Royal Society Range Aeromagnetic Data Processing
DATA PROCESSING STEPS
- Original binary data from each survey flight were converted
to ASCII data. These data (sample frequency 1Hz) contain latitude, longitude,
fiducial, GPS-time, raw magnetic total-field, barometric altitude (if available),
and event mark. Data are organized in individual survey lines. According to
the flight protocol, the beginning and end of each line as well as unusable
sections of a line were identified and edited.
- A first cut quality control was applied by visual
inspection of line data including removal of spikes in all channels. Errors
in time and horizontal positioning were also eliminated and interpolated
(later corrected using differential GPS data).
- Geographical coordinates were converted to the
local Lambert coordinate system used for the survey.
- Positioning data were recorded in-flight independently
using a Trimble GPS receiver. For most flights GPS reference station data
at either Skelton Neve Camp or McMurdo were available and a differential
position was calculated. For gaps in differential positioning, original GPS
Rover data were used, adjusted at both ends of any gap. These data were used
to replace the latitude and longitude values (and the local Lambert coordinates
subsequently) in the line data set.
- Barometric altitudes are available for all flights,
when the altitude did not exceede 10,000 ft. However, since air pressure
was changing during a flight (locally and in time) barometric values were
adjusted to differentially calculated GPS altitudes whenever this was possible.
For the few cases of altitudes of more than 10,000 ft no barometric data are
available so that GPS altitudes, preferably the differentially corrected altitudes,
but in some cases also the simple flight GPS altitudes, were placed in the
barometric altitude channel. Altitude data were smoothed with a simple
low-pass filter (20 points).
- Diurnal correction of the magnetic field values
was primarily done by subtracting the base magnetometer values recorded at
McMurdo, Skelton Neve and Willie Field. At both sites data were collected
in 1-minute intervals, which were smoothed with a 60-minute low-pass filter
to exclude short-period time variations, thus avoiding introduction of anomalies
in the survey data acquired some distance away from the base stations (Maslanyj
and Damaske, 1986). The filtered base-station data were then interpolated
to 1-second intervals and subsequently subtracted from the survey's magnetic
field values.
- The internal (main-field) component of the magnetic
field, calculated for each survey point using the IGRF (International Geomagnetic
Reference Field) model 1995, updated to 12 January 1998 (as the central date
for the whole survey), was subtracted from the magnetic field values.
- After base station correction and IGRF removal,
discrepancies between the magnetic field values at the intersections of profile-
and tie-lines remained. The calculated deviations were minimized using an
iterative levelling approach (Damaske et al., in press). In this way not
only the higher frequency parts of the diurnal variation were accounted for,
but also discrepancies due to differences in elevation or from any other
effect were reduced. The overall root-mean-square error for intersection-differences
in the final data set is less than 4 nT.
- Grids were constructed from the final data set
including all lines- and tie-lines using a cell size of 500 m (i.e.
1/5 of line spacing).
GRID PROJECTION SPECIFICATIONS
- Projection = Lambert conic conformal
- Base parallels = 76 degrees 40 minutes S, 79 degrees
20 minutes S
- Central meridian = 160 degrees E
- Base latitude = 77 degrees 37 minutes 48 seconds S
- Semi-major ellipsoid axis = 6378135 m
- Eccentricity = 0.081818812603
REFERENCES
MASLANYJ, M.P., & DAMASKE, D., 1986, Lessons regarding
aeromagnetic surveying during magnetic disturbances in polar regions: British
Antarctic Survey Bulletin, 73, 9-17.
DAMASKE, D., MARCINKOWSKI, V., MÖLLER, H.-D., in press,
Aeromagnetic survey in central Dronning Maud Land, East Antarctica, during
the GeoMaud expedition 1995/96: Lay-Out, Execution, and Data Processing: Geologisches
Jahrbuch.
For more information about this report
contact: Eric Anderson
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