Abstract
This dataset and its metadata statement were supplied to the Bioregional Assessment Programme by a third party and are presented here as originally supplied.
Collect Magnetic and Radiometric data over the Gloucester Basin in PEL285 - AGL
Dataset History
Airborne Survey (Heli).
Thomson Aviation Pty. Ltd.
GEOPHYSICAL SURVEY DATA REPORT
Date : 28 July 2013
This readme file describes the equipment and specifications of a geophysical
airborne survey conducted by Thomson Aviation Pty. Ltd.
The readme also summarises the data processing parameters and procedures used.
CLIENT DETAILS
Company Flown by : Thomson Aviation Pty. Ltd
Company Processed: Thomson Aviation Pty. Ltd
Client : AGL Energy Limited
Company Job : Thomson 13018
AIRBORNE SURVEY EQUIPMENT:
Aircraft : Bell Jet Ranger
Magnetometer : Geometrics G822A
Magnetometer Resolution : 0.001 nT
Magnetometer Compensation : Post Flight
Magnetometer Sample Interval : 0.05 seconds Hz
Data Acquisition : GeOZ Model 2010
Spectrometer : Radiation Solutions RS 500
Crystal Size : 16.5 lt downward array
Spectrometer Sample Interval : 1 seconds
GPS Navigation System : Novatel OEMV-1VBS GPS Receiver
AIRBORNE SURVEY SPECIFICATIONS
Area: Gloucester, NSW
Flight Line Direction : 090 - 270 degrees
Flight Line Separation : 50 metres
Tie Line Direction : 180 - 000 degrees
Tie Line Separation : 500 metres
Terrain Clearance : 35 metres (MTC)
Survey flown : June 2013
DATUM and PROJECTION
Datum : GDA94
Projection : MGA56
RADIOMETRIC PROCESSING PARAMETERS:
Tot.Count Potassium Uranium Thorium
Height Attn 0.007434 0.009432 0.008428 0.007510
CPS to Eq 29.601 111.508 10.833 5.940
RADIOMETRIC STRIPPING RATIOS:
Alpha = 0.276 a = 0.048
Beta = 0.418 b = 0.003
Gamma = 0.759 g = 0.001
DATA PROCESSING : MAGNETIC DATA
MAGNETIC PROCESSING FLOW
The final magnetic data processing was performed using the following processing flow:
- Aircraft magnetic data QC
- Diurnal magnetic data QC
- System parallax removal
- Diurnal variation removal and addition of the mean diurnal base value
- IGRF removal and addition of mean IGRF value.
- levelling using polynomial Tie line levelling,
- Micro levelling if required
- Reduction to the pole.
- Gridding using Minimum Curvature algorithm
MAGNETIC QUALITY CONTROL
The processing of the magnetic data firstly involved the routine quality control in the field
of both the aeromagnetic and diurnal data during the acquisition phase. Any data found not
meeting the required specifications were reflown.
MAGNETIC PARALLAX CORRECTION
The total magnetic intensity aircraft data was firstly corrected for the effects of system
parallax. The parallax parameters were determined and checked from the results of opposing
test line flights.
MAGNETIC DIURNAL CORRECTION
The base station magnetometer data was edited and merged into the main database. The
aeromagnetic data was corrected for diurnal variations by subtracting the observed magnetic
base station deviations. There were no magnetic storms recorded by the diurnal monitoring
station during the survey. The mean value was then added back to the data.
MAGNETIC IGRF CORRECTION
The data was corrected for the regional gradient of the International Geomagnetic Reference
Field (IGRF). The IGRF was calculated for every point along the lines with respect to
GPS height using the IGRF Model for 2005 with secular variation applied. The mean IGRF
value was then added back to the data.
MAGNETIC PROFILE LEVELLING
The magnetic traverse line data was then statistically levelled from the tie line data using
Intrepid polynomial levelling. The steps involved in the tie line levelling were as
follows:
- A primary tie line was chosen as a reference tie.
- All other ties were levelled to this tie line using 1st degree polynomial adjustment.
- lines were adjusted individually to minimize crossover differences, using 2nd degree
polynomial adjustments.
Any residual flight line effects were removed using Intrepid micro levelling techniques and
the resultant line data saved as a separate field.
MAGNETIC GRIDDING
The data was gridded to a cell size of 20% of line spacing using a Minimum Curvature algorithm.
DATA PROCESSING : RADIOMETRIC DATA
RADIOMETRIC PROCESSING FLOW
Radiometric data processing consists of the following processing flow:
Full spectrum 256 channel Overview:
- Noise Adjusted Singular Value Deconvolution (NASVD) noise reduction
- Dead Time correction
- Energy calibration
- Cosmic and Aircraft background Removal.
- Radon background Removal
- Extraction of IAEA Window data
Windowed data processing Overview:
- Compton Stripping correction.
- Height Attenuation correction using IAEA coefficients.
- Gridding
The specific processing steps are described below:
256 CHANNEL PROCESSING
NASVD Noise Reduction:
Noise-Adjusted Singular Value Decomposition (NASVD) Smoothing. Correction of the radiometric
data involved the reduction of the 256 channels of raw gamma spectrometer data using Noise-Adjusted
Singular Value Decomposition (NASVD) noise reduction method. The signal to noise ratio of the
multi channel spectra can be substantially enhanced using Noise-Adjusted Singular Value
Decomposition (NASVD) as described by Hovgaard and Grasty (1997), Schneider (1998) and Minty (1998).
