SkyTEM Survey: 

Colorado, USA 

Data report 

 

Client: USGS OAG Denver Acquisition Branch 

Date: January 2012 

SkyTEM Surveys ApS, Tofteledet 18, 8330 Beder, Denmark, +45 8620 2050 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 2

Contents 

Introduction ...................................................................................................... 3

Definition of the areas ........................................................................................ 6

Instruments and parameter setup ........................................................................ 6

Synchronizing the data ....................................................................................... 7

Calibration of the TEM system .............................................................................. 7

Special note on Calibration (50/60 Hz) .............................................................. 7

Calibration at the National Danish Reference Site ................................................ 9

High altitude test ............................................................................................. 10

Data acquisition ............................................................................................... 12

Ground Base Stations ....................................................................................... 16

DGPS base station ........................................................................................ 16

Magnetometer base station ............................................................................ 16

Flight reports ................................................................................................... 17

Daily production diary, including weather conditions.......................................... 17

Processed data ................................................................................................ 18

EM processing .............................................................................................. 19

Tilt processing .............................................................................................. 19

Height processing ......................................................................................... 19

DGPS processing .......................................................................................... 20

Digital elevation model .................................................................................. 20

EM GDB-files ................................................................................................ 22

Presentation of GDB-files ............................................................................... 24

Mag processing ............................................................................................. 24

Processing of base station magnetic data ......................................................... 25

Processing and Filtering of airborne magnetic data ............................................ 25

Corrections to the magnetic data .................................................................... 25

IGRF correction ............................................................................................ 26

Tie-line levelling and micro-levelling of magnetic data ....................................... 26

TMI recalculation .......................................................................................... 27

MAG GDB-files .............................................................................................. 28

Gridding of magnetic data .............................................................................. 29

Inversion of SkyTEM data .................................................................................. 31

Initial model and optimization norm ................................................................ 37

Regularization .............................................................................................. 39

Noise model ................................................................................................. 39

Inversion results ........................................................................................... 40

Presentations - Model sections and grids .......................................................... 41

Residuals ..................................................................................................... 41

References ...................................................................................................... 44

Appendix list .................................................................................................... 45

Appendix 1: Instruments ............................................................................... 45

Appendix 2: Time gates ................................................................................. 45

Appendix 3: Calibration ................................................................................. 45

Appendix 4: Control parameters ..................................................................... 45

Appendix 5: Modelling and inversion of TEM data .............................................. 45

Appendix 6: Inversion results ......................................................................... 45

Appendix 7: Digital data ................................................................................ 45

 
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SkyTEM Survey Colorado – January 2012 3

Introduction 

This data report covers the data acquisition pertaining to a time domain 

electromagnetic and magnetic survey carried out in Colorado, USA, by SkyTEM 

Surveys ApS in 2011. 

Contact person at SkyTEM: Bill Brown Tel. +1 519-502-1436 

Email: bb@skytem.com 

From October 11th to 23rd 2011 a combined time domain electromagnetic and 

magnetic survey was performed by SkyTEM Surveys ApS in Colorado, USA. The survey 

outlines seen in Figure 1 represent the Paradox, Buckey, and Doloras Canyon survey 

areas and are outlined in red, orange, and yellow respectively (line spacing 150 m). 

Figure 2 displays the survey outline for the SanLuis project as a single red polygon 

(line spacing 300 m). 

Figure 1 Project overview with the location of the Paradox survey area. The Paradox, 

Buckey, and Doloras Canyon survey areas are outlined in red, orange, and 

yellow respectively. UTM Zone 12N NAD83 
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SkyTEM Survey Colorado – January 2012 4

Figure 2 Project overview with the location of the San Luis survey area. The survey 

boundary is displayed as a red outline. UTM Zone 13N NAD83. 

The survey requested by USGS OAG Denver Acquisition Branch was planned to consist 

of 2657 line kilometers in total. 
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SkyTEM Survey Colorado – January 2012 5

SkyTEM Surveys ApS has agreed to deliver the electromagnetic and magnetic raw data 

measured during the flights together with the standard SkyTEM processing and 

inversion. 

This report does not include any geological interpretations of the geophysical datasets. 

Client USGS OAG Denver Acquisition Branch 

PO Box 25046 

204 Denver Federal Center, Denver CO 

8225-0046 

Field crew Ib Faber 

Jean Christophe Ricard 

Field work October 11th to 23rd 2011 

Flown line km 2732 km 

Flight operation 

 

Helicopter type 

Average flight 

speed 

Nominal terrain 

clearance (above 

any obstacles or 

hazards) 

Eurocopter AS350FX2, operated by 

Abitibi Helicopters Ltd 

50-80 km/h 

30 - 40 m 

Pilot Pierre Otis 

Report Data processing 

and presentation 

QC by 

Russell John Eade 

Rasmus Teilmann 

Contact Person at 

SkyTEM 

 Bill Brown 

Email: bb@skytem.com 
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SkyTEM Survey Colorado – January 2012 6

Definition of the areas 

The survey is obtained by the two areas referred to as San Luis and Paradox (Figure 1 

and Figure 2). In addition to this calibration lines are performed in connection with 

both areas. The calibration lines are described in detail below in Data Acquisition. 

Coordinate systems used are UTM Zone 12N (Paradox) / 13N (San Luis) NAD83. 

Instruments and parameter setup 

The instrumentation includes a time domain electromagnetic system: a data 

acquisition system, a magnetometer, two DGPS’, two inclinometers and two altimeters 

(Figure 3). 

A more thorough description of the setup is given in Appendix 1. 

The equipment setup operates in dual moment configuration including a Low moment 

(LM) with a peak moment of ~3,140 NIA and a High Moment (HM) with a peak 

moment of ~150,000 NIA. 

The main benefit of the dual moment system is the concurrent measurement of early 

time gates (LM mode) and late time gates (HM mode), providing near-surface solution 

as well as deep penetration. 

Figure 3 Sketch showing the frame and the position of the instruments. The red line 

defines the transmitter loop. The horizontal plane is defined by (x, y). 

The location of instruments in respect to the frame is shown in Figure 3. 

 

X and Y define the horizontal plane. Z is perpendicular to (X, Y). X is positive in the 

flight direction, Y is positive to the right of the flight direction, and Z is positive 

downwards. 
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SkyTEM Survey Colorado – January 2012 7

The DGPS systems are mounted in the front of the frame. 

The generator used for powering the transmitter is positioned 10 m below the 

helicopter. 

A more thorough description of the system and individual instruments can be found in 

ref /1/ and Appendix 1. 

Synchronizing the data 

All recorded data are marked with a time stamp used to link the different data types. 

The time stamp is in UTC/GMT. 

The time stamp formats are either 

1. yyyy/mm/dd hh:mm:ss.sss – Values defined as 

year/month/day/hours/minutes/seconds. 

or 

2. Ddddd.ssssssssss - Datetime values defined as the number of days since 

1900-01-01 and seconds of the day. 

Calibration of the TEM system 

Special note on Calibration (50/60 Hz) 

Due to the fact that the electrical power supply grid in North America runs with a

frequency of 60Hz, whereas the European grid uses 50 Hz, the calibration at the 

Danish National Reference site has not been conducted with the exact same timing for 

the transmitter and receiver (referred to as “the script”). This is done in order to avoid 

noise from the 50 Hz power grid while calibrating the system. 

The following table describes the difference between the script used for calibration in 

Denmark and the script used for production in North America. 

Parameter 50 Hz script 60 Hz script

ON-time HM 10000 μs 8000 μs 

OFF-time HM 10000 μs 8667 μs 

ON-time LM 800 μs 800 μs 

OFF-time LM 1450 μs 1283 μs 

Base frq. HM 25 Hz 30 Hz 

Base frq. LM 222.2 Hz 240 Hz 
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The calibration parameters found at the reference site are not dependent on the timing 

and can be used regardless of the frequency setup. The following paragraphs and 

Appendix 3 hence refer to the 50 Hz script calibration, but the parameters are valid for 

the 60 Hz script as well. 
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SkyTEM Survey Colorado – January 2012 9

Calibration at the National Danish Reference Site 

The complete SkyTEM equipment has been tested and calibrated at the Danish 

National Reference Site in March 2011. 

The calibration includes measurements of the transmitter waveform and data level in 

different altitudes. By these measurements it has been documented that the 

instrumentation can reproduce the reference site using constant calibration 

parameters independent of the flight altitude. 

The calibration results and parameters are shown below: 

Low moment: 

Shift factor: 0.96 (on the raw dB/dt data) 

Time shift: -1.1e-6 

High Moment: 

Shift factor: 0.96 (on the raw dB/dt data) 

Time shift: -1.1e-6 

All data has been processed using the above stated calibration parameters. 

SkyTEM inversion software (iTEM) handles time shift calibration during import of data. 

If third party processing or inversion software is used the calibrated gate center times 

in Appendix 2 should be used. 

The waveforms, as well as the reproduced soundings in different altitudes, are shown 

in Appendix 3. 
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High altitude test 

A high altitude test was performed on November 11'th, 2011 prior to survey flying in 

which the system was raised above 1000 masl. The same equipment and configuration 

was used for the duration of the survey. 

