Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 99

Data acquisition Data reduction

Sensor velocity
correction

Tidal correction

Attraction
of Sun & Moon
(tidal correction)

Drift correction

Motion of the sensor
(velocity corr e ction)

Rotation of
the Earth's
(latitude correction)

Instrument effects
(drift correction)

Latitude correction

Attraction of topography
around the station
(Bouguer & terrain correction)

Free-air correction

Distance from Earth's
centre of mass
(free-air correction)

Bouguer & terrain correction

Datum level

Unknown density variations in the subsurface Lateral density variations in the subsurface

below the datum level

Figure 3.12 Schematic illustration of the non-geological causes of variations in measured gravity, and the sequence of their removal in
data reduction.

low-level aeromagnetic surveys are generally to be avoided
in gravity work. The survey heights are chosen to suit the
gravity data and are usually greater than ideal for the
magnetic and radiometric measurements (see Section 2.6 ) .

moving eastward (in the same direction as the Earth s spin)

will experience an increase in centrifugal acceleration,
whereas a body moving westward (against the Earth

'

s spin)
will experience a decrease in centrifugal acceleration. The
vertical component of this acceleration, plus a small accel-
eration related to motion on the curved Earth, is known as
the Eötvös effect. It needs to be accounted for in gravity
measurements made from a continuously moving survey
platform, such as an aircraft. The Eötvös correction (G E ) is:

'

3.4 Reduction of gravity data

The corrections applied to remove unwanted variations
(noise; see Section 2.4 ) in gravity are known by the param-
eter that they correct or compensate, and we consider them
in the order that they are applied to the observed data. The
data reduction sequence is summarised in Fig. 3.12 . Deriv-
ations of the various correction factors can be found in
Lowrie ( 2007 ).

01211V 2 gu

G E ¼ 40 : 40V cos ϕ sin α + 0 : ð 3 : 12 Þ

where is the latitude of the measurement, V the velocity

(in kilometres per hour) of the platform and

ϕ

its heading
direction with respect to true north. The correction is
added to the gravity reading, although its polarity depends
on the direction of motion of the gravity instrument. Error
in the Eötvös correction depends chie y on the accuracy in
determining the platform

α

3.4.1 Velocity effect

' s instantaneous velocity and

The Earth s rotation produces an outward-directed centri-

fugal acceleration which acts on all objects and is compen-
sated by the latitude correction (see Section 3.4.4.1 ).
Objects moving across the rotating Earth experience an
additional acceleration related chie y to the east

'

heading.

3.4.2 Tidal effect

-

west
component of their velocity. The effect is greatest on the
equator and decreases with increasing latitude. A body

Gravity at every location varies with time owing to the
gravitational attraction of the Moon, and to a lesser extent