GRAVITY METHODBalignot, O.R., Bautista, B.R.,Torres, P.

REPORT OUTLINE

• INTRODUCTION• USES• THEORIES/CONCEPTS• MATERIALS• METHODOLOGY• CASE STUDIES• ADVANCEMENTS• LIMITATIONS

INTRODUCTION

WHAT IS GRAVITY?

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)

USES

• Petroleum and Mineral prospecting• Seismology• Geodesy• Metrology

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

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

1. Ocean

2. Reference ellipsoid 5. Geoid

4. Continent

3. Local plumb line

Theories/ConceptsDensities 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.

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.

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.

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

Materials

- a gravimeter that expresses measured g as the difference in g between two sites

• Survey: Measurement of relative gravity as a functionof position on, or under, the surface of the earth or onthe sea floor.

• Stationary: Continuous measurement of changes ingravity or low-frequency earth motion as a functionof time at a fixed location on, or under, the surface ofthe earth or on the sea floor

• Dynamic: Measurement of relative gravity asa function of position from a moving platform suchas an aircraft or a ship.

Relative gravimeters

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

LaCoste ‘G’; steel

Worden ‘Student’; quartz

Sodin; quartzGeneral internal mechanism

LaCoste ‘G’

Scintrex CG-5; quartz

Materials

- an instrument used to measure gravity absolutely, traceable to time and length standards

Absolute gravimeters

LaCoste FG5

Methodology

1. identification of target2. choosing survey parameters for the target

- base stations- general orientation- station spacing-density

3. selecting placement of stations4. station setup (elevation and position measurements and instrument calibration)5. instrument reading6. data processing (gravity reduction/correction, interpretation)

Methodology

1. Measure a base station2. Measure more stations3. Remeasure the base station approximately every two hours4. Record data

Measure: base 1 new base station base 1 new base station base 1

Station Measuring

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, or2. Use a tilt table.

Station Setup

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.

Methodology

1. Latitude - correction for N-S distance2. Free-Air - correction for elevation above the data plane3. Bouguer - correction for excess mass above the data

plane4. Terrain - correction for variations in topography5. Tides - attraction of Sun and Moon6. Eötvös - correction for moving vehicle7. Isostacy - variations in crustal thickness

Data Correction/Reduction

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

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

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

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

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

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 crustPratt 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

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

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

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.

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.

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

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.

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

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

ReferencesBrooks, M., Hill, I., & Kearey, P. (2002). An Introduction to Geophysical

Exploration 3rd Edition. London: Blackwell Science LtdFoulger, 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.

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.