Report on a Helicopter-Borne 

AeroTEM System Electromagnetic 

& Magnetic Survey 

Aeroquest Job # 11-014 

Leach Basin 

Fort Irwin Army Base 

For 

U.S. Geological Survey 

MS 964, Building20, Rm. B2208 

Box 25046, Denver Federal Center 

Denver, Colorado 80225 

by 

7687 Bath Road, 

Mississauga, ON, L4T 3T1 

Tel: (905) 672-9129 

Fax: (905) 672-7083 

www.aeroquest.ca

Report date: April 2011 
Report on a Helicopter-Borne 

AeroTEM System Electromagnetic 

& Magnetic Survey 

Aeroquest Job # 11-014 

Leach Basin 

Fort Irwin Army Base

For 

U.S. Geological Survey 

MS 964, Building20, Rm. B2208 

Box 25046, Denver Federal Center 

Denver, Colorado 80225

By 

7687 Bath Road, 

Mississauga, ON, L4T 3T1 

Tel: (905) 672-9129 

Fax: (905) 672-7083 

www.aeroquest.ca

 

Report date: April 2011
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

i

TABLE OF CONTENTS 

TABLE OF CONTENTS ........................................................................................................................ i

LIST OF FIGURES ................................................................................................................................ 2

LIST OF MAPS SCALE (1:24,000) ...................................................................................................... 2

1. INTRODUCTION .............................................................................................................................. 3

2. SURVEY AREA ................................................................................................................................ 3

3. SURVEY SPECIFICATIONS AND PROCEDURES ....................................................................... 4

3.1. Navigation ................................................................................................................................... 5

3.2. System Drift ................................................................................................................................ 5

3.3. Field QA/QC Procedures ............................................................................................................ 5

4. AIRCRAFT AND EQUIPMENT ....................................................................................................... 6

4.1. Aircraft ........................................................................................................................................ 6

4.2. Magnetometer ............................................................................................................................. 6

4.3. Electromagnetic System .............................................................................................................. 6

4.4. AeroDAS Acquisition System .................................................................................................... 8

4.5. Magnetometer Base Station ...................................................................................................... 10

4.6. Radar Altimeter ......................................................................................................................... 10

4.7. Video Tracking and Recording System .................................................................................... 10

4.8. GPS Navigation System ............................................................................................................ 10

4.9. Digital Acquisition System ....................................................................................................... 11

5. PERSONNEL ................................................................................................................................... 11

6. DELIVERABLES ............................................................................................................................ 11

6.1. Hardcopy Deliverables .............................................................................................................. 11

6.2. Digital Deliverables .................................................................................................................. 12

6.2.1. Final Database of Survey Data (.GDB) ............................................................................. 12

6.2.2. Geosoft Grid files (.GRD) .................................................................................................. 12

6.2.3. Digital Versions of Final Maps (.MAP, .PDF) .................................................................. 12

6.2.4. Google Earth Survey Files (.kmz) ...................................................................................... 12

6.2.5. Free Viewing Software (.EXE) ........................................................................................... 12

6.2.6. Digital Copy of this Document (.PDF) .............................................................................. 13

7. DATA PROCESSING AND PRESENTATION ............................................................................. 13

7.1. Base Map ................................................................................................................................... 13

7.2. Flight Path & Terrain Clearance ............................................................................................... 13

7.3. Electromagnetic Data ................................................................................................................ 14

7.4. Magnetic Data ........................................................................................................................... 14

7.5. Conductivity Data ..................................................................................................................... 15

8. General Comments ........................................................................................................................... 15

8.1. Magnetic Response ................................................................................................................... 15

8.2. EM Anomalies .......................................................................................................................... 15

APPENDIX 1: Survey Boundaries ....................................................................................................... 18
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

ii

APPENDIX 2: Description of Database Fields .................................................................................... 19

APPENDIX 3: AeroTEM Design Considerations ................................................................................ 22

APPENDIX 4: AeroTEM Instrumentation Specification Sheet ........................................................... 28

LIST OF FIGURES 

Figure 1. Project flight path and location ............................................................................................... 4

Figure 2.Helicopter type used during the survey. ................................................................................... 6

