GEOPHYSICS (GRAVITY SKEMANG) Flashcards
Variations is Gravity Field caused by varying densities of subsurface rocks
Gravity Survey
What is Newtons’s Law of Universal Gravitation?
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
Formula of Newtons Law of Gravtitaion?
F= (G m1*m2)/ r2
What is the value of G?
6.67 x10^-8 dyne cm2/g2
6.67 x10^-11 N m^2/kg^2
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
Gravitational Potential (U)
A surface defined by constant U and is horizontal such as the sea level or perpendicular to the force of gravity
Equipotential Surface
Unit of Gravity field strength
Gal (g/cm^3)
Units in gravity surveying
1mGal = 10^-3 Gal = 10^-5 m/s2= 10g.u
Rocks that have greatest range of density and are largely contolled by mineral composition, cementation, porosity and pore fluid
Sed Rocks
Igenous Rock density is largely controlled by ?
Silica Content (Density increases as silica decreases BASALT - DENSEST)
The measuring instrument of gravitational field of Earth at specific locations and works on the principle of measuring constant downward acceleration due to gravity
Gravimeter
LaCoste & Romberg
Scintrex
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
Absolute Gravimeter
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
Relative Gravimeter
The extension of the spring is proportional to the extending force
Hooke’s Law
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
Worden Gravimeter
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.
Gravity Reduction
Gravity variations in terms of latitude
Greater at Poles (Because smaller distance to the center)
Gravity variations in terms of height or elevation
The higher the elevation the smaller the Gravity
Linear Velocity variatons
Fastest on the equator
Shortcomings of gravimenter and is the grdual change in reading with time
Drift
How is Drift corrected?
By periodically returning to the base station
Correction needed for surveys covering a large N-S distance
Latitude Correction
what is the formula used
Clairuts and International Gravity Formula
Apprximate latitue correction
0.8mgal/km
Correction applied when gravity measurements are made in a moveing object thus a centrifugal acceleration is induced
Eotvos correction
Direction of earth rotation
Easting (Counterclockwise)
Effects of rotation to gravity
Easting- less gravity
Westing - more gravity
apprximate eotvos in mid latitue
75 gu
Corrects the decrease in gravity w/ heights resulting from increased distance from the center (always positive)
Free Air Correction
FAC anomaly
Measured+latitude+Free Air
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
Bouguer Correction
Corrections that account for topographic relief in the vicinity of the gravity statin and is always positive
Terrain Correction
Correction that takes into account the effects of tidal variations caused by the combined effects of Sun and Moon gravitaional forc
Tidal Corrections
Bouguer Anomaly
Meas - Lat + FAC - BC +TC+ EC
Types of Gravity Surveys
Land
Marine
Airborne
Satellite
Other applications
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
Mining and Econ Geol
Base Metal Deposits
Chromites and PGEs
Iron Ore
Salt Domes
Hydrocabon accumulations(sed basin)
Structural
Large-Med Scale Geol Structures
Suture Zones
Sed Basins Locations
Karst Sinkhole investigations
Hydrogeol
Geometry of Potential Aquifers
Environmental and Engg
Engg Investigation to locate cavities and voids
