Page 146 - Introduction to Mineral Exploration

7: GEOPHYSICAL METHODS 129

FIG. 7.2 Airborne geophysical installation. The aircraft has been modified for combined magnetic and TEM
surveys. The boom projecting from above the cockpit is the forward mounting of the electromagnetic
transmitter loop, which extends from there to the wingtips and then to the tail assembly. The magnetometer
sensor is housed in the fibreglass “stinger” at the rear, which places it as far as possible from magnetic sources
in the aircraft.

Visual navigation in savannah areas was verification. Velocities can be estimated with
difficult enough, since one clump of trees looks great accuracy, making airborne gravimetry,
much like another when seen sideways on, which requires velocity corrections, usable for
but plotting from vertical photographs in such the first time in mineral exploration.
areas was often easy because tree patterns are Survey lines must not only be flown in the
quite distinctive when viewed from above. right places but at the right heights. Ground
Visual navigation and flight-path recovery clearance is usually measured by radar alti-
were both very difficult over jungles or deserts meter and is often displayed in the cockpits
and were, of course, impossible over water. by limit lights on a head-up display. The extent
If there were no existing radio navigational to which the nominal clearance can be main-
aids, temporary beacons might be set up, but tained depends on the terrain. Areas too rugged
since several were required and each had to for fixed-wing aircraft may be flown by helicop-
be manned and supplied, survey costs were ter, but even then there may be lines that can
considerably increased. Self-contained Doppler only be flown downhill or are too dangerous to
or inertial systems, which could be carried en- be flown at all. Survey contracts define allow-
tirely within the aircraft, produced significant able deviations from specified heights and line
improvements during the period from about separations, and lines normally have to be
1975 to 1990, but airborne work has now been reflown if these limits are exceeded for more
completely transformed by the use of global than a specified distance, but contracts also
positioning satellites (GPS). GPS systems pro- contain clauses emphasizing the paramount
vide unprecedented accuracy in monitoring importance of safety. In a detailed survey the
aircraft position and have largely eliminated radar altimeter data can be combined with GPS
the need for tracking cameras, although these estimates of elevation to produce a high resolu-
are still sometimes carried for back-up and tion digital terrain model (DTM) which may