Basic Field Procedures
diurnal corrections
- drift correction, identical to gravity:
- re-occupy every hour or two
- recording base-station magnetometer
- must avoid magnetic storms
magnetic cleanliness
cultural features
- buildings, power lines, culverts, buried lines, pipes, etc.
- often must be "hand" edited from raw data; e.g., filling in with average
of surrounding values
land surveys
- record: location, time, reading, environment (esp. culture)
- grid or profile
- advantage: closer to anomalous bodies (recall that upward
continuation is low-pass filter); station spacing as close as necessary
aeromagnetic surveys
- instrumental problems: magnetic field of plane (including due to
currents) must be nulled out, or sensor removed from them
- station spacing depends on flight speed and recording rate
- data taken along flight lines
- tie lines used for drift correction and position error
- navigation is critical
- constant elevation (barometric) or constant height above ground
(draped survey)
-
illustrations
marine surveys
- similar to aeromag
- bottle towed as "fish" 500 - 1000' astern; buoyant cable
and bottle
magnetic gradient survey
- two sensors, separated vertically by several feet
- cesium-vapor magnetometer for high accuracy
- diurnal, secular and main field variations don't affect gradient;
local anomalies do