Figure 3.The magnetometer bird (A) and AeroTEM IV EM bird (B) .................................................... 7

Figure 4.Schematic of Transmitter and Receiver waveforms: 90 Hz ..................................................... 7

Figure 5. AeroTEM response to a ‘thin’ vertical conductor ................................................................. 16

Figure 6. AeroTEM response for a ‘thick’ vertical conductor ............................................................. 16

Figure 7. AeroTEM response over a ‘thin’ dipping conductor ............................................................ 17

LIST OF MAPS SCALE (1:24,000) 

• TMI_IGRF removed – Coloured Total Magnetic Intensity (TMI) with IGRF removed 

and line contours 

• Z1-OFF – AeroTEM Z1 Off-Time grid with line contours 

• DTM – Digital Terrain Model grid with line contours 

• EM – AeroTEM Off-Time profiles Z2-Z12 

• RES - Apparent Resistivity – Depth Slice grids, 50 – 200 m, with line contours. 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

3

1. INTRODUCTION 

This report describes a helicopter-borne geophysical survey carried out on behalf of U.S. 

Geological Survey over their Leach Basin property located near Fort Irwin Army Base, 

California. The survey was comprised of one block. 

The principal geophysical sensor is Aeroquest's exclusive AeroTEM IV (‘Papa’ System) 

helicopter time domain electromagnetic system which is employed in conjunction with a 

high-sensitivity caesium vapour magnetometer. Ancillary equipment includes a real-time 

differential GPS navigation system, radar altimeter, video recorder, and a base station 

magnetometer. Full-waveform streaming EM data is recorded at 36,000 samples per second. 

The streaming data comprise the transmitted waveform, and the X component and Z 

component of the resultant field at the receivers. The streaming EM data along with ancillary 

data recorded with AeroDAS acquisition system. 

The total survey coverage for the block was 1785.95 line-km, of which 1747.11 line-km fell 

within the defined project area (Appendix 1). The survey was flown at 400 and 800 metres 

line spacing with a 0o/180o flight line direction. The survey flying described in this report 

took place between December 27th and January 10th, 2010/2011. This report describes the 

survey logistics, the data processing, presentation, and provides a brief overview of the 

results. 

2. SURVEY AREA 

The project area is located at Fort Irwin Army Base, California. The area lies approximately 

10 km N of Fort Irwin, California. The project was made up of one block, Leach Basin (724.3 

km2). 

The base of survey operations and crew accommodation was the Fort Irwin Army Base, 

California. 

The main area, Leach Basin Block, was flown using an AeroTEM base frequency of 90 Hz. 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

4

Figure 1. Project flight path and location 

3. SURVEY SPECIFICATIONS AND PROCEDURES 

The survey specifications are summarised in the following table: 

Project Name Line Spacing 

(metres) 

Line 

Direction 

Survey 

Coverage 

(line-km) 

Dates flown 

Leach Basin (West) 

Leach Basin (East) 

400 

800 

0o/180o 

0o/180o 1785.95 December 27th - January10th, 2010/2011 

Table 1. Survey specifications summary. 

The survey coverage was calculated by adding up the along-line distance of the survey lines 

and control (tie) lines as presented in the final Geosoft database. The survey was flown with a 

line spacing of 400 and 800 metres. The control (tie) lines were flown perpendicular to the 

survey lines with a spacing of 7850 metres. 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

5

The nominal EM bird terrain clearance is 30 metres, but can be higher in more rugged terrain 

due to safety considerations and the capabilities of the aircraft. The magnetometer sensor is 

mounted in a smaller bird connected to the tow rope 33.2 metres above the EM bird and 17.1 

metres below the helicopter (Figure 3). Nominal survey speed over relatively flat terrain is 75 

km/hr and is generally lower in rougher terrain. Scan rates for ancillary data acquisition is 0.1 

second for the magnetometer and altimeter, and 0.2 second for the GPS determined position. 

The EM data is acquired as a data stream at a sampling rate of 36,000 samples per second and 

is processed to generate final data at 10 samples per second. The 10 samples per second 

translate to a geophysical reading about every 1.5 to 2.5 metres along the flight path. 

