week 2 Flashcards
what is newton’s law of gravitation
F = G(m1m2/r^2)
G= gravitational const. 6.67*10^-11
m1 and m2 = masses
r = distance between them
equation for flattening
(Req - R poles)/ Req = (6378 - 6356.6) / 6378
= 1/298.26
since g = GM/R^2, g will be larger where R is smaller
equation for IGF
g = 9.780318 (1 + a sin^2(lambda) - b sin^2 (2lambda))
effect of large-scale inhomogeinetites
- large-scale inhomogeneities produce departures of g from IGF for spheroid
- g does not vary smoothly from the equator to the pole
- geoid is the observed equipotential surface that defines the sea level
- geoid has ‘highs’ and ‘lows’
what is the reference geoid
- a mathematical formula describing a theoretical equipotential surface of a rotating symmetric spheroidal earth model having realistic radial density distribution
describe the geoid
- the equipotential surface that defines sea-level
- to a first approx, V=GM/r
- so if potential V is constant and M increases, r must increase and vice versa
- excess mass, geoid goes up
- deficient mass, geoid goes down
- BUT only true for static earth
what are the highs and lows of the earth
highs = active magmatic regions
- mid-atlantic ridge
- andes
- philippines
lows = old, inactive basins and continents
- antarctica
- canada
- siberia
- india
why are gravity corrections needed
- g can be unexpectedly different due to unknown e.g. subsurface structures
what is the latitude correction
8.1sin(2lambda) g.u. per km
what is the free-air correction for
accounts of the fact that the point of measurement is at elevation H, rather than at sea level on the reference spheroid
what is the bouguer correction for
accounts for the gravitational attraction of the rocks between the point of measurement and the sea level
what is isostacy
the gravitational equilibrium between the lithosphere and aesthenosphere such that tectonic plates ‘float’ at an elevation dependent on their thickness and density
