OR/14/014 Radiometric processing

From Earthwise
Jump to: navigation, search

Energy calibration

Energy recalibration, dead time correction and region of interest selection were performed in field during the field tape extraction process. A peak position statistics file is generated. This is used in the extraction process to select the appropriate channel (fractional) range for calculation of the standard channels, using Specdrift (Proprietary CGG Airborne Survey software).

1.1 ROI Channel KeV Label

1

34 – 224 404 – 2805

Tc

2

115 – 131 1369 – 1574

K

3

139 – 155 1652 – 1863

U

4

202 – 233 2415 – 2812

Th

5

-

-

-

6

-

-

-

7

-

-

-

8

139 - 155 1652 – 1863

UP

Cs-137 was windowed from the 256 channel data using the limits below
Energy Limits Channel Limits
Lower Upper Lower Upper
Cs-137

648

732

54.08

61.09

Dead time correction

The calculated standard windows were time normalized to counts per second. This is done by dividing the count rate by the live time. In the case of the cosmic channel which has different counting circuitry with minimal dead time, the channel was normalised by dividing by the sample time. This is also done during the extraction process.

The extracted file was loaded into the database where subsequent radiometric processing was performed.

Cosmic and aircraft background removal

Before the radiometric processing below was performed, the data was filtered to reduce statistical noise. The filtering applied was typically as follows:

  • Potassium, Uranium, Thorium, Total count channels were filtered with a Gaussian low pass filter with filter length of 5 samples to reduce statistical noise.
  • Radar altitude channel was filtered with a 5 point Gaussian cut off filter so that the altimeter response time matches that of the radiometric data.
  • Cosmic channel was filtered with a running average of 20 terms. Due to the increased volume of upward crystal the number of terms for uranium up may be decreased. This was decided after examination of data statistics.

The time normalized and filtered channels were corrected as follows:

Kbg = KF - KACBG - KCOS_F * Cosmic
Ubg = UF - UACBG - UCOS_F * Cosmic
Thbg = ThF - ThACBG - ThCOS F * Cosmic
TCbg = TCF - TCACBG - TCCOS F * Cosmic
UCAVbg = Uav - UACBG - UCOS_F * Cosmic

where KF, UF, ThF, TCF, UUPF are time normalised filtered potassium, uranium, thorium, uranium up and cosmic channels; KACBG, UACBG, ThACBG, TCACBG are the aircraft backgrounds for potassium, uranium, thorium, total count; Uav and Thav are the channels filtered by means of a running average.

Radon background removal

The radon correction was then computed using Minty’s method.

Minty’s method (1992)[1] is a technique for estimating the background atmospheric (radon) radiation in Airborne Gamma-ray Spectrometry.

The method uses the observations that:

in modern airborne spectrometric systems the resolution of the detectors at the 214Bi photopeaks at 0.609 MeV (from atmospheric radiation) and 1.76 MeV (from uranium in the ground) is well resolved above the Compton continuum, and;
due to the differences in spectral shapes between the two 214Bi photopeaks it is possible to differentiate low energy airborne 214Bi signals (at 0.609 MeV) from the higher energy terrestrial ones (from uranium, at 1.76 MeV): the low energy 214Bi photopeak at 0.609 MeV is less attenuated than the 214Bi peak at 1.76 MeV;
those photopeaks can then be used to estimate the contributions of radon and uranium to the observed spectrum because thorium and potassium sources do not contribute appreciably to these peak count rates.

The technique of determination of background by full spectrum analysis is applied after the aircraft and cosmic backgrounds are removed from the observed spectrum. It involves computing the radon contribution and the uranium contribution to the low energy (0.609 MeV) and high energy (1.76 MeV) peak count rates using the observed count rates in those energy peaks and the computation of three constants determined from the radon and uranium spectra. In turn, the radon contribution to the background in the standard uranium window is computed and added to the aircraft and cosmic backgrounds to get the total uranium channel background. And finally the computed uranium background is used to estimate the total count and potassium backgrounds. Thorium background is computed independently from the aircraft and cosmic backgrounds.

The radon correction values were then removed from the filtered background and cosmic corrected values Kbg, Ubg, Thbg and Tcbg

Calculation of effective height

The height from the filtered height channel was then converted to effective height at standard temperature and pressure as per the following formula:

14014 math2.png
Where:
h = the observed radar altitude in metres.
T = the measured air temperature in degrees Celcius.
P = the barometric pressure in millibars.

