ExplorationGeophysics

7/25/2019 ExplorationGeophysics

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GRAVITYHOME

WORK



GRAVITYGRAVITY





GEOIDGEOID

•What is the shape of the Earth?•The sea-level surface (if un-disturbed) is known as ‘Geoid’. It is

particularly important in gravity surveying as it is horizontal and atright angles to the direction of acceleration due to gravity everywhere.

• Equipotential surface Equipotential surfaceof gravity

•The irregular distribution of mass alter the geoid, which is why geoidis not identical to the ‘ ellipse of rotation ellipse of rotation’



GRAVITY UNITGRAVITY UNIT

•Normal value of g = 980 cm/s2

•Gal = 1 cm/s2

•1 milliGal= 10-3gal

•1 microGal =10-6 gal•In SI gravity is measured µm/s2- gravity unit (g. u.)

•1 g. u. =0.1 mGal

•10 g. u. = 1 mGal



GRAVITY ANOMALYGRAVITY ANOMALY

• Agravity anomaly is the difference between theobserved acceleration ofEarth's Gravity and a value

predicted from a model.

•Models

http://en.wikipedia.org/wiki/Earth%27s_gravity

http://en.wikipedia.org/wiki/Earth%27s_gravity



GEOLOGICALGEOLOGICAL

FACTORFACTOR AFFECTING AFFECTING

DENSITYDENSITY

((IN MEGAIN MEGA

GRAM PERGRAM PER

CUBIC METRECUBIC METRE

OR GRAM PEROR GRAM PER

CUBICCUBICCENTIMETRECENTIMETRE))



GRAVITY MEASUREMENTGRAVITY MEASUREMENT

Relative Gravity Absolute Gravity



GRAVIMETREGRAVIMETRE

LaCoste-Romberg gravimetre

Worden gravimetre



GRAVITY SURVEY TYPESGRAVITY SURVEY TYPES



SURVEY DESIGNSURVEY DESIGN



FACTORS AFFECTING GRAVITYFACTORS AFFECTING GRAVITY

1.Instrumental (spring) Drift

2. Location, hence Latitude of measurement

3.Elevation of the measuring plane

4.Tides

5.Masses between datum and measuring plane

6.Terrain condition

7.Survey vehicle's speed and direction

Need to do some corrections of observed value of ‘g’? Need to do some corrections of observed value of ‘g’?



CORRECTIONCORRECTION

As is true of most all measurement of physicalproperties, there are always effects that change themeasured values that we areNOT interested in andthat we desire to remove (or correct for) as

accurately as possible.

Animportant pointis that wemeasure gravityatwhatever value our gravimeter reads, andTHEN we correct that data for these different effects thatwe are not interested in.



In the case of gravity , there are seven gravityeffects to correct for:

1.Drift correction2.Latitude correction

3.Free-air correction

4.Tide correction5.Bouguer correction

6.Terrain condition

7.Eotvos correction

CORRECTIONCORRECTION



1. DRIFT CORRECTION1. DRIFT CORRECTION

The reading of a gravimeters at a pointchanges with time!

Causes:

•Instrument drift:due to

environmental changes (P,T) andspring creep

•Earth tides:relative rotations of theearth, moon and sun

Correcting procedure:1.Return to base station

periodically

2. Assume drift is linear

3. Correct measurements in loop



2. LATITUDINAL CORRECTIONS2. LATITUDINAL CORRECTIONS

1- It is caused by both rotation of the earth and its slight equatorial bulge.

- !he ma"imum value occurs at latitude #$.

%- L.C. equal to &ero at equator and 'ole.

#- !he correction is added as (e moved to(ard the equator.



