Page 246 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

229
gravity
in
rela-
look
been
of
datum.
tending
deficiency;
center.
(M
because
shows
might
have
variations
Anomalies
the
mass,
sea-level
Earth’s
Fig. 8.5)
back
station
mass
anomaly
anomalies
extra
features.
puts
the
to
mass
Gravity
the
to
in
have
(R)
the
(FAC)
near
station
AR
air
and
/ateral
topographic
radius
that
would
and
free
readings
points
correction
from
Notice
(@
of
increased
changing
areas
effects
function
vary
observation
low
sharp
mass.
mountainous
air
can
elevation
excess,
of
the
free
deficient
mimic
a
corrections, gravity
profile
for
is
the
anomaly
mass
correcting
the
that
and
adding
a
lével,
and
The Nagin,
what
between
the
changes
latitude
of excess
sea
gravity
Anomaly
near
2)
shows
elevation, thereby
Earth; and
near
ot
the
abrupt
elevation
mass
readings
air
8.7
bodies
because
areas
Fig.
free
in
Gravity
also
2"
of the
to
differences
high
after
across
corrected.
The
are
8.5),
Relative
to
tively
bulge
Even
there
Bouguer
lost
Fig.
like
Center of Earth 8.8a). (Fig. gravity the increase to mass the of attraction gravitational the for accounts correction Bouguer The infinite an as mass the approximating by done is datum. This sea-level the above attrac- 8.8b). The (Fig. station the of elevation the to equal (h) thickness with slab, is: slab a of such tion 2npGh = BC where: at than
. acceleration decreases by about 0.308 mGal point, elevated upward | about for the the (g,) spin the
Sl acceleration (g) observation feet) 10 by decreases gravity lower observations to back (m). datum for corrected station. gravity theoretical for accounting
(0.308 mGal/m) Datum lower gravitational a have radius to an (about m 3 every gravity to have therefore gravity compare added be must mGal/m) level sea a gravity, observed FAC + 8, the at observed the subtracting thus /atitude,
= h x ee Level from Earth’s surface, gravitational to tends extended the for that, due acceleration level sea To 8.6b). correction (0.308 X h (s72Gal) above the is - = anomaly acceleration 1) the for
Isostasy =—4R FAC Sea high elevation accounts for illustrates above (Fig. air free = FAC correction the station of anomaly Agy that: corrects
and Observation Point at (FAC) equation the Earth, high level a air station: gravity air gravitational equation (g)
Gravity Rising upward gravity station air correction above the of elevations sea near elevations, 8.6c). free elevation = h air gravity the of free = = g above the gravity
Chapter8 =Ri- Free air correction. a) meter of height. b) A free The surface the at Stations those than different with (Fig. values where: FAC = The free elevation and where: Ag,, in Notice observed the
228 ices AR * 980,625 mGal 6,367,000 m = dg/dR AQ/AR 8.6 lower elevation. c) The level datum.
a g, = R, = FIGURE for every a sea