The test was performed in order to establish that system signal to noise level was 

below an acceptable level on all used gates, and that no drift was present in the 

system. The data acquired was compared to internal standards and was well below the 

maximum allowable signal to noise level. 

Figure 4 displays the data used for the high altitude test. Test data is acquired at a 

height of 1000m above ground level over a period of at least 60 second, often longer. 

Each decay collected at high altitude can be seen in Figure 4 as a grey curve. The solid 

blue line represents the mean of all decays and the sign of each gate is indicated by 

the colored circle with positive and negative values being represented by red and blue 

circles respectively. The dashed blue line indicates the standard deviation of the data 

calculated for each gate. Gates 5 to 26 are displayed in the LM plot and gates 18 to 34 

are displayed in the HM plot. 

In order to confirm that no drift was present in the system ‘semi’ high altitude data is 

collected on each flight. This is done by raising the system to 400m above ground 

level. Inspection of the semi high altitude data from each flight occurs on a daily basis. 

It has been observed that the system reproduces itself and that no drift is present in 

the system. 
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SkyTEM Survey Colorado – January 2012 11

Figure 4 High altitude tests data collected on November 11'th, 2011 at 1000 masl. A 

comparison of all decays acquired during the high alt test (grey curves) with 

the mean response (solid blue curve) and standard deviation (dashed blue 

curve) for each gate. The sign of each gate is indicated by the color fill of the 

circle with positive and negative values being represented by red and blue 

color fill respectively. The LM plot displays Gates 5 to 26 while the HM 

displays gates 18 to 34. 
Data acquisition 

The planned flight lines covering the Colorado area are shown in Figure 5 and Figure 6 

(blue lines). The in-lines of San Luis strike 70 degrees (WSW/ENE), line spacing is 

300 m. The Paradox area is obtained by two sub-areas, the in-lines are oriented 54 

degrees (NW/SW) in the northern area and 44 degrees (NW/SW) in the southern area, 

line spacing is 150 m. Calibration lines (calc-lines) are performed in connection with 

both areas. The table below contains an explanation of the flight line numbers. 

Block In-line Tie-Line Calc-line 

San Luis 

- Regional 

100101 - 110801 

300101 - 300501 

200201 - 200401 990101 - 990106 

Paradox 

-Buckey 

-Doloras 

-Regional 

100202 - 106302 

150101 - 152401 

350101 – 351301 

600100 - 600103 

200101 - 200801 

250101 - 250301 

990101 - 990106 

The nominal terrain clearance is 30 m above any obstacles or hazards, with an 

increase over forests, power lines, etc. The safe flying height during survey is always 

based on the pilot’s assessment of risk and deviations from nominal values are at the 

discretion of the pilot. 

Figure 5 Planned flight lines (blue) for the Paradox survey area. Coordinate system: 

UTM Zone 12N NAD83. 
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SkyTEM Survey Colorado – January 2012 13

Figure 6 Planned flight lines (blue) for the San Luis survey area. Coordinate system: 

UTM Zone 13N NAD83. 
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SkyTEM Survey Colorado – January 2012 14

The planned helicopter airspeed is 85 km/h for flat topography with no wind. This may 

vary in areas of rugged terrain and/or windy conditions. 

 

The surveyed flight path (what was actually flown) can be seen as red lines in Figure 7 

for the Paradox area and Figure 8 for the San Luis area. Discrepancies from the 

planned lines occur when possible noise sources are present or cultural features such 

as roads, buildings, and antennas necessitate a diversion. This can also result in the 

survey lines being broken into pieces and flown in multiple passes or flights. 

Figure 7 Red lines represent the GPS recorded flight path of the survey instrument 

while blue lines indicate the planned flight lines. Coordinate system: UTM 

Zone 12N NAD83. 
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SkyTEM Survey Colorado – January 2012 15

Figure 8 Red lines represent the GPS recorded flight path of the survey instrument 

while blue lines indicate the planned flight lines. Coordinate system: UTM 

Zone 13N NAD83. 

 
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SkyTEM Survey Colorado – January 2012 16

Ground Base Stations 

The DGPS and magnetic base stations were positioned at the closest accessible place 

to the survey areas. 

DGPS base station 

Utmost effort was made to ensure that the DGPS base station was placed at a location 

of maximum possible view to satellites and out of any metallic objects that could 

influence the GPS antenna. 

Table showing DGPS base station location (Lat/Lon (WGS84)): 

Area Lat Lon Ell. Height

San Luis 37° 41' 38.84" -105° 32' 56.49" 2403.5 m

Paradox 38° 19' 10.03" -108° 51' 07.40" 1497.6 m

Magnetometer base station 

Great effort was made to ensure that the base station magnetometer was placed in a 

location of low magnetic gradient, away from electrical transmission lines and moving 

metallic objects, such as motor vehicles and aircrafts. 

The location of the magnetic base stations can be seen in the table below (Lat/Lon, 

WGS84, degrees, minutes, seconds). 

Magnetometer 

Base station 

Lat Lon 

San Luis 37° 41' 39.11" -105° 32' 55.99" 

Paradox 38° 19' 10.50" -108° 51' 07.94" 

 
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Flight reports 

For each flight, a report with key information regarding the data gathering was made. 

Listed in the reports are details on the weather, special data parameters and other 

events which may influence the data. Selected information from the flight reports are 

shown in the table below: 

Daily production diary, including weather conditions 

Date Temperat

ure (0C) 

Wind 

(m/s) 

Visibility Description 

20111011.06 14-18 4 - 6 Good Production

Calibration and recon 

20111012.01 14-18 2 - 6 Good Production 

20111012.02 14-18 1 - 4 Good Production 

20111012.03 14-18 1 - 4 Good Production 

20111013.01 14-18 1 - 4 Good Production 

20111013.02 3-8 2 - 8 Good Production 

20111013.03 16-20 2 - 8 Good Production 

20111014.01 16-20 1 - 4 Good Production 

20111014.02 4-10 1 - 4 Good Production 

20111014.03 12-15 2 - 4 Good Production 

20111015.01 14-18 1 - 3 Good Production 

20111015.02 3-8 1 - 4 Good Production 

20111015.03 15-20 1 - 4 Good Production 

20111016.01 15-22 1 - 2 Good Production 

20111016.02 5-12 3 - 7 Good Production 

20111020.01 18-22 1 - 3 Good Production 

20111020.02 5-19 2 - 4 Good Production 

20111020.03 15-22 4 - 7 Good Production 

20111021.02 15-22 4 - 7 Good Production 

20111021.03 15-22 4 - 7 Good Production 

20111022.01 15-22 4 - 7 Good Production 

20111022.02 15-22 4 - 7 Good Production 

20111022.03 15-22 4 - 7 Good Production 

20111023.01 15-22 4 - 7 Good Production 

20111023.02 15-22 4 - 7 Good Production 
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Processed data 

All files needed for processing of EM data in Aarhus Workbench are delivered digitally, 

including, EM data files (skb-files), auxiliary data files (sps-files), mask file (lin-file) 

and geometry file (geo-file) describing all settings and the geometry of the system. 

Selected control parameters are plotted in Appendix 4. The plots contain information 

about the flight altitude, speed, angle of the frame, transmitted current, transmitter 

voltage and transmitter temperature. 

Mean values and standard deviations of control parameters are found in the table 

below. 

 

Control parameter Mean Value Standard Deviation 

San Luis Paradox San Luis Paradox 

Ground speed*) 80.4 km/h 71.8 km/h 10 km/h 13.4 km/h 

Processed height 32 m 42.7 m 7.1 m 15.7 m 

Tilt angle X -2.6 deg. 1.2 deg. 4.5 deg. 7.3 deg. 

Y -0.9 deg. -1.1 deg. 2.2 deg 2.8 deg. 

Tx Voltage**) Tx_off 70 V - 

Tx_on 67 V - 

Low moment Current**) 9.46 A 9.41 A 0.1 A 0.1 A 

High Moment Current**) 111.9 A 110.9 A 1.4 A 1.4 A 

Tx temperature**) 45 °C - 

*) Actual speed varies as a function of day and flight direction due to different wind directions 

and magnitude. 

**) Few spikes are seen in the temperature, current and voltage data. These are not caused by 

errors in the instruments but are a matter of digital drop outs. 
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SkyTEM Survey Colorado – January 2012 19

EM processing 

In the following the processing and inversion performed by SkyTEM Surveys are 

presented. 

All data are resampled to 10 Hz using the SkyTEM in-house software SkyPRO. 

The data are normalized in respect to effective Rx coil area, Tx coil area, number of 

turns and current giving the unit: pV/(m4·A). 

The raw HM EM data are filtered using a third order polynomial filter with varying filter 

width increasing at late gate times. 

The raw LM EM data are filtered using a Box-car filter with a width of 3.6 s 

All auxiliary devices (DGPS, Laser altimeters, inclinometers) are moved to the center 

of the frame as based on the values stated in Appendix 1. 