3.1. NAVIGATION 

Navigation is carried out using a GPS receiver, an AGNAV2 system for navigation control, 

and AeroDAS data acquisition system which records the GPS coordinates. The x-y-z position 

of the aircraft, as reported by the GPS, is recorded at 0.2 second intervals. The system has a 

published accuracy of less than 3 metres. A recent static ground test of the Mid-Tech WAAS 

GPS yielded a standard deviation in x and y of under 0.6 metres and for z under 1.5 metres 

over a two-hour period. 

3.2. SYSTEM DRIFT

Unlike frequency domain electromagnetic systems, the AeroTEM IV system has negligible 

drift due to thermal expansion. The operator is responsible for ensuring the instrument is 

properly warmed up prior to departure and that the instruments are operated properly 

throughout the flight. The operator maintains a detailed flight log during the survey noting the 

times of the flight and any unusual geophysical or topographic features. Each flight included 

at least two high elevation ‘background’ checks. During the high elevation checks, an internal 

5 second wide calibration pulse in all EM channels was generated in order to ensure that the 

gain of the system remained constant and within specifications. 

3.3. FIELD QA/QC PROCEDURES

On return of the pilot and operator to the base, usually after each flight, the AeroDAS 

streaming EM data and ancillary data are carried on removable hard drives and FlashCards, 

respectively and transferred to the data processing work station. At the end of each day, the 

base station magnetometer data on FlashCard is retrieved from the base station unit. 

Data verification and quality control includes a comparison of the acquired GPS data with the 

flight plan; verification of the base station magnetometer data and conversion to ASCII 

format XYZ data; and loading, processing and conversion of the steaming EM data from the 

removable hard drive. All data is then merged to an ASCII XYZ format file which is then 

imported to an Oasis database for further QA/QC and for the production of EM, magnetic 

contour, and flight path maps. 

Survey lines which show excessive deviation from the intended flight path are re-flown. Any 

line or portion of a line on which the data quality did not meet the contract specification was 

noted and re-flown. 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

6

4. AIRCRAFT AND EQUIPMENT 

4.1. AIRCRAFT

A Eurocopter (Aerospatiale) AS350 B3 "A-Star" helicopter - registration N352SA was used 

as survey platform. The helicopter was owned and operated by Sinton Helicopters. 

Installation of the geophysical and ancillary equipment was carried out by Aeroquest Surveys 

personnel in conjunction with a licensed aircraft engineer. The survey aircraft was flown at a 

nominal terrain clearance of 80 metres. 

Figure 2.Helicopter type used during the survey. 

4.2. MAGNETOMETER

The Aeroquest airborne survey system employs the Geometrics G-823A caesium vapour 

magnetometer sensor installed in a two metre towed bird airfoil attached to the main tow line, 

17.1 metres below the helicopter (Figure 3). The sensitivity of the magnetometer is 0.001 

nanoTesla at a 0.1 second sampling rate. The nominal ground clearance of the magnetometer 

bird is 63.2 metres. The magnetic data is recorded at 10 Hz by the AeroDAS system. 

4.3. ELECTROMAGNETIC SYSTEM

The electromagnetic system is an Aeroquest AeroTEM IV time domain towed bird system 

(Figure 3). The current AeroTEM IV transmitter dipole moment is 220 kNIA at 90 HZ. The 

AeroTEM bird is towed 50.3 metres below the helicopter. More technical details of the 

system can be found in Appendix 5. 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

7

For the Leach Basin Block the wave-form was triangular with a symmetric transmitter ontime pulse of 1945 μs and a base frequency of 90 Hz (Figure 4) The current alternates 

polarity every on-time pulse. During every Tx on-off cycle 180 per second, 200 contiguous 

channels at 90 HZ of raw X and Z component (and a transmitter current monitor, itx) of the 

received waveform are measured. The channel time widths are defined in Section 4.4 below. 

This 200 channel data is referred to as the raw streaming data. The AeroTEM system has one 

EM data recording streams, the newly designed AeroDAS system which records the full 

waveform (Figure 4). 

