# Gravity

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16-Jan-2016Category

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Gravity Exploration Method In gravity surveying subsurface geology is investigated on the basis of variations in Earths gravitational field arising from density contrast between the subsurface rocks

Basics The basis of gravity survey method is Newtons Law of Gravitation.where G = universal gravitational constant = 6.670 10-11 N-m2/kg M1, M2 = masses 1 and 2, respectively R = distance between center of masses F = force

This equation is also known as the inverse square law, since F varies with 1/R2

Gravity Units Gravity is the acceleration on a unit mass. Objects fall to Earth with an acceleration of about 980 cm/s2. The unit "centimeter per second square" (cm/s2) is known as a gal in honor of Galileo. In gravity exploration, the acceleration of gravity is the fundamental quantity measured, and the basic unit of acceleration is the milligal (mGal). Thus, the acceleration of a body near the Earth's surface is about 980,000 mGal.

Gravity and Geology The attraction of gravity is not uniform at every point on the earths surface. There are small variations from place to place because of irregularities in rock densities.

Exploration geophysicist hopes to distinguish different kind of rocks by detecting density variations from measurements of gravity.

ExampleBuried Salt dome penetrates layers of shale would produce small but measurable decrease in observed gravity value, because average salt density (2.0 g/cm3) is lesser than shale density (2.6 g/cm3)

Application of Gravity Survey in Oil ExplorationMost of gravity survey are carried out for reconnaissance of large, unexplored areas.

Gravity method initially was used for locating salt domes in Gulf Coast of USA. Gravity method can be used to detect some structure traps for hydrocarbon.

Gravity survey can help geophysicists to determine thickness of the sedimentary basinMost of the sedimentary basins have lower densities as compared to basement rocks. This density contrast can help us to know the depth of basement.

Gravity MeasurementsEarth gravitational attraction can be measured with portable instrument known as gravimeter. The instrument consists small object supported by very sensitive spring. The stretch of spring changes because of variation of gravity at different places.Land Gravity Survey???Marine Gravity Survey????

Gravity Survey DesignThe spacing of observation sites depends upon the size of structure/feature of the interest. To detect anomalies cause by relatively small structures such as salt dome or buried reefs, which are usually less than few kilometers in size, we require gravity meter readings at intervals closer at the interval of one or two kilometers. As a rule of thumb, minimum observations interval should be of half of the size of the structure/geological feature.

Processing of gravity dataThe attraction of the earth is about 980 gals (980,000 mGal). Gravity anomalies of exploration interest sometime of order of magnitude 1 mGal, which means we are interested in variations in earth gravity field of about one part in million.

Before the interpretation, we have to remove all other variations, which are not due to the difference of the densities of subsurface rocks. This process is known as gravity reductions.

Gravity ReductionsDrift Correction The change in the gravity field with the passage of time at one point is removed by drift corrections. This change in the gravity observation is because of changes in spring properties with temperature and passage of time.

Gravity ReductionsDrift Correction (continued.)Correction for instrumental drift is based on repeated readings at base stations after regular time through out the survey. The meter reading is plotted against time and drift is assumed to be linear between two consecutive base station readings.

ExcerciseFollowing gravity values were obtained from gravity survey. Measure readings were made using a warden gravimeter with a dial constant of 0.3801 mgal per dial division. Relative gravity values can be obtained by multiplying each dial reading by the instrument dial constant. Observed gravity at the base station is 979700 mgal. Calculate the drift correction

STATIONTIME (A.M)DIAL READINGB8.102896.31F1 8.262925.93F28.452907.89F39.002908.92B9.172897.03F49.402906.63F59.572921.65F6 10.202920.49B 10.352998.26F7 10.562911.94F8 11.152905.05F9 11.472905.60F10 12.102904.26B 12.452900.26

Gravity ReductionsTidal CorrectionDuring different timings of the day the position of the sun & moon change w.r.t position of earth, changing the gravitational effects exerted by sun & moon. These are called tidal effects. These effects can be removed by knowing the time of the day and phase of the moon. Tidal effects are removed along with the drift effect of the gravimeter.

