GEOPHYSICS (GRAVITY SKEMANG)

Question 1

Variations is Gravity Field caused by varying densities of subsurface rocks

Answer 1

Gravity Survey

Question 2

What is Newtons’s Law of Universal Gravitation?

Answer 2

Forces of Attraction Between two objects is proportional to the product of their masses and inversely to the square of the distance between their centers

Question 3

Formula of Newtons Law of Gravtitaion?

Answer 3

F= (G m1*m2)/ r2

Question 4

What is the value of G?

Answer 4

6.67 x10^-8 dyne cm2/g2
6.67 x10^-11 N m^2/kg^2

Question 5

Gravitational Field is defined in terms this value which is defined as WORK needed by the gravitational force in moving a unit from infinity to the final position P

Answer 5

Gravitational Potential (U)

Question 6

A surface defined by constant U and is horizontal such as the sea level or perpendicular to the force of gravity

Answer 6

Equipotential Surface

Question 7

Unit of Gravity field strength

Answer 7

Gal (g/cm^3)

Question 8

Units in gravity surveying

Answer 8

1mGal = 10^-3 Gal = 10^-5 m/s2= 10g.u

Question 9

Rocks that have greatest range of density and are largely contolled by mineral composition, cementation, porosity and pore fluid

Answer 9

Sed Rocks

Question 10

Igenous Rock density is largely controlled by ?

Answer 10

Silica Content (Density increases as silica decreases BASALT - DENSEST)

Question 11

The measuring instrument of gravitational field of Earth at specific locations and works on the principle of measuring constant downward acceleration due to gravity

Answer 11

Gravimeter
LaCoste & Romberg
Scintrex

Question 12

Type of gravimeter that measures local gravity by directly measuring the acceleration of a mass during freefall in a vacuum while an accelerometer is rigdly attached to the ground

Answer 12

Absolute Gravimeter

Question 13

Extremely sensitive, spring based instruments carryinga fixed mass (m) and works by the principle that the changes in gravity will result in a cange in weight of fixed mass with change of location

Answer 13

Relative Gravimeter

Question 14

The extension of the spring is proportional to the extending force

Answer 14

Hooke’s Law

Question 15

A widely used instrment which is a compact temp-compensate unit with a precision of <0.1mGal and is a shiborne system extensively used in Marine gravity surveys for mapping ocean floor

Answer 15

Worden Gravimeter

Question 16

Routine corrections of field gravity contrast between a arbitrary reference point and a series of ground stationds influenced by extraneous effects not related to subsurface geol.

Answer 16

Gravity Reduction

Question 17

Gravity variations in terms of latitude

Answer 17

Greater at Poles (Because smaller distance to the center)

Question 18

Gravity variations in terms of height or elevation

Answer 18

The higher the elevation the smaller the Gravity

Question 19

Linear Velocity variatons

Answer 19

Fastest on the equator

Question 20

Shortcomings of gravimenter and is the grdual change in reading with time

Answer 20

Drift

Question 21

How is Drift corrected?

Answer 21

By periodically returning to the base station

Question 22

Correction needed for surveys covering a large N-S distance

Answer 22

Latitude Correction

Question 23

what is the formula used

Answer 23

Clairuts and International Gravity Formula

Question 24

Apprximate latitue correction

Answer 24

0.8mgal/km

Question 25

Correction applied when gravity measurements are made in a moveing object thus a centrifugal acceleration is induced

Answer 25

Eotvos correction

Question 26

Direction of earth rotation

Answer 26

Easting (Counterclockwise)

Question 27

Effects of rotation to gravity

Answer 27

Easting- less gravity
Westing - more gravity

Question 28

apprximate eotvos in mid latitue

Answer 28

75 gu

Question 29

Corrects the decrease in gravity w/ heights resulting from increased distance from the center (always positive)

Answer 29

Free Air Correction

Question 30

FAC anomaly

Answer 30

Measured+latitude+Free Air

Question 31

Correction taking into account the gravitational effects of the rock present between the observation point and datum and assumes that the surface is flat horizontal

Answer 31

Bouguer Correction

Question 32

Corrections that account for topographic relief in the vicinity of the gravity statin and is always positive

Answer 32

Terrain Correction

Question 33

Correction that takes into account the effects of tidal variations caused by the combined effects of Sun and Moon gravitaional forc

Answer 33

Tidal Corrections

Question 34

Bouguer Anomaly

Answer 34

Meas - Lat + FAC - BC +TC+ EC

Question 35

Types of Gravity Surveys

Answer 35

Land
Marine
Airborne
Satellite

Question 36

Other applications

Answer 36

Large-Med Scale Geol Structures
Ancient Suture Zones
Location of Sed Basins
Hydrogeol investigation to determine GEOMERTY of potenial Aquifers
Engg investigation to locate cavities and voids
Karst Sinkhole investigations

Question 37

Mining and Econ Geol

Answer 37

Base Metal Deposits
Chromites and PGEs
Iron Ore
Salt Domes
Hydrocabon accumulations(sed basin)

Question 38

Structural

Answer 38

Large-Med Scale Geol Structures
Suture Zones
Sed Basins Locations
Karst Sinkhole investigations

Question 39

Hydrogeol

Answer 39

Geometry of Potential Aquifers

Question 40

Environmental and Engg

Answer 40

Engg Investigation to locate cavities and voids