Gravity Exploration Method

In gravity surveying subsurface geology is investigated on the basis

of variations in Earth’s gravitational field arising from density

contrast between the subsurface rocks

Basics

The basis of gravity survey method is Newton’s 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 earth’s 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.

Example

Buried 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 Exploration

Most 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 basin– Most 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 Measurements

Earth 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 Design

The 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 data

The 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 Reductions

• Drift 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 Reductions

• Drift 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.

Excercise• Following 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

• STATION TIME (A.M) DIAL READING• B 8.10 2896.31• F1 8.26 2925.93• F2 8.45 2907.89• F3 9.00 2908.92• B 9.17 2897.03• F4 9.40 2906.63• F5 9.57 2921.65• F6 10.20 2920.49• B 10.35 2998.26• F7 10.56 2911.94• F8 11.15 2905.05• F9 11.47 2905.60• F10 12.10 2904.26• B 12.45 2900.26

Gravity Reductions

• Tidal Correction

During 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 Reductions

Latitude 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 Reductions

• Because 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 Reduction

• Latitude correction

The theoretical gravity is given in milligals by the International Gravity Formula :

g( Ө ) = 978 031.846 (1 + 0.005 278 895 sin2Ө +0.000 023 462 sin4Ө )????

based on the 1980 Geodetic Reference System, where θ 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

Gravity Reduction

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

• Free Air CorrectionAccording to Newton’s 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 mGal·m-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 + (δg/δz) h where:

go = observed gravity (mGal)gt = theoretical gravity (mGal)δg/δz = vertical gradient of gravity (0.3086 mGal·m-1)h = elevation above mean sea level (m).

Gravity Reductions

• Bouguer 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 Reductions

• Bouguer Correction

BA = go - gt + (δg/δz - 2πGρc) h

where:go = observed gravity (mGal)gt = theoretical gravity (mGal)δg/δz = vertical gradient of gravity (0.3086 mGal·m-1)G = gravitational constant (6.672 x 10-11 m³·kg-1s-2 or 6.672 x 10-6 m²·kg-1·mGalρc = density of crustal rock (kg·m-3)h = elevation above mean sea level (m).

Gravity Reductions

• Terrain Correction

In 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 Reductions

•Terrain correction

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

Gravity Reductions

• Terrain 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( Ө ) = 978 031.846 (1 + 0.005 278 895 sin2Ө +0.000 023 462

sin4Ө )

g(30o) = 979 324.012 mGal is the expected

value for gravity at the base of the tower.

Using the free-air gradient, gӨa = 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 Data

Gravity 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 Data

Gravity anomaly over anticline

When subsurface geology contains successive formations having significant density contrast, folding should be reflected in gravity map. If lithologies having relative greater densities are brought near the surface at the crest of an anticline, the crest line of an anticline will have greater gravity value.

Fig. Bouguer Gravity Map of Ghawar Anticline and Comparison with Field Outline (H. Stewart Edgell )

Interpretation of Gravity Data

Gravity anomalies due to regional structures

Gravity surveys sometime provides good subsurface geological information where there are no exposures on the subsurface. Regional gravity surveys can provide reconnaissance information about little explored areas of economic interest. This type of information can help geophysicist in planning of other detail geophysical and geological surveys.

Interpretation of Gravity Data

• Gravity map of Pakistan

Interpretation of Gravity Data

Data Enhancement Technique

Regional-Residual Separation

Bouguer Anomaly maps contain:• Regional anomaly: long wavelength features due to deep crustal

features • Residual anomaly: short wavelength anomalies due to shallow

structures and small structures

In order to perform proper interpretation residual gravity anomalies must be separated for interpretation. This can be done fitting smooth trend to Bouguer anomaly graphically or by computer

Data Enhancement Technique

Data Enhancement Technique

• Upward Continuation

Transformation of gravity data measured on one surface to some higher surface is known as upward continuation. It is the operation that smoothes the original data by removing the short wavelengths.. This is low pass filter and help to recognize the regional structures.

Data Enhancement Technique

Upward Continuation

Data Enhancement Technique

Data Enhancement Technique

Directional filters

Sometimes interpreter is interested in only some particular direction anomalies. In this special type filter anomalies of particular directions are retained and anomalies of other directions are suppressed. Suppose user is only wants to see the anomalies in direction of NE-SW.

Data Enhancement Technique

• Vertical Derivative

Derivative are calculated to enhance the local gravity anomalies. The derivatives are regarded as high pass filter.

Quantitative Interpretation (Gravity Modeling)

Gravity modeling is divided into two broad categories: • Forward modeling • Inverse modeling

Forward Modeling

Commonly used technique for quantitative interpretation of processed gravity data involves the direct calculation gravity effect of assumed densities, depth and size of the body. The assumed parameters then modified by trial & error. Some constraints can be placed using available geological and other geophysical data.

Quantitative Interpretation (Gravity Modeling)

Quantitative Interpretation (Gravity Modeling)

Inverse Modeling

In gravity forward modeling we iterate with different parameters to match the calculated and observed gravity values. It is also possible sometimes to directly infer directly from the observed gravity data. The model can be prepared by keeping the inferred parameter constant.

Direct Interpretation

Information can be directly obtained from gravity anomalies. The possible information which can be interpreted are

1. Depth of the body producing the anomaly

2. Excess mass determination

Direct Interpretation

Estimation of depth of anomalous body

There are various methods which are helpful in calculating the depth of buried body causing gravity anomaly.

Half Width Method

Half-width, X1/2 , is the distance from the centre of an anomaly at which amplitude has decreased to half its peak value.

Direct Interpretation

• If anomaly is spherical:• If anomaly is horizontal cylinder:

• If anomaly is vertical cylinder:

Direct Interpretation

Depth Estimation by Gradient-Amplitude Method

Depth of the body causing gravity anomaly can also be estimated by calculating maximum slope.

For 3D body

For 2D body

Direct Interpretation

Excess mass Determination

Me = 1/2π ∑ni=1 ∆gi ∆tti

M = ρ1 Me / (ρ1 - ρ2 )

EXERCISE

Figure is a Bouguer anomaly

Map, contoured at an interval of

50 gu, of a drift-covered area.

a) On the map, sketch is what you

consider to be the regional field and

then remove it from the observed

field to isolate anomalies, which be

represented on the map as contours

drawn in a different colour.

b) Construct gravity profiles along line

A – A- illustrating the observed, regional

And residual anomalies.

Exercise

At Artic the ice has the density of 0.91 g/cm3. At a location of 80 degrees north. The ice surface is 1530 m above sea level and ice layer is 2470m thick. The value of observed gravity is 983.061 gals. The density of rock beneath ice is 2.67 g/cm3 .Calculate the free air and bouguer gravity values.