Figure 3.The magnetometer bird (A) and AeroTEM IV EM bird (B) 

Figure 4.Schematic of Transmitter and Receiver waveforms: 90 Hz 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

8

4.4. AERODAS ACQUISITION SYSTEM

The 200 channels at 90 Hz of raw streaming data are recorded by the AeroDAS acquisition 

system onto a removable hard drive. In addition the magnetic, altimeter and position data are 

also recorded in it (Figure 5), six channels of real time processed off-time EM decay in the Z 

direction and one in the X direction can be viewed on a color monitor on board, these 

channels are derived by a binning, stacking and filtering procedure on the raw streaming data. 

The primary use of the displayed EM data (Z1 to Z6, X1), magnetic and altimeter is to 

provide for real-time QA/QC on board. 

Figure 5 AreoTEM IV Instrument Rack 

The streaming data are processed post-survey to yield 33 stacked and binned on-time and offtime channels at a 10 Hz sample rate. The timing of the final processed EM channels is 

described in the following tables: 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

9

Average TxOn -21.3880 us 

Average TxSwitch 990.6583 us 

Average TxOff 1926.5496 us 

Average TxPeak 399.0434 A 

OTC Gate 67 

Gate Shift 36 

[Channel Data] 

Channel Sample Range Time Width (us) Time Center (us) Time After TxOn (us) 

 On1 4 - 4 27.8 97.2 118.6 

 On2 5 - 5 27.8 125.0 146.4 

 On3 6 - 6 27.8 152.8 174.2 

 On4 7 - 7 27.8 180.6 201.9 

 On5 8 - 8 27.8 208.3 229.7 

 On6 9 - 9 27.8 236.1 257.5 

 On7 10 - 10 27.8 263.9 285.3 

 On8 11 - 11 27.8 291.7 313.1 

 On9 12 - 12 27.8 319.4 340.8 

 On10 13 - 13 27.8 347.2 368.6 

 On11 14 - 14 27.8 375.0 396.4 

 On12 15 - 15 27.8 402.8 424.2 

 On13 16 - 16 27.8 430.6 451.9 

 On14 17 - 17 27.8 458.3 479.7 

 On15 18 - 18 27.8 486.1 507.5 

 On16 19 - 19 27.8 513.9 535.3 

Channel Sample Range Time Width (us) Time Center (us) Time After TxOff (us) 

 Off0 73 - 73 27.8 2013.9 87.3 

 Off1 74 - 74 27.8 2041.7 115.1 

 Off2 75 - 75 27.8 2069.4 142.9 

 Off3 76 - 76 27.8 2097.2 170.7 

 Off4 77 - 77 27.8 2125.0 198.5 

 Off5 78 - 78 27.8 2152.8 226.2 

 Off6 79 - 81 83.3 2208.3 281.8 

 Off7 82 - 84 83.3 2291.7 365.1 

 Off8 85 - 87 83.3 2375.0 448.5 

 Off9 88 - 90 83.3 2458.3 531.8 

 Off10 91 - 95 138.9 2569.4 642.9 

 Off11 96 - 100 138.9 2708.3 781.8 

 Off12 101 - 106 166.7 2861.1 934.6 

 Off13 107 - 116 277.8 3083.3 1156.8 

 Off14 117 - 130 388.9 3416.7 1490.1 

 Off15 131 - 152 611.1 3916.7 1990.1 

 Off16 153 - 187 972.2 4708.3 2781.8 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

10

4.5. MAGNETOMETER BASE STATION

The base magnetometer was an ENVI/MAG (Scintrex) proton precession magnetometer with 

internal clock synchronised with GPS time. Data logging and UTC time synchronisation was 

carried out within the magnetometer. The data logging was configured to measure at 2.0 

second intervals. Digital recording resolution was 0.1 nT The sensor was placed on a tripod in 

an area of low magnetic gradient and free of cultural noise sources. A continuously updated 

display of the base station values was available for viewing and regularly monitored to ensure 

acceptable data quality and diurnal variation. 

4.6. RADAR ALTIMETER

A Terra TRA 3500/TRI-30 radar altimeter is used to record terrain clearance. The antenna 

was mounted on the outside of the helicopter beneath the cockpit. Therefore, the recorded 

data reflect the height of the helicopter above the ground. The Terra altimeter has an altitude 

accuracy of +/- 1.5 metres. 