After merging auxiliary data together with EM data in SkyPRO additional filters in Oasis 

Montaj Geosoft has been applied. This include for both LM and HM: 

1. Gaps from HM/LM series are interpolated using B-Spline filter 

a) Smoothness= 0.55 

b) Tension= 0.0 

2. Transferring data channels into Oasis Montaj Geosoft Array channels 

Tilt processing 

The X and Y angle processing involves manual and automated routines using a 

combination of the SkyTEM in-house software SkyPRO and Oasis Montaj Geosoft. 

The processing involves the following steps: 

1. 3 sec box filter (SkyPRO) 

2. Manual editing for spikes (Geosoft) 

3. Akima interpolation of edited gaps (Geosoft) 

4. Low pass filtering of 4.0 sec. (Geosoft) 

Height processing 

The height processing involves manual and automated routines using a combination of 

the SkyTEM in-house software SkyPRO and Oasis Montaj Geosoft. 

The processing involves the following steps: 

1. Keeping the 2 highest values pr. second and discarding the rest to correct for 

the canopy effect (treetop filter) 

2. 2 sec running box filter (smoothing filter) 

3. Tilt correction 
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SkyTEM Survey Colorado – January 2012 20

4. Averaging of the two laser values 

5. Additional filters in Geosoft involving: 

a. Editing of spurious data (i.e. missing data over lakes etc.) 

b. Small data gaps interpolated (Akima interpolation) 

c. Low pass filter of 4.0 sec 

DGPS processing 

The DGPS has been processed using the Waypoint GrafNav Lite Differential GPS 

processing tool. The standard airborne settings have been used. 

1. Import of base station (Master) 

2. Import of Airborne files (Rover) 

3. Calculation of forward and reverse DGPS solution 

4. Export as .txt file 

The DGPS.txt files are used as input to the SkyPRO software assuring DGPS corrected 

data in the processed files. 

The ground speed, altitude, latitude and longitude from the processed DGPS are 

merged into the final GDB. Afterwards the coordinates are transformed into UTM Zone 

12N (Paradox) / 13N (San Luis) NAD83. 

A low pass filter of 4.0 sec has been applied to the above mentioned parameters. 

Digital elevation model 

A digital elevation model (DEM) channel has been calculated by subtracting the filtered 

laser altimeter data from the DGPS elevation. 

The Processing to the final DEM involves the following steps: 

1. Filtering and processing of the laser altimeter height as described above 

2. DEM data received by subtraction of final filtered laser data from final 

processed DGPS altitude data 

3. Grids produced using the minimum curvature method – grid cell sizes are 50 

m for Paradox, and 100 m for San Luis. 

The DEM channel was gridded (see Figure 9 and Figure 10) in Geosoft format and 

included in the data delivery catalogue. 
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SkyTEM Survey Colorado – January 2012 21

Figure 9 Digital Elevation Model of meters above sea level, cell size 50m UTM Zone 

12N NAD83.
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SkyTEM Survey Colorado – January 2012 22

Figure 10 Digital Elevation Model of meters above sea level, cell size 100m UTM Zone

13N NAD83.

EM GDB-files 

The EM GDB files are the final result of the SkyTEM survey, containing all the collected 

and processed EM data and information used for the interpretation and inversion. 

Data in the files are split at the beginning and end of each planned flight line. 

The raw EM data and auxiliary data are filtered and processed as described above. All 

parameters in the GDB-file hence refer to the origin of the frame. 

The GDB can be used as input for further processing and gridding and as input to 

inversion and interpretation software. 

The projection of the GDB is given as Latitude/longitude, WGS84 and UTM Zone 12N 

(Paradox) / 13N (San Luis) NAD83. 
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SkyTEM Survey Colorado – January 2012 23

The header of the EM GDB-file gives the following information: 

Parameter Explanation Unit 

Fid Unique Fiducial number seconds 

Line Line number LLLLLL 

Flight Name of flight yyyymmdd.ff 

DateTime DateTime format Decimal days 

Date Date yyyymmdd 

Time Time hhmmss.zzz 

AngleX Angle in flight direction Degrees 

AngleY Angle perpendicular to flight direction Degrees 

Height Filtered height measurement Meters 

DEM Digital Elevation Model Meters above 

mean sea level 

Lon Latitude/longitude, WGS84 Decimal degrees 

Lat Latitude/longitude, WGS84 Decimal degrees 

E UTM Zone 12N (Paradox) / 13N (San 

Luis) NAD83 

Meter 

N UTM Zone 12N (Paradox) / 13N (San 

Luis) NAD83 

Meter 

Alt DGPS Altitude Meters above 

mean sea level 

GdSpeed Ground Speed [km/h] 

Curr_1 Current, high moment Amps 

Curr_2 Current, low moment Amps 

LM_Z_G5[xx] Normalized LM Z-coil value: gate 5-

26. [xx] refer to Geosoft array 

channel number* 

pV/(m4*A) 

HM_Z_G18 [xx]** Normalized HM Z-coil value: gate 16-

34. [xx] refer to Geosoft array 

channel number*

 

pV/(m4*A) 

LM_X_G5[xx] Normalized LM X-coil value: gate 10-

26. [xx] refer to Geosoft array 

channel number* 

pV/(m4*A) 

HM_X_G18[xx]** Normalized HM Z-coil value: gate 18-

34. [xx] refer to Geosoft array 

channel number* 

pV/(m4*A) 

*) If Geosoft array channels are exported, the numbers in the brackets starts from [0]. I.e. 

LM_Z_G5[4] corresponds to LM Z gate 9. The grids are named after the actual gate number I.e. 

InnerBoxHM_Z_G21_Mask.grd.gi is Inner Box, high moment Z, gate 21 (Gate21=Z[20])of the EM 

channels. 
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SkyTEM Survey Colorado – January 2012 24

Presentation of GDB-files 

High and low moment z coil gates from the GDB-file have been exported as Geosoft 

.grd files. The files are included in the data delivery catalogue. Figure 11 shows an 

example of the HM data. 

Please note that no height correction has been applied to the raw EM data. This can 

cause striations in the data set when looking at the grids. This is due to the fact that 

variations in height will change the magnitude of the EM signal. 

Figure 11 Plot of raw HM Z coil data from Gate 21 of the Outer Box. Gate plots can be 

found as Geosoft Montaj .grd files in the data delivery catalogue. Warm colors 

(red) represent high signal and cold colors (blue) represent low signal. UTM 

Zone 12N NAD83. 

Mag processing 

Final processing of the magnetic data involved the application of traditional corrections 

to compensate for diurnal variation and heading effects prior to gridding. Advanced 

full processing of magnetic data was implemented in Geosoft’s Oasis Montaj software 

as follows: 
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SkyTEM Survey Colorado – January 2012 25

• Processing of static magnetic data acquired on magnetic base station 

• Pre-processing of airborne magnetic data 

o Stacking of data from 60 Hz to 10 Hz in SkyPro. 

o Moving positions to the center of the sensor in SkyPro. 

o Adapting auxiliary data channels from EM GDB (processed height, 

Angles, Speed and DEM) 

• Processing and filtering of airborne magnetic data 

• Standard corrections to compensate the diurnal variation and heading effect 

• IGRF correction 

• Levelling 

• Gridding 

Processing of base station magnetic data 

The base station magnetometer data was transferred into the base station Geosoft 

GDB database on a daily basis for further processing. A non-linear filter was applied to 

remove spikes and a low-pass filter used to smooth the magnetic data. 

IGRF was calculated and subtracted from TMI data to obtain residual magnetic field 

and remove secular variation. 

Diurnal variation was calculated from residual magnetic field by subtracting the mean 

value averaged from all observations received on magnetic base station in course of 

the survey. 

Processing and Filtering of airborne magnetic data 

No spikes or data out of range was observed on airborne TMI data therefore no 

manual editing or non-linear filtering of the data was required. TMI data was filtered 

and interpolated as follows: 

 Adjacent record at the beginning and end of each 0.3 sec gap in magnetic data 

not measured during low moment TEM data acquisition was deleted. These 

records may still be influenced by B-field generated during low moment TEM 

data acquisition. 

 Bi-cubic spline (tension of 1.0 and smoothness of 0.6 was applied as low-pass 

filter – this filter also interpolates the gaps in magnetic data not acquired 

during low moment TEM data acquisition (0.3 sec gaps) 

Corrections to the magnetic data 

The processing of the data involved the application of the following corrections: 

• Airborne magnetometer data was corrected for diurnal variations. Calculated 

diurnal variation was subtracted from the filtered airborne magnetic data. 
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SkyTEM Survey Colorado – January 2012 26

• No time lag correction is necessary since the positions are shifted to the sensor 

location to account for the distance between the GPS position and the position 

of the magnetic sensor. 

The heading correction test flown during the survey shows the heading errors as 

indicated in the following table. 

Direction Heading Correction 

0 deg -0.294 

90 deg -0.613 

180 deg 0.128 

270 deg 0.779 

The coefficients listed above were so low that no heading correction was applied to the 

data.

