that will
assist calculations o f mass-withdrawal , fluid redistribution,
reservoir performance, and recharge. This data together with
additional measurements made outside the geothermal field ,
have been compiled t o construct a Bouguer anomaly map o f
the Tongonan area.

33. Case Studies
• Microgravity surveying conducted at Pu’u O’o, which is a flank
vent of Kilauea, Hawaii. Microgravity surveying involves making
repeated, super-accurate gravity surveys together with geodetic
surveys for elevation, in order to seek mismatches between
changes in elevation and changes in gravity. The mismatches
can be interpreted as changes in the mass distribution beneath
the surface. This method has been applied to various active
volcanoes in an effort to detect the movement of magma and
gas in and out of chambers, thereby contributing to volcanic
hazard reduction.
• A gravity survey was conducted on Taylor Glacier, Antarctica to
determine ice and subglacial sediment layer thickness.

34. Case Studies
• NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission
is comprised of two spacecraft, named Ebb and Flow, flying in
precision formation around the Moon. The mission’s purpose is
to recover the lunar gravitational field in order to investigate the
interior structure of the Moon from the crust to the core. The
spacecraft were launched together on September 10, 2011 and
began science operations and data acquisition on March 1,
2012. The Lunar Gravity Ranging System (LGRS) flying on NASA’s
Gravity Recovery and Interior Laboratory (GRAIL) mission
measures fluctuations in the separation between the two GRAIL
orbiters

35. Advancements
•Airborne gravity gradiometer
- Traditional gravimeters measure the force exerted on
them from one direction only, usually straight down. If a
survey does not fly directly over an anomaly but slightly to
one side, the odds it will detect that anomaly decrease
sharply. Gravity gradiometers, on the other hand, measure
forces from the sides as well, greatly improving the ability
to detect objects.

36. Advancements
•Exploration Gravity Gradiometer
- utilizes the concept of superconductivity and operates at
4 degrees above absolute zero (-269°C), which allows
greater sensitivity and stability
• Integrated quantum sensors
-uses ultracold atoms
-can be used in making very compact, highly
sensitive gravimeter

37. Limitations
•A sufficient density contrast between the background
conditions and the feature being mapped must exist for the
feature to be detected.
• Some significant geologic or hydrogeologic boundaries may
have no field-measurable density contrast across them, and
consequently cannot be detected with this technique.
•Ambiguity of the interpretation of the anomalies almost
always present. Accurate determination usually requires
outside geophysical or geological information
• Each station has to be precisely surveyed for elevation and
latitude control. This could be costly and time consuming,
especially in surveys covering large areas

38. References
Brooks, M., Hill, I., & Kearey, P. (2002). An Introduction to Geophysical
Exploration 3rd Edition. London: Blackwell Science Ltd
Foulger, G. R., & Peirce, C. (n/a). Geophysical Methods in Geology.
Gupta, H. K. (Ed.). (2011). Encyclopedia of Solid Earth Geophysics. The
Netherlands: Springer.
Lowrie, W. (2007). Fundamentals of Geophysics 2nd Edition. United
Kingdom: University Press, Cambridge.
Milsom, J. (2003). Field Geophysics 3rd Edition. West Sussex, England:
John Wiley & Sons Ltd.
Murray, A. S., & Tracey, R. M. (2001). Best Practices in Gravity
Surveying.
Reynolds, J. M. (1997). An Introduction to Applied and Environmental
Geophysics. West Sussex, England: John Wiley & Sons Ltd.

39. Image Sources
http://earthobservatory.nasa.gov/Features/GRACE/page3.php.
Gravimetry map from the Gravity Recovery and Climate Experiment—
GRACE, a joint mission of NASA and the German Aerospace Center.
http://en.wikipedia.org/wiki/File:Geoid_height_red_blue.png.
Deviation of the Geoid from the idealized figure of the Earth.