4.7. VIDEO TRACKING AND RECORDING SYSTEM

A high resolution digital colour 8 mm video camera is used to record the helicopter ground 

flight path along the survey lines. The video is digitally annotated with GPS position and time 

and can be used to verify ground positioning information and cultural causes of anomalous 

geophysical responses. 

Figure 6. Digital video camera typical mounting location 

4.8. GPS NAVIGATION SYSTEM

The navigation system consists of an Ag-Nav Incorporated AG-NAV2 GPS navigation 

system comprising a PC-based acquisition system, navigation software, a deviation indicator 

in front of the aircraft pilot to direct the flight, a full screen display with controls in front of 

the operator, a Mid-Tech RX400p WAAS-enabled GPS receiver mounted on the instrument 

rack and an antenna mounted on the magnetometer bird. WAAS (Wide Area Augmentation 

System) consists of approximately 25 ground reference stations positioned across the United 

States that monitor GPS satellite data. Two master stations located on the east and west coasts 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

11

collect data from the reference stations and create a GPS correction message. This correction 

accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere 

and ionosphere. The corrected differential message is then broadcast through one of two 

geostationary satellites, or satellites with a fixed position over the equator. The corrected 

position has a published accuracy of less than 3 metres. 

Survey co-ordinates are set up prior to the survey and the information is fed into the airborne 

navigation system. The co-ordinate system employed in the survey design was WGS84 

[World] using the UTM zone 11N projection. The real-time differentially corrected GPS 

positional data was recorded by the AeroDAS system in geodetic coordinates (latitude and 

longitude using WGS84) at 0.2 s intervals. 

4.9. DIGITAL ACQUISITION SYSTEM

The AeroTEM received waveform sampled during on and off-time at 200 channels per decay, 

180 times per second (90 Hz) was logged by the proprietary AeroDAS data acquisition 

system. The channel sampling commences at the start of the Tx cycle and the width of each 

channel is 27.78 μs. In addition the positional and secondary geophysical data, (i.e. magnetic, 

radar altimeter, GPS position, and UTC time) was recorded on a removable hard-drive and 

later backed-up onto DVD-ROM from the field-processing computer. 

5. PERSONNEL 

The following Aeroquest personnel were involved in the project: 

 Manager of Operations: Lee Harper 

 Field Data Processors: Thomas Wade 

 Field Operator: John Douglas 

 Data Processing and Reporting: Chris Kahue, Marion Bishop 

The survey pilot, Barry Wilson, was employed directly by the helicopter operator – Sinton 

Helicopters. 

6. DELIVERABLES 

6.1. HARDCOPY DELIVERABLES

The report includes maps at a scale of 1:24,000. The survey area is covered by one map sheet 

for both the main area with extension and one map sheet to the infill area, and five 

geophysical data products are delivered as listed below: 

• TMI_IGRF removed – Coloured Total Magnetic Intensity (TMI) with IGRF removed 

and line contours 

• Z1-OFF – AeroTEM Z1 Off-Time grid with line contours 

• DTM – Digital Terrain Model grid with line contours 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

12

• EM – AeroTEM Off-Time profiles Z2-Z12 

• RES - Apparent Resistivity – Depth Slice grids, 50 – 200 m, with line contours. 

The coordinate/projection system for the maps is NAD83 – UTM Zone 11N. For reference, 

the latitude and longitude in WGS84 are noted on the maps. 

6.2. DIGITAL DELIVERABLES

6.2.1. Final Database of Survey Data (.GDB) 

The geophysical profile data is archived digitally in four Geosoft GDB binary format 

databases. The first database includes all final data, a second includes all intermediate 

processed EM data, the third database includes the raw gated and compensated EM data and 

the fourth includes all CDI data. A description of the contents of the individual channels in 

each the database can be found in Appendix 2. 

6.2.2. Geosoft Grid files (.GRD) 

Levelled Grid products used to generate the geophysical map images. Cell size for all grid 

files is 80 metres. Grid names reflect corresponding channels in the database. 

 Total Magnetic Intensity (maguf.grd) 

 Total Magnetic Intensity IGRF removed (maguf_igrf.grd) 

 Digital Terrain Model (dtmf.grf) 

 Radar Altimeter (ralt_lag.grd) 

 Laser Altimeter (lalt.grd) 

 AeroTEM Off-Time Channel 1 (zoff_miclev1.grd) 

 Apparent Resistivity Grids, 50 – 200m depth (Resdepthm.grd) 

6.2.3. Digital Versions of Final Maps (.MAP, .PDF) 

Map files in Geosoft .map and Adobe PDF format. 