2. LATITUDINAL CORRECTIONS2. LATITUDINAL CORRECTIONS

2 2 3 6

1 2 3 1 2 3( ) (1 sin sin 2 ), 9.78031, 5.3024 , 5.900 . g e eθ γ γ θ γ θ γ γ γ − −

= + + = = =

Gravity varies from 9.78 m/s2 at the equator (lat=0°) to 9.83 m/s2 at the poles (lat: north =+90°; south = -90°). This is a huge change: a 0.052 m/s2 variation equals 5200 mgals! This ismuch larger than other gravitational effects. The gravity varies with latitude for two

reasons:

)The Earth is not a sphere, but a flattened spheroid with an equatorial radius of 6,378 kmand a polar radius of 6,356 km (21 km different). Thus, the gravity isLESS at the equatorbecause it isFARTHER AWAYfrom the Earth’s center of mass.

)The Earth isa non-inertial reference framebecause it is a rotating body that spins once per

day. At the equator any object has a rotational velocity of 465 m/s, whereas at the poles therotational velocity is zero! Physics requires that a rotational reference frame has non-inertial (fictitious) forces such as the outward directedcentrifugal force. The centrifugalforce is the force that any mass rotating with the planet ‘feels’ in response to the centripetalforce that the planet’s gravity field provides to continually curve an object’s path on theearth intoa circular path. Recall Newton’s first law says that all masses go in a straight line

in aINTERTIAL reference frame unless acted on by an unbalanced force (it is gravity thatprovides the unbalanced force as acentripetalacceleration).

The International gravity formula that describes latitudinal (Ѳ) gravityvariations in m/s2units is:

An a''ro"imate latitudinal equation (hen survey is small.



3. FREE-AIR CORRECTION3. FREE-AIR CORRECTIONTo apply an elevation correction to our observed gravity, we need to know the

elevation of every gravity station. If this is known, we can correct all of the observed

gravity readings to a common elevation (usually chosen to be sea level) by adding-0.3086 times the elevation of the station in meters to each reading. Given the

relatively large size of the expected corrections, how accurately do we actually need to

know the station elevations?

If we require a precision of 0.01 mgals, then relative station elevations need to be

known to about 3 cm. To get such a precision requires very careful location surveying

to be done. In fact, one of the primary costs of a high-precision gravity survey is inobtaining the relative elevations needed to compute the Free-Air correction.

a- It is a correction for change in elevation.

b- F.A. is calculated by

F. A. = 0.3086 x h mgal/m

c- The F.A.C. is added to the field reading

when the station is above the datum and

subtracted when below.



4. TIDE CORRECTION4. TIDE CORRECTION

1- It is the change of gravity due to movement of the sun and moon.2- These variation has amplitude as large as 0.3 mgal.

3- The amplitude depend on latitude and time.



5. BOUGUER CORRECTION5. BOUGUER CORRECTION

1- It is account for attraction of materials between thestations and the datum plane.

2- We have to consider that the stations are located on a

plateau of horizontal extent

has uniform thickness and density.

3- B.C. is calculated by

B.C. = 0.04191 ph mgal

4- B.C. is applied in the opposite sense to F.A.C. , it is

subtracted when the stations are above the datum and viceversa.



Also notice that to a''ly

the *ouguer +lab

correction (e need to,no( the elevations of

all of the observation

'oints and the density

of the slab used to

a''ro"imate the e"cess

mass. In choosing adensity use an average

density for the roc,s in

the survey area. or a

density of ./0

gm=cm% the *ouguer

+lab Correction is about:11mgals=m.

5. BOUGUER CORRECTION5. BOUGUER CORRECTION



6. TERRAIN CORRECTION6. TERRAIN CORRECTION1- It is applied only in mountainous area.

2- The reading is applied to surface irregularity in the vicinity of the station.3- The measurements decreases in both cases :

a- Upward attraction due to hill.

b- Downward attraction due to valleys.

4- The terrain correction is always added.

Like Bouguer Slab Corrections, when computing Terrain Corrections we need toassume an average density for the rocks exposed by the surrounding topography.Usually, the same density is used for the Bouguer and the Terrain Corrections.Thus far, it appears as though applying Terrain Corrections may be no moredifficult than applying the Bouguer Slab Corrections. Unfortunately, this is not the

case.