Special stripping

The background corrected count rates in the three windows must be stripped to give the counts in the potassium, uranium and thorium windows that originate solely from potassium, uranium and thorium. The stripping ratios a, b, g, a and g must be determined from measurements over calibration pads. The three principal stripping ratios (a, b, and g) increase with altitude above the ground as shown in Table 2. The background corrected data is then stripped. Before stripping the coefficients are corrected for variation from height as follows:

Where:

The background corrected data was then stripped.

Before stripping the coefficients were corrected for variation from height as follows:

αe = α + 00049 * he
βe = β + 00065 * he
γe = γ + 00069 * he

Table 6 – Stripping Ratios

Where:

he is equivalent height above ground at STP.

α, β and γ are the stripping ratios calculated at ground level.

αe, βe and γe are the corresponding co-efficients at height he and,

a, b, g are the reverse stripping co-efficients derived from the PAD calibrations.

Stripping is then applied as follows:

NK,K = [NTH (αeγe - βe) + Nu (aβe - γe) + NK (1 - aαe)] / A
NU, = [NTH (gβe - αe) + Nu (1 - bβe) + NK(bαe - g)] / A
NTH, TH = [NTH (1 - Gγe) + Nu (bγe - a) + NK (ag - b)] / A
Where:


A = 1 - gγe - a (αe - g βe) - b (βe - αe γe).
NK = Observed potassium counts corrected for background.
NU = Observed uranium counts corrected for background.
NTH = Observed thorium counts corrected for background.
NK,K = Stripped counts in K.
NU,U = Stripped counts in U.
NTH,TH = Stripped counts in Th, and,
αe, βe, γe are stripping coefficients derived from the pad calibration, and a, b, g are coefficients as described above.

Height correction

The background corrected and stripped count rates are corrected for variations in the altitude of the detector using the following equation:

Ncorr = Nobse — μ(ho - h)
Where:
Ncorr = the count rate normalised to the nominal survey altitude, h0.
Nobs = the background corrected, stripped count rate at STP height h and
μ = the attenuation coefficient for that window.

Conversion to concentration

The corrected window count rate data will be converted to ground concentrations of potassium, uranium and thorium using the following expression:

C = NS
Where:
C = concentration of the radioelement (K%, U ppm or Th ppm).
S = broad source sensitivity for the window; and,
N = count rate for each window, after dead-time, background, stripping and height correction.

Levelling

Some leveling was required after radiometric correction. This was done using proprietary tie-line leveling and micro-leveling software routines where required.

Noise adjeusted singular value decompostion (NASVD)

Noise adjusted singular value decomposition (NASVD) was used to considerably enhance low signal to noise regions of the survey.

The technique is an enhancement of the principal component (PC) technique commonly used in Remote Sensing for processing of Landsat and Spot images. The PC technique is a linear transformation of multiband data that generates uncorrelated components.

Each successive component contributes successively less variance to the total response. The first PC can be considered the spectral shape that contributes most to the overall response.

This technique is enhanced by adjusting the variances to fit that expected for a Poisson distribution. This yields components that have physical meaning.

Products delivered

The following products have been delivered. It is noted that all products are digital and have been presented in the project coordinates reference system.

Geosoft Grids

Magnetic Grids
• tmi.grd Total Magnetic Intensity (TMI) - nT
• tmi_hg_enhanced.grd TMI with Horizontal Gradient enhancement (nT)
• tmi_igrf.grd IGRF Correcxted TMI (nT)
• tmi_igrf_1VD.grd First Vertical Derivative of TMI (nT/m)
• tmi_igrf_analytical_signal.grd Analytical Signal of TMI-( nT/m)
• tmi_igrf redp.grd Reduction to the Pole of TMI (nT)
Radiometric Grids
• Potassium masked.grd Processed Potassium with offshore data masked out (%)
• Potassium.grd Processed Potassium (%)
• Thorium masked.grd Processed Thorium with offshore data masked out (ppm)
• Thorium.grd Processed Thorium (ppm)
• Total Count masked.grd Processed Total Count with offshore data masked out (cps)
• Total Count.grd Processed Total Count (cps)
• Uranium masked.grd Processed Uranium with offshore data masked out (ppm)
• Uranium.grd Processed Uranium (ppm)
Other Grid
• Terrain.grd Digital Terrain Model (m)