IGRF correction 

The International Geomagnetic Reference Field (IGRF) is a long-wavelength regional 

magnetic field calculated from permanent observatory data collected around the world. 

The IGRF is updated and determined by an international committee of geophysicists 

every 5 years. Secular variations in the Earth’s magnetic field are incorporated into 

the determination of the IGRF. The IGRF model for all blocks was calculated before 

levelling using the following parameters for the survey area: 

IGRF model year: IGRF 11th generation 

Date: variable according to date channel in database 

Position: variable according to GPS WGS84 longitude and latitude 

Elevation: variable according to magnetic sensor altitude derived from DGPS data 

Tie-line levelling and micro-levelling of magnetic data 

After applying the above corrections to the profile data, statistical levelling of control 

lines followed by full levelling of traverse lines and micro-levelling is usually applied as 

a standard procedure. The following steps were adapted on the data: 

• Statistical levelling on control lines applied 

• Statistical levelling on trend lines applied 

• Full levelling on traverse lines applied 

• Micro levelling applied on traverse lines 

o Decorrugation cutoff wavelength: 

3600 m /1200 m / 700 m (San Luis/Paradox / Buckley) 

o Max amplitude limit: 

2 nT / 1.5 nT / 1.0 nT (San Luis/Paradox / Buckley)

o Naudy filter: 
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SkyTEM Survey Colorado – January 2012 27

800 / 300 / 300 (San Luis / Paradox / Buckley) 

The corrected data were then used to generate the final grids free of line directional 

noise. 

TMI recalculation 

Residual magnetic field (RMF) was the outcome of processed magnetic data after all 

corrections and levelling was applied. Total magnetic intensity was recalculated to add 

back the IGRF using the following parameters. 

IGRF model year: IGRF 11th generation 

Date: variable as flown 

Position: variable according to GPS WGS84 longitude and latitude 

Elevation: variable according to magnetic sensor altitude derived from DGPS data 
MAG GDB-files 

The GDB files are the main result of the magnetic survey, containing all the processed 

magnetic data and information for the interpretation and gridding. The projection of 

the GDB-files are in NAD83, UTM Zone 12N (Paradox) / 13N (San Luis) NAD83. The 

header of the magnetic GDB-file gives the following information: 

Channel Name Description Units 

Line Line number LLLLLS 

Flight Flight number YYYYMMDD.FF 

Date UTC date YYYYMMDD 

Time UTC time HH:MM:SS.S 

Lon Longitude using WGS84 datum Decimal-deg. 

Lat Latitude using WGS84 datum Decimal-deg. 

E Easting in UTM Zone 12N (Paradox) / 

13N (San Luis) NAD83 

Meter 

N Northing in UTM Zone 12N (Paradox) / 

13N (San Luis) NAD83 

Meter 

Alt Mag sensor GPS altitude – mean sea 

level altitude – geoid EGM96 

Meter 

Height Processed laser altimetry – mag sensor 

above ground level 

Meter 

DEM Calculated digital elevation model – 

mean sea level 

Meter 

IGRF_TMI calculated IGRF-11 - total magnetic 

intensity 

nT 

IGRF_Inc calculated IGRF-11 - magnetic inclination Degrees 

IGRF_Dec calculated IGRF-11 - magnetic 

declination 

Degrees 

Bmag_TMI Total Magnetic Intensity – raw magnetic 

data – magnetic base station 

nT 

Bmag_diur Diurnal variation– magnetic base station 

data 

nT 

mag_raw raw magnetic data – total magnetic 

intensity - despiked 

nT 

Mag_cor residual magnetic field - corrected for 

diurnal, lag, heading and IGRF-11 

nT 

RMF Residual magnetic field – IGRF removed 

- final corrected and levelled magnetic 

data 

nT 

TMI Total magnetic intensity – final corrected 

and levelled magnetic data; IGRF 

recalculated. 

nT 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 29

Gridding of magnetic data 

The corrected data was used to generate the Residual Magnetic Field (RMF) and Total 

Magnetic Intensity (TMI) grid. Corrected magnetic line data was interpolated between 

survey lines using a minimum curvature gridding algorithm to yield x-y grid values for 

a standard grid cell size of 50/100 . A Hanning filter was used to remove residual 

noise. 

Figure 12 (Paradox) and Figure 13 (San Luis) both show gridded data after processing 

data from the magnetometer. All grids from the areas (RMF and TMI) can be found in 

the data delivery folder. 

Figure 12 RMF grid of San Luis, cell size 100m. UTM Zone 13N NAD83. 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 30

Figure 13 RMF grid of Paradox, cell size 50 m. UTM Zone 12N NAD83. 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 31

Additional EM processing 

30Hz Noise Investigation 

During the inspection of the acquired data, an anomalous and detrimental signal 

behavior was identified in the high moment TEM data of both the Paradox and San Luis 

target areas. A thorough investigation was carried out in order to identify the cause of 

this problem and to assess the options for removing it through post processing. In the 

following, data from the flight 20111022.02 measured in the Paradox survey area are 

used for the presented examples. The reason for choosing this specific dataset for 

presentation is that it contains the most anomalous signal behavior in the entire 

Paradox area. 

 

Identifying the problem 

Figure 14 displays an interval of high moment data with low signal levels, which has 

been smoothed along every gate by a spline interpolator. All of the presented gates 

display an oscillatory behavior with a variable period ranging from just a few to several 

seconds. These oscillations are related neither to geological variability nor carrier 

height and angle variations. Furthermore, the oscillations are clearly correlated 

between gates, and they are especially evident in the latest gates which contain little 

to no earth response. Regular low pass filtering of the data is unsuited to remove 

these oscillations due to their low frequency character. Such averaging filters would 

necessarily be very wide and thereby significantly distort the features of the target 

earth response. 

 

Figure 14 An example of the oscillatory signal behaviour. Many of the initial inversions 

exhibited numerous narrow "conductive" and "resistive" artefacts due to this. 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 32

 Figure 15 HyTEM inversion result of affected data on line 104502 illustrating the 

general way in which these oscillations effect the inversion result introducing 

narrow "conductive" and "resistive" artefacts. 

Noise signal characteristics 

A spectral analysis tool applied to the latest gate (gate 26) of the acquired low 

moment Z data was developed in order to obtain an image of the frequency content of 

the noise signal and its temporal variations. The low moment measurements have 

been performed in blocks of 120 consecutive measurements, effectively setting an 

upper limit on the achievable frequency resolution. The period between transmitter 

pulses was 2.083 ms yielding a sampling frequency of 480Hz. The resulting Nyquist 

frequency allows us to resolve frequencies from 0 to 240 Hz, however, aliasing 

phenomena may occur as the dominant low-pass filtering effect is due to the gate 

integration (the gate width is 258.6 μs for gate 26) which is insufficient to avoid 

aliasing. The low moment data were acquired using a sign pattern of [+ + – –] 

meaning that the earth response appears as a signal component at 120 Hz in the 

resulting spectrograms. Figure 16 displays a time interval of a typical spectrogram 

where the power line noise at 60 and 180 Hz is clearly visible, while a spurious and 

persistent noise component is visible at, or near, 30 Hz which appears to be correlated 

with the 60 Hz signal. 

Figure 16 A typical spectrogram over a 900 second interval of survey flying. The power 

line noise at 60 and 180 Hz is clearly visible, while a spurious and persistent 

noise component is visible at, or near, 30 Hz which appears to be correlated 

with the 60 Hz signal.
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 33

The averaged low moment measurements are unaffected by the 30 Hz signal because 

all signal components away from 120 Hz are heavily attenuated by the applied 

synchronous stacking. 

Unfortunately, the high moment measurements are performed using a sign pattern of 

[+ –] and a period between transmitter pulses of 16.67 ms yielding a sampling 

frequency of 60 Hz. The signal is therefore located at the Nyquist frequency of 30 Hz, 

which is deliberately done in order to maximize power line noise immunity. As 

indicated by Figure 17 synchronous stacking of the high moment data in the presence 

of a 30 Hz noise signal will introduce a constant offset of each gate with an amplitude 

that depends on the relative phase between the sampling pattern and the noise signal. 

Figure 17 HM gate centre times plotted on a sinusoid wave of 30 Hz.

If the amplitude, frequency or phase of the noise signal changes slowly in time, these 

offset levels will likewise vary and a situation similar to that observed in Figure 14 may 

occur. 

Suppressing the noise signal 

In an effort to determine and subtract the time varying, noise-generated gate offsets, 

an automated sinusoid fitting and subtraction tool was introduced and applied to the 

individual soundings prior to stacking. Assuming a constant noise frequency of 30 Hz, 

while using only the last four gates to calculate a least squares fit (in terms of 

amplitude and phase), it was possible to remove a significant part of the problematic 

oscillations. The most important prerequisite for the successful application of this noise 

compensation technique is that the 30 Hz noise signal must be the dominating signal 

contribution for the last 4 gates. This requires that the earth response is negligible in 

order for the sinusoid fit to be unbiased, while unrelated noise contributions will 

likewise tend to worsen the fit. Examples of fits and resulting compensated soundings 

are demonstrated in Figure 18. 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 34

Figure 18 Examples of random decays displaying original and compensated soundings 

after the 30 Hz signal removal.

