Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 98

aerogravity survey of the Timmins area in central Canada,
equating the results with a ground survey with station
spacing of 1 km.

For a ground gravity survey, it is essential to establish a
base station close to, or within, the survey area and per-
manently mark it for later reoccupation. This is the loca-
tion of repeat readings used to monitor instrument drift
and detect tares (see Section 3.3.1 ) and the origin point for
determining the latitudinal gradient of normal gravity
( Section 3.4.4.1 ). Furthermore, because gravity meters
measure the difference in gravity between survey stations,
the base station is the essential common point of reference
for all the survey measurements (see Section 2.2.1 ) . For
larger surveys, logistical constraints may dictate that a
network of base stations be established throughout the
survey area. In these cases, a separate survey is needed to
establish the relative differences in gravity between the
stations comprising the network. Strict adherence to meas-
urement procedures is necessary in order to minimise
errors in the network, as these will ultimately propagate
into the survey data. One base station can be taken as the
master base station for the entire survey, and often this is
tied to a national gravity station where absolute gravity and
height are accurately known. As an ongoing check for
errors, repeat readings at various stations within the survey
area is normal practice.
Absolute measurements of gravity have been made at
numerous reference stations around the world to form a
global network known as the International Gravity Stand-
ardisation Net 1971 (IGSN71). Often the stations coincide
with geodetic survey marks. To determine absolute gravity
at a local base station, the difference in gravity between it
and the reference station is measured. Tying a gravity
survey to a national network is not necessary for identify-
ing the types of responses of interest to mineral explorers.
However, it does allow different surveys to be compared
and merged at a later time.
In addition to the gravity measurement, the time of the
reading, its location and height above sea level to centi-
metre accuracy are required for data reduction. These
ancillary data can be obtained using GPS-based survey
methods ( Fig. 3.10 ) .
Airborne gravity and gravity gradiometer surveys are
usually conducted at a terrain clearance ranging from 80
to 150 m with survey lines spaced from 100 m to several
kilometres apart depending on the nature of the survey.
A magnetometer and sometimes radiometric equipment
are carried on the aircraft. Both helicopters and fixed-wing
aircraft are used but the gravity sensors are sensitive to
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3.3.2 Measuring gravity gradients

A variety of purpose-built gravity gradiometers have been
developed for use in moving platform surveys. The sensors
making up the gradiometer equally experience the acceler-
ations related to platform motion, and any difference
between them will be due to the gradient of the Earth s

gravity field. The higher sensitivity of gradiometers to
shorter wavelengths in the gravity field means that gradi-
ometer data are more suitable for mineral exploration
targeting than normal gravity systems measuring gravity
directly. Importantly, gradiometer data can be transformed
to normal gravity data showing the required shorter-
wavelength variations with less error than obtained from
normal gravity systems, or that would otherwise be unob-
tainable (Drans eld, 2007 ).
Two gravity gradiometer systems in use at the time of
our writing are FALCON (Drans eld, 2007 ) and Air-
FTG TM (Murphy, 2004 ) . FALCON produces the vertical
gradient of gravity from which normal gravity is com-
puted. Drans eld ( 2007 ) provides examples of data from
several types of mineral deposits. Air-FTG determines the
gradient in each of the three perpendicular directions of all
three components of the gravity field to produce full-tensor
gravity (FTG) measurements (see Section 2.2.3 ). Hatch
( 2004 ) describes its application to kimberlite exploration.
Figure 3.11 shows the detection limit of the FALCON
system, which is also representative of other existing gra-
diometer systems. The ability of the higher-sensitivity gra-
diometers to detect a wide range of mineral deposits,
compared with gravity systems, is clearly demonstrated.
Gradiometer measurements are very sensitive to near-
surface features so the data are also very sensitive to vari-
ations in survey height and topography. Airborne
gradiometer systems carry laser survey instruments for
the acquisition of digital terrain data to centimetre accur-
acy for post-survey data reduction (see Section 3.4.5 ) .

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3.3.3 Gravity survey practice

Comprehensive descriptions of all aspects of geophysical
data acquisition and survey design are given in Section 2.6 .
Details pertaining speci cally to gravity surveying are
described here.

aircraft manoeuvres, so the tight turns at the end of
flight-lines and the rapid elevation changes common in

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