GEOPHYSICS (GRAVITY SKEMANG) Flashcards
(40 cards)
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