In order to avoid removing the earth response in regions with signal in the last four 

gates, a check on the fitted sinusoid amplitude was made and the compensation was 

"turned off" if the sinusoidal amplitude exceeded a certain threshold. 

 

Results and potential issues 

 

Figure 19 shows a "before and after" noise compensation plot of the high moment Z 

data prior to stacking of the principal measurements (a principal measurement being 

the difference between a plus and the subsequent minus measurement). It is clear 

that a large part of the oscillatory noise phenomena has been removed in the lower 

plot. 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 35

 Figure 19 3D illustration of HM Z data before and after compensation (upper and 

lower plots respectively). Each decay is referenced to single point in time 

based on the turnoff for that sounding and is used to plot the data along the y 

axis ‘time(s)’. The centre time for each gate determines the position on the x 

axis ‘time(ms)’ and the amplitude of the gate value is represented by the z 

axis ‘signal amplitude(pV/Am4

)’. 

An example of the resulting improvement in inversion results is shown in Figure 20. 

 Figure 20 This image displays the improvement in the inversion result achieved 

through the use of the sinusoid compensation. The upper profile was 

generated from uncompensated data while the lower profile displays the 

improved results after compensation. Both images have the same colour scale 

applied.

The main concern in using the sinusoid fitting and subtraction procedure is the 

unavoidable small-scale distortion of late gates in conductive areas. The sinusoid 

fitting procedure will inevitably start to fit the earth response and remove it for very 

small levels of earth response before the threshold level is reached, yielding slight 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 36

distortions at the onset and trailing edge of isolated conductive areas. Nevertheless, it 

is found that the data improvement offered by the presented noise compensation 

scheme to significantly outweigh the mentioned drawbacks. 
Inversion of SkyTEM data 

In this section, the particulars of modeling and inversion of SkyTEM data from 

Colorado, USA will be described with reference to the more general material found in 

Appendix 5. The inversion code is named SELMA, ref /2/ and /3/. However, recent 

developments including the lateral parameter correlation, not yet published, have 

enhanced the accuracy of the code. 

Initial model and optimization norm 

The inversion is performed as a regularized, damped, least-squares inversion on 

individual sounding data along the profiles with a one-dimensional (1D), multi-layer 

model (MLM) with 30 layers. In the inversion, the thickness of the layers are kept 

constant and only the layer resistivities are allowed to vary in order to let the model fit 

the measured data. 

To obtain laterally smooth model sections, the Lateral Parameter Correlation (LPC) 

procedure is used (/3/ and /4/). 

In the inversion the thickness of the first layer is 5 m and the depth to the deepest 

layer boundary is 500 m. Thicknesses and depths to top of layers for all layers are 

stated in the table below. In the top of the model, the layer thickness increases slowly, 

giving a linear sampling of the subsurface, while layer thickness increases 

exponentially at the deeper parts of the model. 

The input data to the inversion is the z-component of the EM-data described in the 

chapter ‘Processed data’. 

In the Colorado survey the resistivity of the initial model for the inversion is set to 500 

Ωm. Resistivities are allowed to vary within the interval of 0.1 to 10000 Ωm. 

Optimization is performed using the L2-norm. 

In the Colorado area the inversions are based on a 3 Hz input file giving a model for 

approx. every 4 m. 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 38

Layer # Layer Thickness [m] Layer depth [m] 

1 5.00 0.00 

2 5.06 5.00 

3 5.17 10.06 

4 5.34 15.22 

5 5.56 20.56 

6 5.85 26.12 

7 6.21 31.97 

8 6.63 38.18 

9 7.13 44.81 

10 7.70 51.93 

11 8.36 59.63 

12 9.11 67.99 

13 9.97 77.11 

14 10.93 87.08 

15 12.02 98.01 

16 13.24 110.03 

17 14.60 123.26 

18 16.13 137.86 

19 17.83 153.99 

20 19.74 171.82 

21 21.86 191.56 

22 24.22 213.41 

23 26.85 237.64 

24 29.78 264.49 

25 33.04 294.27 

26 36.66 327.31 

27 40.70 363.97 

28 45.18 404.67 

29 50.16 449.84 

30 N/A 500.00 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 39

Regularization 

A statistical broadband approach is used in the regularization of the multi-layer model. 

Nine different correlation lengths with a maximum of 10 000 km and a standard 

deviation of 1 were used to define the correlation matrix. (See Appendix 5 for more 

detail). 

Noise model 

In the Colorado survey, the noise parameters for both inversions were chosen as: 

Low moment 

V0 = 2.5e-12 in field units normalized with Tx moment

t0 = 1 ms 

slope = -0.5 

High Moment 

V0 = 2.5e-13 in field units normalized with Tx moment

t0 = 1 ms 

slope = -0.5 

Negative data values caused by e.g. capacitive coupling and values lower than 

0.01*noise level, were excluded in the inversion. 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 40

Inversion results 

The results of the inversion are presented in a GDB file included in the data delivery 

catalogue. The file contains the resistivities for each layer in the model. The header of 

the GDB file is described in the table below (also see Appendix 6 for more detail). 

Parameter Explanation Unit 

FID Fiducial number 

LINE Line number 

E UTM Zone 12N (Paradox) / 13N (San 

Luis) NAD83 

Meter 

N UTM Zone 12N (Paradox) / 13N (San 

Luis) NAD83 

Meter 

DTM Digital Elevation Model Meters above mean 

sea level 

ResI1 Residual of data - 

ResI4 Residual total - 

Height Height above ground Meter 

Layer Number of layers in model - 

DOI Depth of Investigation Meter 

Elev[xx] Elevation of top of layer. [xx] refer to 

Geosoft channel number. 

Meter 

Res[xx] Resistivity of layer. [xx] refer to Geosoft

channel number. 

Ωmeter 

RUnc[xx] Relative uncertainty of layer. [xx] refer 

to Geosoft channel number. 

- 

 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 41

Presentations - Model sections and grids 

The models resulting from the inversion are presented as model sections/profiles 

including analytic sections that display the normalized standard deviation of the 

resistivity sections along with the DOI (Figure 23) and as grids of resistivity in each 

model layer for Inner Box (Figure 21) and Outer Box (Figure 22). 

The model sections and grids are enclosed in digital form. A brief description is given 

in Appendix 6. 

The model sections have a large vertical exaggeration which will make the structures 

look more vertical than they are. 

Residuals 

The quality of the inversion results can be evaluated by inspecting the residuals. 

The data residual is calculated by comparing the measured data with the response of 

the resulting model after inversion. If the residual is in the range of 1, the misfit 

between the response of the final model and the data is, on average, equal to the 

noise. If the residual is high, it might be caused by data that are noisier than the noise 

model takes into account. This can be seen where resistivities are very high and the 

signal consequently very low. A high data residual can also be due to the inconsistency 

between the model assumed in the inversion and the 2D/3D character of the real 

world. These are found primarily at the edges of sharp lateral conductivity contrasts. 

Finally, coupling effects due to power lines and other manmade conductors can also be 

a source of a high residual. 

The total residual is a weighted sum of the data residual and the model residual, 

where the latter is a measure of the roughness of the model, i.e. the deviation of the 

final model from the initial homogeneous half space model. 

 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 42

Figure 21 The image above is a low resolution example of the enclosed PDF maps 

displaying the inversion results for the Paradox survey area for layer 7 (32.0-

38.2m, cell size 50m) UTM Zone 12N NAD83. Geosoft grids are found in the 

data delivery folder. 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 43

Figure 22 The image above is a low resolution example of the enclosed PDF maps 

displaying the inversion results for the San Luis survey area layer 7 (32.0-

38.2m, cell size 100m) UTM Zone 13N NAD83. Geosoft grids and PDF’s are 

found in the data delivery folder. 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 44

Figure 23 Sample of the model section plots enclosed as PDF's. Top plot: Resistivity 

section with flight height (red) and depth of investigation (white line) 

indicated. Data and total residuals are displayed in the second plot. The third 

plot show the analysis section. The bottom plots are the low and high moment 

data (dots) and model response (full line). Profile section for flown lines are 

found as PDF’s in the data delivery folder. 

References 

/1/ Sorensen, K. I. and Auken, E. (2004). SkyTEM - A new high-resolution 

helicopter transient electromagnetic system, Exploration Geophysics, 35, 191-199. 

/2/ Christensen, N. B. (2002). A generic 1-D imaging method for transient 

electromagnetic data. Geophysics, 67, 438-447. 

