Abstract
During 1992 and 1993 select portions of existing government airborne radiometric data covering almost 91 000 km2 of central Namibia were compiled into a master digital data set. This compilation involved the interactive, semi-automated digital recovery of approximately 42 000 line kilometres of original analogue chart traces. A further 49 000 line kilometres of digital data were also reprocessed. Available data represented ten different surveys collected over 12 years with a variety of spectrometers, spectral windows and survey parameters. Preliminary digital grids of each radioelement were compiled, verified and used to select representative sites for ground measurements within each survey block. Results obtained from the ground program were used to back-calibrate the airborne data, standardize the various surveys and convert airborne measurements into equivalent ground concentrations of uranium, thorium and potassium. The quality and consistency of final map products conclusively demonstrates that existing analogue radiometric data, in various states of preservation, can be successfully recovered, combined with modern digital data, and utilized to assist exploration, mapping and environmental studies.
1. INTRODUCTION
Airborne geophysical data have been collected over much of Namibia since 1968. Although these government sponsored regional surveys were generally of good quality, data collected prior to 1977 were not in a digital format. Furthermore, in the case of radiometric data, existing map products were neither consistent in form (ranging from simple redball anomaly selections and stacked profiles to contour presentations) nor particularly suited for mineral exploration, geological interpretation or environmental applications. The different map presentations also hindered geophysical and geological correlation between survey blocks.
In an attempt to overcome this problem, the Geological Survey of Namibia (GSN), in conjunction with the Bundesanstalt fur Geowissenschaften und Rohstoffe (BGR, Germany), selected an area of central Namibia (Fig. 1) from which all available airborne radiometric data would be recovered, standardised and compiled into a master digital data set. This digital data would then be used to produce maps of a) exposure rate, b) ground concentrations of potassium, uranium and thorium, c) elemental ratios and d) ternary images. A contract for radiometric data recovery, compilation, back-calibration and map plotting was awarded to UDL (Urquhart Dvorak Limited, Canada) in conjunction with DMT (Gesellschaft fur Forschung und Pruning mbH, Germany). The project was funded by the German Government in a technical cooperation agreement with the Government of Namibia, and jointly supervised by BGR and GSN geophysicists.
2. SURVEY AREAS AND DATA SOURCES
The selected area consists of four (4) 1:250000 scale map sheets — namely 2014 Fransfontien, 2016 Otjiwarongo, 2114 Omaruru and 2116 Okahandja. Covering almost 91 000 km2
of west central Namibia, the area includes varied physiographic regions (coastal plain (Namib desert), escarpment and highlands), typical geological environments and a representative sampling of the different types of radiometric data.
59
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F/G, 1. Project location — Namibia.
This radiometric data resource comprises ten different surveys flown between 1968 and 1980;
approximately 42 000 line kilometres of analogue chart records and about 49 000 line kilometres of digital data. Flight line orientation and mean terrain clearance vary, but line spacing remains constant at 1000 m (Fig. 2). Although each survey recorded four radiometric data channels, the contractor, survey equipment, detector crystal volume, energy windows, sensitivities and calibration procedures varied from year to year and survey to survey (see Table I for a summary). In addition to these known variations, it became apparent that parameters were occasionally changed during the course of a single survey (e.g. survey aircraft, equipment, location of calibration sources, correction coefficients, etc.). Unfortunately, in some cases (particularly the older surveys) only incomplete records of survey procedures were available.
3. DATA RECOVERY, BACK-CALIBRATION AND PROCESSING
Original analogue chart records (approximately 46% of the total line kilometres) were digitized with a proprietary interactive trace recovery procedure (RE-TRACE) developed by UDL.
This interactive, semi-automated system scans and vectorizes each pen trace, capturing a perfect digital reproduction of both high and low frequency components, thereby ensuring profile fidelity for
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all radiometric channels plus the altimeter information. Analogue data recovery formed the main thrust of this project. Raw digital data was recovered from the original survey tapes, reformatted and written to databases. The degree of early data manipulation (stripping, etc.) done by the initial contractor varied from survey to survey for both analogue and digital data sets.
Processing commenced once the recovery stage was complete, and can be divided into two main parts, an initial data verification phase followed by a second, intensive processing phase. Initial data verification consisted of preparing rough maps that were used to check completeness of the recovered databases for each survey area. Radiometric anomaly positions were compared with known locations of river valleys and granitic bodies. These rough maps were also used to identify data errors, adjust for failings in the original surveys and to determine back-calibration sites.