Gravity ReductionsLatitude correction

Gravity value varies with latitude because of non-spherical shape of the earth. Earth's diameter is approximately 20 km smaller from pole to pole than through the equator, the force of gravity increases the closer we get to the poles.

Gravity ReductionsBecause angular velocity of a point on Earth increases zero at poles to maximum at equator. The centripetal forces causes the gravity to decrease at equator.

Gravity ReductionLatitude correction

The theoretical gravity is given in milligals by the International Gravity Formula : g( ) = 978 031.846 (1 + 0.005 278 895 sin2 +0.000 023 462 sin4 )????

based on the 1980 Geodetic Reference System, where is the latitude in degrees of any point on the Earth. The effect of latitude is removed by subtracting the theoretical value of gravity from the observed values

lmkr1102 - Re-write the equation for gravity latitude

Gravity ReductionElevation correctionElevation correction is applied in two steps i.e Free correction and Bouguer correction

Free Air CorrectionAccording to Newtons law gravity value decreases as the distance of observation point increases from center of the Earth. The gravity values are observed over the surface of the Earth having different elevations, in order to remove the effect of elevation, these observed gravity values need to be calculated at one datum.

Gravity Reductions Free Air Correction

To correct for variations in elevation, the vertical gradient of gravity (vertical rate of change of the force of gravity, 0.3086 mGalm-1) is multiplied by the elevation of the station and the result is added, producing the free-air anomaly. The free-air gravity anomaly is given by the formula: FA = go - gt + (g/z) h where: go = observed gravity (mGal) gt = theoretical gravity (mGal) g/z = vertical gradient of gravity (0.3086 mGalm-1) h = elevation above mean sea level (m).

Gravity ReductionsBouguer Correction

To isolate the effects of lateral variations in density on gravity, it is also necessary to correct for the gravitational attraction of the slab of material between the observation point and the mean sea level. This is the Bouguer gravity anomaly,

Gravity ReductionsBouguer Correction

BA = go - gt + (g/z - 2Gc) h where: go = observed gravity (mGal) gt = theoretical gravity (mGal) g/z = vertical gradient of gravity (0.3086 mGalm-1) G = gravitational constant (6.672 x 10-11 mkg-1s-2 or 6.672 x 10-6 mkg-1mGal c = density of crustal rock (kgm-3) h = elevation above mean sea level (m).

Gravity ReductionsTerrain CorrectionIn Bouguer correction it is assumed that topography around observation point is flat. This is the rare case further correction is made to account for the topographic relief in the vicinity of observation point. The gravitational pull of the surrounding terrain reduces the observed gravity.

Gravity ReductionsTerrain correction

Terrain correction can be calculated as T=0.4191 (density(r2-r1+

lmkr1102 - Insert correct equation

Gravity ReductionsTerrain Correction

Terrain corrections can be computed using transparent template, called Hammer Chart, which is placed over a topographic map.

Exercise

What is the expected value of gravity at the top of a 400 m hill located at latitude of 30 degrees?

Solution Solution: g( ) = 978 031.846 (1 + 0.005 278 895 sin2 +0.000 023 462 sin4 ) g(30o) = 979 324.012 mGal is the expected value for gravity at the base of the tower. Using the free-air gradient, ga = 400m(.3086 mGal/m); gravity is 123.440 mGal less at the top of the tower. The expected value of gravity at the top is 979 200.572 mGal.

Interpretation of Gravity Data After applying all corrections, we obtain gravity values, depends only upon the density variations due to subsurface lithological and structural changes. The final gravity value obtained is called Bouguer gravity. The patterns of Bouguer gravity variations are usually displayed on contour maps just like topographic map.

Interpretation of Gravity DataGravity anomaly over salt dome Average density of salt, is less than most sediments in a basin, so salt often rises in diapir due to its bouyancy. Makes good target for gravity surveys, and will show up as a bullseye (relatively low gravity anomaly over the salt)

Interpretation of Gravity DataGravity anomaly over anticline

When subsurface geology contains successive formations having significant density contrast, folding should be reflected in gravity map. If lithologi

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