/3/ Christensen, N.B., Reid, J.E. and Halkjær, M. (2009). Fast, laterally smooth 

inversion of airborne time-domain electromagnetic data, Near Surface Geophysics, 7, 

599-612 

/4/ Christensen N.B. and Tølbøll R.J. 2009, A lateral model parameter correlation 

procedure for one-dimensional inverse modeling. Geophysical Prospecting 57, 919-

929. DOI: 10.1111/j.1365-2478.2008.00756.x 
SkyTEM Surveys ApS 

SkyTEM Survey Colorado – January 2012 45

Appendix list 

Appendix 1: Instruments 

Appendix 2: Time gates 

Appendix 3: Calibration 

Appendix 4: Control parameters 

Appendix 5: Modeling and inversion of TEM data 

Appendix 6: Inversion results 

Appendix 7: Digital data 
SkyTEM Survey Colorado – January 2012 1- 1

Appendix 1: Instruments
SkyTEM Survey Colorado – January 2012 1 - 2

Instrument positions 

The instrumentation involves a time domain electromagnetic system, two 

inclinometers, two altimeters and two DGPS’. 

The measurements were carried out, using a setup as described below. 

Figure 1 Sketch showing the frame and the position of the basic instruments. The 

red line defines the transmitter loop. The horizontal plane is defined by (x, 

y). 

The location of instruments in respect to the frame is shown in Figure 1 and is 

given in (x, y, z) coordinates in the table below. 

X and y define the horizontal plane. Z is perpendicular to (x, y). X is positive in the 

flight direction, y is positive to the right of the flight direction, and z is positive 

downwards. 

The generator used for powering of the transmitter is 10 m below the helicopter. 

 
SkyTEM Survey Colorado – January 2012 1 - 3

Device X Y Z 

DGPS1 (EM) 11.85 1.01 -0.23 

DGPS2 (EM) 11.85 -0.60 -0.23 

HE1 (altim.) 5.15 7.72 0.02 

HE2 (altim.) 5.15 -7.72 0.02 

Inclinometer 1 -11.96 -0.50 -0.37 

Inclinometer 2 -11.96 0.50 -0.37 

RX (Z Coil) -12.64 0.00 -2.10 

RX (X Coil) -13.8 0.00 0.00 

TX (transmit.) 12.73 0.16 -0.74 

Condensator 12.73 -0.16 -0.74 

For the location of instruments see Figure 1. 

Transmitter 

The time domain transmitter loop can be described as an octagon with the corners 

listed below: 

X Y 

-11.87 -2.03 

 -5.68 -8.22 

 5.68 -8.22 

 11.87 -2.03 

 11.87 2.03 

 5.68 8.22 

 -5.68 8.22 

-11.87 2.03 

The total area of the transmitter coil defined by the corner points is 314 m2 and 

65.9 m in circumference. 
SkyTEM Survey Colorado – January 2012 1 - 4

The key parameters defining the transmitter set up are: 

Low moment 

Parameter Value 

Number of transmitter turns 1 

Transmitter area 314 m2 

Peak current 5 

Peak moment ~3,140 NIA 

Repetition frequency 240 Hz 

On-time 800 μs 

Off-time 1283 μs 

Duty cycle 38 % 

Wave form Square 

Turn on wave form exp. decay constant 44000 sˉ1 

Turn off linear ramp 4.46e6 A/s 

Turn off current end avalanche mode 1.5 A 

Turn off free decay exp. decay constant -3.00e6 sˉ1

High Moment 

Parameter Value 

Number of transmitter turns 4 

Transmitter area 314 m2 

Peak current 114.1 

Peak moment ~150,000 NIA 

Repetition frequency 30 Hz 

On-time 8000 μs 

Off-time 8667 μs 

Duty cycle 48 % 

Wave form Square 

Turn on wave form exp. decay constant 410 sˉ1 

Turn off linear ramp 2.38e6 A/s 

Turn off current end avalanche mode 1.0 A 

Turn off free decay exp. decay constant -1.29e6 sˉ1
SkyTEM Survey Colorado – January 2012 1 - 5

Figure 2 The 314 m2 frame in production mode. 

Receiver system 

The decay of the secondary magnetic field is measured using two independent 

active induction coils. The Z coil is the vertical component, and the X coil is the 

horizontal in-line component. Each coil has an effective receiver area of 105 m2 . 

The receiver coils are placed in a null-position: 

Z coil (x, y, z) = (-12.80 m, 0.0 m, -2.22) 

X coil (x, y, z) = (-13.75 m, 0.0 m, 0.0 m) 

In the null-position, the primary field is damped with a factor of 0.01. 
SkyTEM Survey Colorado – January 2012 1 - 6

Figure 3 Rudder containing the Z coil located approximately in the top part of the 

tower. 

The key parameters defining the receiver set up are: 

Receiver parameters 

Sample rate All decays are measured 

Number of output gates 34 (HM) and 26 (LM) 

Receiver coil low pass filter 450 kHz 

Receiver instrument low pass filter 300 kHz 

Repetition frequency LM 

HM 

240 Hz 

30 Hz 

Front gate LM 

HM 

0.0 μs 

60.0 μs 

Receiver gate times are measured from the start of the transmitter current turn-off. 

A complete list describing gate open, close and centre times are listed in Appendix 

2. 
SkyTEM Survey Colorado – January 2012 1 - 7

Inclination 

Instrument type: Bjerre Technology 

The inclination of the frame is measured with 2 independent inclinometers. The x 

and y angles are measured 2 times per second in both directions. The inclinometers 

are placed in the rear of the frame as close to the z coil as possible, see Figure 1. 

 

The angle data are stored as x, y readings. X is parallel to the flight direction and 

positive when the front of the frame is above horizontal. Y is perpendicular to the 

flight direction and negative when the right side of the frame is above horizontal. 

The angle is checked and calibrated manually within 1.0 degree by use of a level 

meter. 

DGPS airborne unit and base stations 

Chipset: OEMV1-L1 14-channel rate. 

Antenna: Trimble, Bullet III GPS Antenna 

The differential GPS receiver is on top of the boom in front of the frame. 

The DGPS delivers one dataset per second. The raw coordinates are given in 

Latitude/longitude, WGS84. 

The uncertainty in the xyz-directions is ± 1 m after processing. 

The processed DGPS data is combined with the EM data in the xyz-files, giving the 

precise position. 

DGPS parameters 

Sample rate 1 Hz 

Uncertainty ± 1 m 
SkyTEM Survey Colorado – January 2012 1 - 8

Altimeter 

Instrument type: MDL ILM300R 

Two independent laser units mounted on each side of the frame measure the 

distance from the frame to the ground, see Figure 1. 

Each laser delivers 30 measurements per second, and covers the interval from 1.5 

m to approximately 130 m. 

Dark surfaces including water surfaces will reduce the reflected signal. 

Consequently, it may occur that some measurements do not result in useful values. 

The altimeter measurements are given in meters with two decimals. The 

uncertainty is 10 - 30 cm. The lasers are checked on a regular basis against well 

defined targets. 

Laser parameters 

Sample rate 30 Hz 

Uncertainty 10 - 30 cm 

Min/ max range 1.5 m / 130 m 
SkyTEM Survey Colorado – January 2012 1 - 9

Magnetometer airborne unit 

Instrument type: Geometrics G822A sensor and Kroum KMAG4 counter. 

The Geometrics G822A sensor and Kroum KMAG4 counter is a high sensitivity 

cesium magnetometer. The basic of the sensor is a self-oscillating split-beam 

Cesium Vapor (non-radioactive) Principe, which operates on principles similar to 

other alkali vapor magnetometers. 

The sensitivity of the Geometrics G822A sensor and Kroum KMAG4 counter is 

stated as <0.0005 nT/√Hz rms. Typically 0.002 nT P-P at a 0.1 second sample rate, 

combined with absolute accuracy of 3 nT over its full operating range. 

The magnetometer is synchronized with the TEM system. When the TEM signal is 

on, the counter is closed. In the TEM off-time the magnetometer data is measured 

from 100 microseconds until the next TEM pulse is transmitted. The data are 

averaged and sampled as 60 Hz. 

Parameter Value 

Sample frequency 60 Hz (in between each HM EM pulse) 

Magnetometer on HM Cycles 

Magnetometer off LM Cycles 

Magnetometer base station 

Instrument type: GEM Overhauser. 

The GEM Overhauser is a portable high-sensitivity precession magnetometer. 

The GEM Overhauser is a secondary standard for measurement of the Earth’s 

magnetic field with 0.01 nT resolutions, and 1 nT absolute accuracy over its full 

temperature range. 