Back-calibration of radiometric data is based on the comparison of airborne counts rates with ground concentrations measured beneath the flight path by a calibrated portable gamma-ray 61
TABLE I. SUMMARY OF AIRBORNE SURVEY ACQUISITION PARAMETERS Survey Year Digital/ Line Line Flight Crystal
Number Flown Analogue Direction Spacing Height Volume
(a or d) (metres) (metres) (litres) 51 1968 a NW/SE 1000 ?? ??
52 1971 a NW/SE 1000 150 ??
54 1973 a NW/SE 1000 150 ??
55 1973 a * N/S 1000 150 ??
56 1974 a N/S + E/W 1000 125 11.12 57 1975 a N/S + E/W 1000 100 16.68 S10 1977 a* E/W 1000 100 16.68
S13 1979 d E/W 1000 100 16.68 CDM80 1980 d N/S 1000 100 16.68
S15 1980/81 d N/S + E/W 1000 100 16.68
?? No Documentation Available
* Digital data collected for area S10 was not available
spectrometer [2]. Aircraft sensitivity factors calculated by this technique can be used to correlate survey results obtained from radiometric systems with different detector crystal volumes and, to a lesser extent, energy windows.
Systematic ground radiometric measurements in Namibia were made at 20 calibration sites within each of the ten airborne survey areas. Preferred ground calibration sites were situated at the intersection of flight lines and roads, in areas of uniformly elevated radiometric signatures and subdued topography. By choosing calibration sites located at the same features used by the survey aircrews for navigation (fiducial markers recorded on flight records) good data correlation was assured. Each site was evaluated with respect to potential cultural disturbance or physiographic influences prior to taking ground concentration readings. Four measurements were made at every calibration site; two on either side of the road and typically 25 m apart. A laptop computer was used to store the full digital site spectrum measured by the portable spectrometer. Preliminary databases for each survey area (generated during the data verification stage) were available on the same computer, allowing each days ground data to be processed, evaluated and compared with the airborne results while the field crew was still in the area.
Navigation in the field was via published 1:50 000 scale topographic maps, assisted by satellite GPS (Global Positioning System). GPS readings were used to verify location at each calibration site, and provided positioning control in areas of featureless terrain (e.g. Namib desert).
A typical series of ground calibration sites are shown in Fig. 3.
Airborne count rates at the different calibration sites showed variations in radioactivity.
Airborne and ground measurements were therefore compared separately at each site to derive potassium, uranium and thorium sensitivities. Sensitivity errors were computed from the calculated statistical error, based on the mean value of the airborne count rates and the four ground
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FIG. 3. Ground calibration sites with corresponding flight path (survey area S10).
measurements. The total count channel was calibrated by comparing the airborne measurements with the calculated exposure rate derived from the ground concentrations of potassium, uranium and thorium using the following standard relationships [1]:
1 percent K = 1.505 uR/h, 1 ppm eU - 0.625 uR/h, 1 ppm eTh = 0.31 uR/h
Table II provides an example of the preliminary total count sensitivities for survey S10. The final sensitivity was calculated from the individual sensitivities, taking into consideration errors associated with calibrations at each measurement location. Fig. 4 graphically compares the exposure rates (uR/h) at each of the calibration sites. During the final intensive stage of data processing, individual surveys were fine-levelled using a variation of microlevelling adapted for radiometrics.
Each survey was processed independently following standard radiometric correction procedures consisting of background determination and removal. Compton stripping (where required — some
analogues were already stripped) and altimeter correction. All surveys were continued to a common
flight height of 150 m above terrain. Optimal radiometric filters were applied to reduce noise effects.Intermediate grids of each survey area were prepared throughout the process, and checked for internal consistency with an imaging program.
Airborne sensitivities were recalculated for each survey area, and applied to generate equal area ground concentration grids for each component. Individual survey areas were merged together on central meridians of 15 and 17° at a 500 m grid cell size. Decorrugated [3] of the data was used
as a guide, with any discrepancies between the pre and post decorrugation mitigated by the earlier
application of fine levelling.4. FINAL PRODUCTS
The back calibration and re-processing of the radiometric data facilitated merging of the individual survey data to provide a master digital grid, for each of the components (potassium, 63
TABLE II. SAMPLE CALIBRATIONS DERIVED FOR TOTAL COUNT DATA (SURVEY AREA SIO)