6.2.4. Google Earth Survey Files (.kmz) 

Flight navigation lines, EM anomalies, EM Profiles, geophysical grids and contours in 

Google earth KMZ format. Double click to view flight lines in Google Earth. 

6.2.5. Free Viewing Software (.EXE) 

 Geosoft Oasis Montaj Viewing Software 

 Adobe Acrobat Reader 

 Google Earth Viewer 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

13

6.2.6. Digital Copy of this Document (.PDF) 

Adobe PDF format of this document. 

7. DATA PROCESSING AND PRESENTATION 

All in-field and post-field data processing was carried out using Aeroquest proprietary data 

processing software and Geosoft Oasis Montaj software. Maps were generated using 36-inch 

wide Hewlett Packard ink-jet plotters. 

7.1. BASE MAP

The geophysical maps accompanying this report are based on positioning in the NAD83 

datum. The survey geodetic GPS positions have been projected using the Universal 

Transverse Mercator projection in Zone 11 North. A summary of the map datum and 

projection specifications is given following: 

 Ellipse: GRS 1980 

 Ellipse major axis: 6378137m eccentricity: 0.081819191 

 Datum: North American 1983 - Canada Mean 

 Datum Shifts (x,y,z) : 0, 0, 0 metres 

 Map Projection: Universal Transverse Mercator Zone 11 (Central Meridian 117oW) 

 Central Scale Factor: 0.9996 

 False Easting, Northing: 500,000m, 0m 



For reference, latitude and longitude ticks in WGS84 are noted on the maps. 

7.2. FLIGHT PATH & TERRAIN CLEARANCE

The position of the survey helicopter was directed by use of the Global Positioning System 

(GPS). Positions were updated five times per second (5 Hz) and expressed as WGS84 latitude 

and longitude calculated from the raw pseudo range derived from the C/A code signal. The 

instantaneous GPS flight path, after conversion to UTM co-ordinates, is drawn using linear 

interpolation between the x/y positions. The terrain clearance was maintained with reference 

to the radar altimeter. The raw Digital Terrain Model (DTM) was derived by taking the GPS 

survey elevation and subtracting the radar altimeter terrain clearance values. The calculated 

topography elevation values are relative and are not tied in to surveyed geodetic heights. 

Each flight included at least two high elevation ‘background’ checks. These high elevation 

checks are to ensure that the gain of the system remained constant and within specifications. 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

14

7.3. ELECTROMAGNETIC DATA

The raw streaming data, sampled at a rate of 36,000 Hz was reprocessed using a proprietary 

software algorithm developed and owned by Aeroquest Surveys. Processing involves the 

compensation of the X and Z component data for the primary field waveform. Coefficients 

for this compensation for the system transient are determined and applied to the stream data. 

The stream data are then pre-filtered with a 3 point filter, stacked utilizing a box-car stacking 

algorithm of 12 half cycles with a 50% overlap, binned to the 33 on and off-time channels 

and checked for the effectiveness of the compensation and stacking processes. The stacked 

data is then filtered with a 19pt de-spiking filter followed by a 21pt Hanning filter. The 

background levelling picks are then applied when the system was at height and out of ground 

effect. The data are then split up into the individual line segments. Further base level 

adjustments may be carried out at this stage. 

The final field processing step was to merge the processed EM data with the other data sets 

into a Geosoft GDB file. The EM fiducial is used to synchronize the two datasets. The 

processed channels are merged into ‘array format; channels in the final Geosoft database as 

Zon, Zoff, Xon, and Xoff 

The filtering of the stacked data is designed to remove or minimize high frequency noise that 

cannot be sourced from the geology. Apparent bedrock EM anomalies were interpreted with 

the aid of an auto-pick from positive peaks and troughs in the on-time Z channel responses 

correlated with X channel responses. The auto-picked anomalies were reviewed and edited by 

a geophysicist on a line by line basis to discriminate between thin and thick conductor types. 