The base station data are sampled with 1 Hz frequency. 
SkyTEM Survey Colorado– January 2012 2 - 1

Appendix 2: Time gates 

 
SkyTEM Survey Colorado– January 2012 2 - 2

Gate GateOpen 

(μs) 

Gatewidth 

(μs) 

GateClose 

(μs) 

Raw 

GateCenter 

(μs) 

GateCenter 

Applied time 

shift calibration 

for HM and LM 

(μs) 

Comment 

1 0.390 5.610 6.000 3.195 2.095 Not used 

2 6.390 1.610 8.000 7.195 6.095 Not used 

3 8.390 1.610 10.000 9.195 8.095 Not used 

4 10.390 1.610 12.000 11.195 10.095 Not used 

5 12.390 1.610 14.000 13.195 12.095 LM only 

6 14.390 1.610 16.000 15.195 14.095 LM only 

7 16.390 1.610 18.000 17.195 16.095 LM only 

8 18.390 3.610 22.000 20.195 19.095 LM only 

9 22.390 4.610 27.000 24.695 23.595 LM only 

10 27.390 6.610 34.000 30.695 29.595 LM only 

11 34.390 7.610 42.000 38.195 37.095 LM only 

12 42.390 9.610 52.000 47.195 46.095 LM only 

13 52.390 12.610 65.000 58.695 57.595 LM only 

14 65.390 15.610 81.000 73.195 72.095 LM only 

15 81.390 20.610 102.000 91.695 90.595 LM only 

16 102.390 25.610 128.000 115.195 114.095 LM only 

17 128.390 31.610 160.000 144.195 143.095 LM only 

18 160.390 41.610 202.000 181.195 180.095 LM & HM 

19 202.390 50.610 253.000 227.695 226.595 LM & HM 

20 253.390 64.610 318.000 285.695 284.595 LM & HM 

21 318.390 81.610 400.000 359.195 358.095 LM & HM 

22 400.390 102.610 503.000 451.695 450.595 LM & HM 

23 503.390 129.610 633.000 568.195 567.095 LM & HM 

24 633.390 162.610 796.000 714.695 713.595 LM & HM 

25 796.390 205.610 1002.000 899.195 898.095 LM & HM 

26 1002.390 258.610 1261.000 1131.695 1130.595 LM & HM 

27 1261.390 325.610 1587.000 1424.195 1423.095 HM only 

28 1587.390 409.610 1997.000 1792.195 1791.095 HM only 

29 1997.390 516.610 2514.000 2255.695 2254.595 HM only 

30 2514.390 649.610 3164.000 2839.195 2838.095 HM only 

31 3164.390 818.610 3983.000 3573.695 3572.595 HM only 

32 3983.390 1030.610 5014.000 4498.695 4497.595 HM only 

33 5014.390 1297.610 6312.000 5663.195 5662.095 HM only 

34 6312.390 1632.610 7945.000 7128.695 7127.595 HM only 

 

Note: The first gates are not used in any of the moments in the present survey as it 

is in the transition zone. 
SkyTEM Survey Colorado– January 2012 2 - 3

SkyTEM inversion software (iTEM) handles time shift calibration during import of 

data. 

If third party processing software is used the calibrated Gate centre times should 

be used. 
SkyTEM Survey Colorado– January 2012 3 - 1

Appendix 3: Calibration of the TEM system 

 
SkyTEM Survey Colorado– January 2012 3 - 2

As described in the main document the system has been calibrated in a 50 Hz 

power supply grid setting (In Denmark), but the data was recorded in a 60 Hz 

environment (USA). 

The wave form is measured with the 60 Hz script with a repetition frequency of 

240 Hz for LM and with a repetition frequency of 30 Hz HM. Figure 1 to Figure 4 

show the up and down ramp, respectively. 

 

Figure 1 Ramp up at 240 Hz. Blue curve is the measured wave form. Red curve is 

the function that fits the data. The current is 10 A and the decay 

constant τ = 44000 sˉ1. 

0

2

4

6

8

10

12

0.00E+00

2.00E-044.00E-046.00E-048.00E-041.00E-03

Amp

Time
SkyTEM Survey Colorado– January 2012 3 - 3

Figure 2 Ramp up at 30 Hz. Blue curve is the measured wave form. Red curve is 

the function that fits the data. The current is 117 A and the decay 

constant τ = 410 sˉ1. 

 

Figure 3 Ramp down at 240 Hz. Blue curve is the measured wave form. Red 

curve is the piecewise linear function that fits the data. Decay constant -

3.00e6 sˉ1. 

0

20

40

60

80

100

120

0.00E+00

2.00E-034.00E-036.00E-038.00E-031.00E-02

Amp

Time

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0.00E+00

1.00E-062.00E-063.00E-064.00E-065.00E-06

Amp/sec

Time
SkyTEM Survey Colorado– January 2012 3 - 4

Figure 4 Ramp down at 30 Hz. Blue curve is the measured wave form. Red curve 

is the piecewise linear function that fits the data. Decay constant -

1.29e6 sˉ1. 

 

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

110.00

120.00

130.00

0.00E+00

1.00E-052.00E-053.00E-054.00E-055.00E-056.00E-05

Amp

Time
SkyTEM Survey Colorado– January 2012 3 - 5

LM 

Parameter Value 

Ramp 

up 

Repetition frequency 240 Hz 

Decay constant, τ 44000 sˉ1 

Ramp 

Down 

Avalanche mode 1.96 μs 

Linear ramp dI/dt 4.46e6 A/s 

End avalanche mode current 1.5 A 

Decay const exp mode, τ -3.00e6 sˉ1 

HM 

Parameter Value 

Ramp 

up 

Repetition frequency 30 Hz 

Decay constant, τ 410 sˉ1 

Ramp 

Down 

Avalanche mode 48.9 μs 

Linear ramp dI/dt 2.38e6 A/s 

End avalanche mode 1.0 A 

Decay const exp mode, τ -1.29e6 sˉ1 

The complete SkyTEM equipment has been calibrated at the National Danish 

Reference Site. The following plots, Figure 5 to Figure 8, show the measured data 

as well as the expected response in altitudes 5 m, 10 m, 15 m, 20 m and 30 m. 

The reference data for both LM and HM data are shown as blue curves and the 

measured data for LM and HM as red curves. 

 

Figure 5 The frame is in 5 m altitude. Blue curves with 5% error bars are the 

expected response, and red curves are the actual measurements. 

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02

Time (sec)

Apparent resistivity (Ohmm)
SkyTEM Survey Colorado– January 2012 3 - 6

Figure 6 The frame is in 10 m altitude. Blue curves with 5% error bars are the 

expected response, and red curves are the actual measurements. 

Figure 7 The frame is in 15 m altitude. Blue curves with 5% error bars are the 

expected response, and red curves are the actual measurements. 

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02

Time (sec)

Apparent resistivity [Ohmm]

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02

Time (sec)

Apparent resistivity [Ohmm]
SkyTEM Survey Colorado– January 2012 3 - 7

Figure 8 The frame is in 20 m altitude. Blue curves with 5% error bars are the 

expected response and red curves are the actual measurements. 

Figure 9 The frame is in 30 m altitude. Blue curves with 5% error bars are the 

expected response and red curves are the actual measurements 

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02

Time (sec)

Apparent resistivity [Ohmm]

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02

Time (sec)

Apparent resistivity [Ohmm]
SkyTEM Survey Colorado – January 2012 4 - 1

 Appendix 4: Control parameters 
SkyTEM Survey Colorado – January 2012 4 - 2

The following plots show the speed, altitude and the angle of the frame for every 

flight. Variations in the current, voltage on the transmitter and transmitter 

temperature are also shown. 

The green line, depicting processed frame height, shows the SkyPRO input from 

HE1 and HE2 after the frame has been corrected from deviations, away from the 

horizontal plane and any obstacles on the ground e.g. trees. 

Turns at the end of flight lines and transport are shown as gaps in the bottom of 

the display. 

The ground speed in the uppermost window displays the signal from both gps GP1 

and GP2. 
20111011.06

SkyTEM printed 14/12/2011
20111012.01

SkyTEM printed 14/12/2011
20111012.02

SkyTEM printed 14/12/2011
20111012.03

SkyTEM printed 14/12/2011
20111013.01

SkyTEM printed 14/12/2011
20111013.02

SkyTEM printed 14/12/2011
20111013.03

SkyTEM printed 14/12/2011
20111014.01

SkyTEM printed 14/12/2011
20111014.02

SkyTEM printed 14/12/2011
20111014.03

SkyTEM printed 14/12/2011
20111015.01

SkyTEM printed 14/12/2011
20111015.02

SkyTEM printed 14/12/2011
20111015.03

SkyTEM printed 14/12/2011
20111016.01

SkyTEM printed 14/12/2011
20111016.02

SkyTEM printed 14/12/2011
20111020.01

SkyTEM printed 08/12/2011
20111020.02

SkyTEM printed 08/12/2011
20111020.03

SkyTEM printed 08/12/2011
20111020.03

SkyTEM printed 13/12/2011
20111021.02

SkyTEM printed 08/12/2011
20111021.03

SkyTEM printed 08/12/2011
20111022.01

SkyTEM printed 08/12/2011
20111022.02

SkyTEM printed 08/12/2011
20111022.03

SkyTEM printed 08/12/2011
20111023.01

SkyTEM printed 08/12/2011
20111023.02

SkyTEM printed 08/12/2011
SkyTEM Survey Colorado – January 2012 5 - 1

Appendix 5: Modelling and inversion of TEM 

Data 
SkyTEM Survey Colorado – January 2012 5 - 2

This appendix gives a brief introduction to modelling and inversion of SkyTEM 

data. 

The model 

The model used for inversion of SkyTEM data is a 1D multi-layer model (MLM) with 

typically 30 layers. The layer thicknesses increase downwards as a hyperbolic sine 

function of the layer number. This means that the depth to the layer boundaries 

increases linearly for small depths, so that the top layers are all of approximately 

the same thickness. For large depths, the depth to the layer boundaries increases 

exponentially with depth, so that the thickness of a layer is a factor times the 

previous one. 