6. TERRAIN CORRECTION6. TERRAIN CORRECTION"a//er pproach2not for details could be im'ortant for thesis 3

'ro4ect (or, at #-13#- stage5



7. EOTVOS CORRECTION7. EOTVOS CORRECTION

For a gravimetre mounted on a vehicle, such as a ship or a helicopter, themeasured gravitational acceleration is affected by the vertical component of theCoriolis acceleration which is function of the speed and the direction in whichthe vehicle is travelling.

To compensate for this, gravity data are adjusted by applying the Eotvos

correction (named after Baron von Eotvos).

δgEC= 75.08 cosφ sinα + 0.0416 V2 (g. u.)

Where,φ is the degree of geographical latitde,

! is the a"i#th in degrees,

and $ is the speed of the %ehicle in &nots per hor.



CORRECTION ALL TOGETHERCORRECTION ALL TOGETHER

Adding or subtracting the gravity corrections:It is very important to keepphysical track of the sign of the corrections; if you do not, you will get thewrong answer. Remember, we are correcting the measured gravity datato remove unwanted eects.

• he free!air eect is added if you are above sea!level and is subtractedif you are below sea!level.

• he "ouguer eect is subtracted if you are above sea!level #$h% andadded if you are below sea!level #!h%.

• otal "ouguer correction & "ouguer ' observed ( latitude $)! free!air $)!"ouguer

• otal correct to *ree!air& *ree!air ' observed ( latitude $)! free!air

he sign of the free!air and "ouguer correction depends on whether themeasurements was made above or below ones datum.*ouguer Gravity6 2after all corrections to observed gravity data is often called *ougur gravity5

*ouguer anomaly can be obtained by6



REGIONAL AND RESIDUAL ANOMALIESREGIONAL AND RESIDUAL ANOMALIES

+ote that there are three dierent

structures- ! dyke, granite, dipping strata !associated with mass anomalies that createdierent gravitational eects.

ften, we surveying at a small scale #e.g.,for the dyke and granite bodies%, we are+ interested in the larger scale regional-gravity eects #e.g., the dipping strata%.

hus, we reduce- the data by subtracting aneyeball- estimate of the regional gravitytrend he dotted lines show two possibleregional trends-.

/fter subtracting the regional gravity trends,we can more easily see the short scaleresidual- features we are interested instudying.



SEPARATING LOCAL AND REGIONALSEPARATING LOCAL AND REGIONAL

GRAVITY ANOMALIESGRAVITY ANOMALIES



INTERPRETATIONINTERPRETATION

Duly corrected observed gravitydata can be interpreted in twodifferent way:

1.Direct interpretation of the

observed data

2.Indirect or inverse (model

based) interpretation

Selection of interpretationtechniques depends on the

project objectives.

Di44erent 5o0ies i7inDi44erent 5o0ies i7in

i0entical ano/alies.i0entical ano/alies.



USESUSES

•Depth estimates

•Mass determination

•Identification of geological structure

•Mineral exploration

•Basin configuration

•Detection of underground cavities

• Volcanic hazards

•Basement configuration and nature etc.



GRAVITYGRAVITY

ANOMALIES ANOMALIESOVER GIVENOVER GIVEN

GEOMETRICGEOMETRIC

FORMSFORMS



GRAVITY ANOMALIES OVER GIVENGRAVITY ANOMALIES OVER GIVEN

GEOMETRIC FORMSGEOMETRIC FORMS





WHAT THISWHAT THIS

MAP TELLS US?MAP TELLS US?



WHYWHY

GRAVITY ISGRAVITY IS

CHANGINGCHANGINGON AON A

TEMPORALTEMPORAL

BASIS?BASIS?



THANK YOUTHANK YOU

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