Final databases

FORMAT OF GEOSOFT MAGNETIC DATABASE
Channel Description Units
X Easting (X) in WGS84 UTM Zone 30N m
Y Northing (Y) in WGS84 UTM Zone 30N m
X_BNG Easting (X) OSGB 1936 m
Y_BNG Northing (Y) OSGB 1936 m
Lat Latitude in WGS84 ddd.mm.ss.ss
Lon Longitude in WGS84 ddd.mm.ss.ss
fid Fiducial number
flight Survey flight number
date Survey flight date yyyy/mm/dd
humidity Humidity Degrees C
Fluxgate X Fluxgate X millisecs
Fluxgate Y Fluxgate Y millisecs
Fluxgate Z Fluxgate Z millisecs
Igrf International Geomagnetic Reference Field nT
diurnal Magnetic Ground Base Station nT
diurnal_igrf IGRF Corrected Magnetic Ground Base Station nT
gps_time Time in seconds after midnight seconds
gps_height Altitude above WGS84 Datum metres
altimeter Radar altimeter height from surface metres
pressure Outside air pressure mb
temperature Outside air temperature Degrees C
compensated_mag_left_sensor Total Magnetic Intensity (compensated) nT
compensated_mag_right_sensor Total Magnetic Intensity (compensated) nT
compensated_mag_tail_sensor Total Magnetic Intensity (compensated) nT
raw_horizontal_gradient Horizontal Gradient (compensated) nT
processed_horizontal_gradient Horizontal Gradient (processed) nT/m
levelled_mag Total Magnetic Intensity (Levelled) nT
levelled_mag_igrf_corrected Residual Magnetic Intensity (Levelled) nT
inclination Magnetic Inclination degrees
declination Magnetic Declination degrees
terrain Calculated digital terrain model m
srtm_data SRTM m
FORMAT OF GEOSOFT RADIOMETRIC DATABASE
Channel Description Units
X Easting (X) in WGS84 UTM Zone 30N m
Y Northing (Y) in WGS84 UTM Zone 30N m
X_BNG Easting (X) OSGB 1936 m
Y_BNG Northing (Y) OSGB 1936 m
fid Fiducial number
flight Survey flight number
date Survey flight date yyyy/mm/dd
STime Sample time millisecs
LTime Live time millisecs
gps_time Time in seconds after midnight seconds
gps_height Aircraft height above geoid metres
pressure Outside air pressure mb
temperature Outside air temperature Degrees C
cosmic Cosmic radiation counts
uranium_up Upward looking Uranium counts
normalised_total_count Total count (normalised) counts
normalised_total_count_nasvd_processed NASVD corrected total count (normalised) counts
processed_total_count_nasvd Final corrected total count cps
normalised_potassium_nasvd Potassium (normalised) counts
normalised_potassium_nasvd_processed NASVD corrected potassium (normalised) counts
processed_potassium_nasvd Final corrected potassium  %
normalised_uranium Uranium (normalised) counts
normalised_uranium_nasvd_processed NASVD corrected uranium (normalised) counts
processed_uranium_nasvd Final corrected uranium ppm
normalised_thorium Thorium (normalised) counts
normalised_thorium_nasvd_processed NASVD corrected thorium (normalised) counts
processed_thorium_nasvd Final corrected thorium ppm
raw_256_down Downward looking radiometric spectra
raw_256_up Upward looking radiometric spectra
FORMAT OF GEOSOFT TESTLINE DATABASES
Channel Description Units
X Easting (X) in WGS84 UTM Zone 30N m
Y Northing (Y) in WGS84 UTM Zone 30N m
Lat Latitude in WGS84 ddd.mm.ss.ss
Lon Longitude in WGS84 ddd.mm.ss.ss
fid Fiducial number
flight Survey flight number
date Survey flight date yyyy/mm/dd
gps_time Time in seconds after midnight seconds
gps_height Aircraft height above geoid metres
altimeter Radar altimeter height from surface metres
pressure Outside air pressure mb
temperature Outside air temperature Degrees C
STime Sample time millisecs
LTime Live time millisecs
TC_nasvd Total count (normalised) counts
K_nasvd Potassium (normalised) counts
U_nasvd Uranium (normalised) counts
Th_nasvd Thorium (normalised) counts
TC_processed Final corrected total count cps
K_processed Final corrected potassium  %
U_processed Final corrected uranium ppm
Th_processed Final corrected thorium ppm
raw_256_down Downward looking radiometric spectra
raw_256_up Upward looking radiometric spectra

Reference

  1. Minty, B R S. 1992. Airborne gamma-ray spectrometric background estimation using full spectrum analysis. Geophysics, Vol 57, No. 2, PP. 279–287.