Anomaly picks locations were migrated and removed as required. This process ensures the 

optimal representation of the conductor centres on the maps. 

At each conductor pick, estimates of the off-time conductance have been generated based on 

a horizontal plate source model for those data points along the line where the response 

amplitude is sufficient to yield an acceptable estimate. Some of the EM anomaly picks do not 

display a Tau value; this is due to the inability to properly define the decay of the conductor 

usually because of low signal amplitudes. Each conductor pick was then classified according 

to a set of seven ranges of calculated off-time conductance values. For high conductance 

sources, the on-time conductance values may be used, since it provides a more accurate 

measure of high-conductance sources. Each symbol is also given an identification letter label, 

unique to each flight line. Conductor picks that did not yield an acceptable estimate of offtime conductance due to a low amplitude response were classified as a low conductance 

source. Please refer to the anomaly symbol legend located in the margin of the maps. 

7.4. MAGNETIC DATA 

Prior to any levelling the magnetic data was subjected to a lag correction of -0.1 seconds and 

a spike removal filter. The filtered aeromagnetic data were then corrected for diurnal 

variations using the magnetic base station and the intersections of the tie lines. Corrections 

for the regional reference field (IGRF) were applied to the corrected profile data and were 

interpolated on to a grid using bi-directional gridding with a grid cell size of 80 metres. The 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

15

final levelled grid provided the basis for threading the presented contours which have a 

minimum contour interval of 10nT. 

7.5. CONDUCTIVITY DATA

The apparent conductivity is computed using both the on- and off-time z component 

of dB/dt. Multiple apparent conductivities were calculated from a series of time gates 

extracted from the levelled EM data at each measurement location. Conductivity 

values were determined using a table look-up procedure based on the analytical 

solution over a homogenous halfspace (Huang and Rudd, 2008). The outputted 

conductivity and depth information were used to create the depth slice images. 

8. GENERAL COMMENTS 

The survey was successful in mapping the magnetic and conductive properties of the geology 

throughout the survey area. Below is a brief interpretation of the results. For a detailed 

interpretation please contact Aeroquest Surveys. 

8.1. MAGNETIC RESPONSE

The magnetic data provide a high resolution map of the distribution of the magnetic mineral 

content of the survey area. This data can be used to interpret the location of geological 

contacts and other structural features such as faults and zones of magnetic alteration. The 

sources for anomalous magnetic responses are generally thought to be predominantly 

magnetite because of the relative abundance and strength of response (high magnetic 

susceptibility) of magnetite over other magnetic minerals such as pyrrhotite. 

8.2. EM ANOMALIES

The EM anomalies on the maps are classified by conductance (as described earlier in the 

report) and also by the thickness of the source. A thin, vertically orientated source produces a 

double peak anomaly in the z-component response and a positive to negative crossover in the 

x-component response (Figure 5). For a vertically orientated thick source (say, greater than 

10 metres), the response is a single peak in the z-component response and a negative to 

positive crossover in the x-component response (Figure 6). Because of these differing 

responses, the AeroTEM system provides discrimination of thin and thick sources and this 

distinction is indicated on the EM anomaly symbols (N = thin and K = thick). Where 

multiple, closely spaced conductive sources occur, or where the source has a shallow dip, it 

can be difficult to uniquely determine the type (thick vs. thin) of the source (Figure 7). In 

these cases both possible source types may be indicated by picking both thick and thin 

response styles. For shallow dipping conductors the ‘thin’ pick will be located over the edge 

of the source, whereas the ‘thick’ pick will fall over the downdip ‘heart’ of the anomaly. 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

16

-15

-10

-5

0

5

10

15

0 100 200 300 400 500 600

Conductance 50 S

Depth 50 m

Width 300 m

Length 300 m

Conductance 50 S

Depth 50 m

Width 300 m

Length 300 m

Distance (m) RESPONSE (nT/s)

Figure 5. AeroTEM response to a ‘thin’ vertical conductor 

-150

-100

-50

0

50

100

150

200

250

300

350

0 100 200 300 400 500 600

Conductance 50 S

Depth 50 m

Thickness 50 m

Dip 90o

Width 300 m

Length 300 m

Conductance 50 S

Depth 50 m

Thickness 50 m

Dip 90o

Width 300 m

Length 300 m

DISTANCE (m)