This method involves a general linear transformation of groups of spectra (a whole line or flight),
using NASVD to compute the different spectral shapes that make up the measured multi-channel
spectra. New multi-channel spectra are created by recombining the statistically significant
spectral components. Each spectral component contributes an unequal amount to the features
observed in the measured multi-channel spectrum, until a point is reached where the spectral
components represent only noise.
The 1st spectral component is the spectral shape that represents most of the features in the
measured multi-channel spectra. The 2nd spectral component represents those features not
described by the 1st spectral component, etc. By excluding from the recombination those spectral
components that do not represent significant features in the measured multi-channel spectra, the
resulting reconstructed multi-channel spectra have a much larger signal to noise ratio than the
measured multi-channel spectra.
Dead Time Corrections:
The raw 256 channel spectra were corrected for spectrometer dead time using the recorded live time
and the standard formula.
N = n / (1 - t)
N = corrected counts in each second;
n = all counts processed in each second by the ADC; and
t = the recorded dead time
Where the live time (L) is recorded, the dead time t is replaced by (1 - L).
Energy Calibration:
Energy calibration was undertaken line by line using a maximum of 3 calibration peaks; and a
minimum of 2 calibration peaks dependent upon their clear identification in the spectra. The 3
calibration peaks used were Bi 214 at 0.609 Mev, K-40 at 1.46 Mev and Tl-208 at 2.615 Mev
Cosmic and Aircraft Background Correction:
Cosmic and aircraft background removal utilised the data recorded from a series of calibration flights
over water. These flight produce a normalised cosmic spectra for the system installation, together with
a 256ch spectra for the aircraft background.
The combined correction is calculated using:
N = a + bC,
where:
N = the combined cosmic and aircraft background in each spectral window;
a = the aircraft background in the window
C = the cosmic channel count; and
b = the cosmic stripping factor for the window.
The values of a and b for each window are determined from the calibration flights over the sea.
Cosmic coefficients and aircraft background coefficients were derived using INTREPID CAL256 program.
Atmospheric Radon:
The influence of atmospheric radon has been minimised using the spectral ratio method described by
Minty (1992). However the effect of radon in the Uranium channel can be considerable; and some
effects of the radon are visable in the character of the final processed data.
Extraction of Four Standard Windows:
The fully processed 256 channel spectra were reduced to the four IAEA (1991) standard windows or
Regions of Interest (ROI): As given by the following Energy windows and channel numbers:
Total Count 0.41 to 2.81 Mev (channels 33 to 238)
Potassium 1.37 to 1.57 Mev (channels 116 to 133)
Uranium 1.66 to 1.86 Mev (channels 140 to 158)
Thorium 2.41 to 2.81 Mev (channels 205 to 238)
WINDOW PROCESSING
Spectral Stripping of Standard Window Data:
Corrections for Compton stripping and height attenuation were applied to the windowed
data using constants supplied by Radiation Solutions Inc.
Due to scattering of gamma rays in the air, the three principle stripping ratios
( Alpha, Beta and Gamma) increase with altitude above the ground:
Stripping Ratio Increase at STP per metre
Alpha 0.00049
Beta 0.00065
Gamma 0.00069
Following adjustment of the stripping ratios for altitude, the technique for producing the corrected
(stripped) count rates in the potassium, uranium and thorium channels (NKC, NUC and NThC) are given
by Grasty and Minty (1995)
The Compton coefficients for the system are given above:
Height Corrections
The stripped count rates vary exponentially with aircraft altitude. Adjustments for variation
in altitude were made using the formula:
Nc = No e^ -u(H-h)
Where No = uncorrected counts,
Nc = count rate normalised to height H,
h = measured height above the ground,
H = nominal flight height,
u = attenuation coefficient for the channel being corrected.
Calculation of Effective Height
The Effective Height, which is the aircraft terrain clearance corrected to Standard Temperature
and Pressure was determined as follows:
- Filtering of the temperature field was applied to remove spikes and smooth out the
instrument noise.
- Filtering of the barometric pressure field was applied to remove spikes and to smooth
out the instrument noise.
- Filtering of the radar altimeter was applied to remove spikes, spurious reflections from
groups of tree and very narrow gullies and to smooth out the instrument noise.
- The formula option in the spread sheet editor was used to combine the terrain clearance,
pressure and temperature.
h x P x 273
E_height = _____________________
1013 x (T + 273)
Where:
E_height= the effective height;
h = the observed radar altitude in metres;
T = the measured air temperature in degrees C;
P = the barometric pressure in millibars.
Reduction to Ground Concentrations:
The fully corrected window data is then converted to effective ground concentrations by dividing
by the conversion coefficient to produce the following equivalent concentrations for each element.
Total Count : Dose Rate
Potassium : Percent
Uranium : PPM
Thorium : PPM
Radiometric gridding
The data was gridded to a cell size of 20% of line spacing using a Minimum Curvature algorithm.
Dataset Citation
AGL (2014) AGL - 2013 Gloucester Airborne Survey. Bioregional Assessment Source Dataset. Viewed 31 May 2018, http://data.bioregionalassessments.gov.au/dataset/5cffc19a-0ff4-402c-824a-88935f70931a.