Inversion - The initial model 

The initial model for the deep inversion is a 30-layer MLM with a homogeneous 

resistivity for all layers, i.e. the initial model is essentially a homogeneous half 

space. Model optimization can be carried out in both a L1- and a L2- norm 

formulation where the former produces more blocky models than the latter. 

Data and noise model 

The inaccuracy of TEM data is influenced by the ambient noise. This noise is 

reduced by selective stacking of delay time series, and by applying appropriate 

filters in the receiver system. 

Experience with SkyTEM data suggests that the noise voltage most often can be 

described with a simple model: log(noise) is a linear function of log(time). When 

the width of the time gates increases proportional to delay time - as is the case 

with the SkyTEM system - the slope of the linear function is close to -0.5. The 

noise model used can therefore be described as: 

α

















= ⋅

0

0

t

t

V V

Where V is the noise voltage, V0 is the noise voltage at time t0 and α is the slope 

of the noise voltage as a function of time in a double logarithmic plot. Choosing t0

= 1 ms, the noise model is defined by the values of V0 and α. These values are 

chosen pragmatically by inspection of a subset of the data volume. 

V0 = 2.5e-12 in field units normalized with Tx moment (LM) 

V0 = 2.5e-13 in field units normalized with Tx moment (HM) 

t0 = 1 ms 

slope = -0.5 
SkyTEM Survey Colorado – January 2012 5 - 3

Regularization 

Inversion of TEM data is highly non-linear which means regularization is needed in 

order to guide the inversion routine to produce feasible geological models. In the 

initial inversion, a vertical smoothness constraint is implemented through a 

broadband model covariance matrix. This matrix is constructed by stacking singlescale exponential covariance functions with different correlation lengths, describing 

the covariance between any two points in the sub-surface. This approach has 

proven to be very robust and stable as the expected subsurface variability can be 

described through the prior covariance matrix (/3/). 

To obtain laterally smooth model sections, the Lateral Parameter Correlation (LPC) 

procedure is used (/3/ and /4/). Through an inversion process, a smooth version 

of the resistivity variation is predicted from the results of the initial inversion. In 

this approach, all parameter values are correlated with all other values in the 

plane. After the LPC procedure, data are subjected to a final inversion constrained 

by the LPC models to improve the data fit. 

Data insufficiency 

For SkyTEM data, the insufficiency lies primarily in the limited delay time range 

that can be obtained. The earliest obtainable time gate is determined by the 

turnoff of the Tx current, and the latest useful time gate is determined by the 

signal to noise ratio. Increasing the Tx moment will give better measurements at 

late times, and thus improve the depth penetration, but also increase the turnoff 

time and thus remove early-time gates, thereby making the near-surface 

resolution poorer. This trade-off is solved by transmitting an alternating sequence 

of (1) a low moment that can be turned off quickly to give good near-surface 

resolution, and (2) a high moment that will improve the signal-to-noise ratio at 

late times, thus improving depth penetration. 

Model inconsistency 

When using 1D models in the interpretation of SkyTEM data, inconsistency arises 

where the lateral gradient of conductivity is not small, e.g. typically in mining 

applications. However, also in environmental investigations, inconsistencies can 

arise, typically where near-surface good conductors have abrupt boundaries. 

Often such inconsistency is indicated by the data residual being high and one 

should look upon the inversion results with some caution at these locations. 3D 

effects can also reveal themselves by the so-called ‘pant legs’, i.e. conductive or 

resistive structures projecting at an angle of approximately 30 degrees from the 

horizontal at the edges of high contrast structures. 
SkyTEM Survey Colorado– January 2012 6 - 1

Appendix 6: Model sections and resistivity 

intervals 
SkyTEM Survey Colorado– January 2012 6 - 2

Model sections and analysis sections are delivered in digital form as PDF files. 

Model sections 

The Model sections can be found in the data delivery folder as PDF’s. 

The model section plot consists of five subplots. The top plot shows the inverted 

models, with topography, where the resistivity of the individual layers is colour 

coded according to the colour bar. The resistivity is shown on a logarithmic scale 

and conductive and resistive features appear with the same weight. The actual 

flight elevation is shown with a red line above the model section. The white line in 

the model section indicates the estimated depth of investigation (DOI). Starting 

from the bottom layer of the model, the DOI is equal to the depth of the first layer 

having a conductance uncertainty of less than 0.5. If the resistivity uncertainty is 

too high, the layer resistivity is unresolved. 

Below the model section is a plot of the normalized data residual (red line) and 

normalized total residual (black line) of the inversions. The total residual is a 

weighted sum of the data residual and the model residual, where the latter is a 

measure of the roughness of the model, i.e., the deviation of the final model from 

the initial homogeneous halfspace model. 

Below the residual section is the analysis section. The resistivity of the inverted 

models is determined partly by the measured data and partly by the regularization 

– the vertical and horizontal smoothness constraints – used in the inversion. To 

illustrate the relative importance of the data and the smoothness constraints an 

analysis section is produced. The analysis section has the same appearance as the 

model section, but rather than plotting the layer resistivities the normalized 

relative uncertainty of the layer resistivities are plotted. The values of the 

normalized relative uncertainty are colour coded according to the colour scale. The 

colour scale consists of four colours: red, yellow, blue, and blue fading into white. 

The red colour indicates that data have contributed considerably to the inverted 

resistivity, i.e., the resistivity is well determined. 

The yellow colour indicates that data has had more influence on the inverted 

resistivity than the regularization, i.e., the resistivity is fairly well determined. 

The blue colour indicates that the regularization has had more influence than the 

data in determining the inverted resistivity, i.e., the resistivity is poorly 

determined. 
SkyTEM Survey Colorado– January 2012 6 - 3

Where the blue colour fades into the white, the inverted resistivity is determined 

almost exclusively by the regularization, i.e., the resistivity is essentially 

undetermined. 

In short, one can say that data has had more influence than the regularization 

when values are below 1 – the red and yellow colours – and that the regularization 

has had more influence than the data where the values are above 1 – the blue and 

white colours. 

Please take note that in some parts of the analysis sections, where the nearsurface resistivity is very high, the top part of the model can be seen as 

undetermined. In this situation the TEM method cannot determine the resistivity. 

Below the analysis section are two plots of the measured data (dots) together with 

the response of the inverted models (solid lines). M1 is low moment data and M2 

is high moment data. For both plots, every second gate is plotted starting with the 

earliest gate, and data are plotted with a density of 8 points per centimetre along 

the profile. 

Layer Resistivity Maps 

The Model sections can be found in the data delivery folder as PDF’s as well as 

geosoft . grd files. 

The resistivity maps show the inverted resistivity for each of the model layers. 

As the thickness of the model layers increases downwards the maps represent a 

varying thickness interval. The depth interval is stated on the pdf files and is in 

meters below the surface. 
SkyTEM Survey Colorado – January 2012 7 - 1

Appendix 7: Digital data 

The digital data are listed in the following folders. 

Data 

delivery 

folder 

Sub folder Sub 

folder 

File format Comment 

01_TEM_data 01_Data Geosoft.gdb EM database ready for 

import in Geosoft 

02_EM_Channels_ 

grid 

Paradox_HM

_Z 

Geosoft.grd Channel plots of raw 

data. Gate 18-34 

Paradox_LM

_Z 

Geosoft.grd Channel plots of raw 

data. Gate 5-26 

SanLuis_HM

_Z 

Geosoft.grd Channel plots of raw 

data. Gate 18-34 

SanLuis_LM_

Z 

Geosoft.grd Channel plots of raw 

data. Gate 5-26 

03_WB_Files Geofile .geo Geometry file 

Maskfile .lin Files masking flown 

lines 

Rawdata .skb 

.sps 

Em and Aux data 

02_MAG_data 01_Data Geosoft.gdb Mag database ready for 

import in Geosoft 

02_Grids Geosoft.grd Grids of TMI, RMF 

03_Maps .pdf 

.map 

Maps and PDF’s of 

RMF, TMI 

03_Inversion 01_Data Geosoft.gdb Database containing 

EM inversion results, 

read for import in 

Geosoft 

02_Layer_Resis

tivity_Grids 

Paradox Geosoft.grd Grids of the resistivity 

in each layer

SanLuis 

03_Layer_Resis

tivity_Maps 

Paradox .pdf 

.map 

Maps and pdf’s of the 

resistivity in each layer

SanLuis 

04_Sections Paradox .pdf 

.png 

Layer mean resistivity 

and analysis of all lines 

SanLuis 

04_MISC DEM Geosoft.grd 

.pdf 

.map 

Digital Elevation Model 

(DEM) 

Linepath .pdf 

.map 

Line Path. 

PlannedFlightLines .pdf 

.map 

Planned flight lines 

05_Report .pdf The report and 

appendices 

06_AGG_Report .pdf Aarhus Geophysics 

group report and pdf 

plots