RESPONSE (nT/s)

Figure 6. AeroTEM response for a ‘thick’ vertical conductor 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

17

-30

-20

-10

0

10

20

30

40

0 100 200 300 400 500 600

Conductance 50 S

Dip 45o

Depth 50 m

Width 300 m

Length 300 m

Conductance 50 S

Dip 45o

Depth 50 m

Width 300 m

Length 300 m

DISTANCE (m)

RESPONSE (nT/s)

Figure 7. AeroTEM response over a ‘thin’ dipping conductor 

All cases should be considered when analyzing the interpreted picks and prioritizing for 

follow-up. Specific anomalous responses which remain as high priority should be subjected 

to numerical modeling prior to drill testing to determine the dip, depth and probable geometry 

of the source. 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

18

APPENDIX 1: SURVEY BOUNDARIES 

The following table presents the block boundaries. All geophysical data presented in this 

report have been windowed to 100m outside these outlines. X and Y positions are in NAD83 

UTM Zone 11. 

Leach Basin (West) Leach Basin (East) 

X Y X Y 

 511406 3945137 547461 3945269 

 547073 3945254 556850 3945310 

 547158 3929172 556818 3929205 

 511427 3929055 547547 3929170 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

19

APPENDIX 2: DESCRIPTION OF DATABASE FIELDS 

The GDB file is a Geosoft binary database. In the database, the Survey lines and Tie Lines 

are prefixed with an "L" for "Line" and "T" for "Tie". 

Structure based on folders within the accompanying DVD. 

/Data/11-014_LeachBasin.gdb 

COLUMN UNITS DESCRIPTOR 

line Line number 

flight Flight # 

emfid AERODAS Fiducial 

utctime hh:mm:ss.ss UTC time 

x m UTM Easting (NAD83, Zone 11) 

y m UTM Northing (NAD83, Zone 11) 

Lat_wgs84 o Geographic Easting (NAD83, Zone 11) 

Long_wgs84 o Geographic Northing (NAD83, Zone 11) 

galt m GPS altitude of Mag bird 

bheight m Terrain clearance of EM bird 

dtm m Digital Terrain Model 

ralt m Radar Altimeter 

balt m Barometric altitude 

Lalt M Laser Altimeter 

Ralt_Gps_Separation m Separation between Radar Altimeter and GPS sensor 

Tow_Cable_Length m Length of the Tow Cable 

basemag nT Base station total magnetic intensity 

Magu_raw nT Raw Magnetic intensity 

Magu_filt nT Filtered Magnetic intensity 

Magu nT Diurnally corrected Mag 

magUf nT Final levelled total magnetic intensity 

IGRF nT IGRF channel 

Maguf_Igrf nT Final levelled total magnetic intensity IGRF removed 

Inclination o Magnetic Inclination 

Declination o Magnetic Declination 

lZon nT/s Levelled Streaming On-Time Z component Channels 1-16 

lZoff nT/s Levelled Streaming Off-Time Z component Channels 0-16 

lXon nT/s Levelled Streaming On-Time X component Channels 1-16 

lXoff nT/s Levelled Streaming Off-Time X component Channels 0-16 

Zon_raw nT/s Raw Streaming On-Time Z component Channels 1-16 

Zoff_raw nT/s Raw Streaming Off-Time Z component Channels 0-16 

Xon_raw nT/s Raw Streaming On-Time X component Channels 1-16 

Xoff_raw nT/s Raw Streaming Off-Time X component Channels 0-16 

Zon_lag nT/s Lagged Streaming On-Time Z component Channels 1-16 

Zoff_lag nT/s Lagged Streaming Off-Time Z component Channels 0-16 

Xon_lag nT/s Lagged Streaming On-Time X component Channels 1-16 

Xoff_lag nT/s Lagged Streaming Off-Time X component Channels 0-16 

Zoff_miclev nT/s Micro levelled Streaming Off-Time Z component Channels 0-16 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

20

Zon_miclev nT/s Micro levelled Streaming On-Time Z component Channels 1-16 

Zoff_correction nT/s Corrected Off-Time Z component Channels 0-16 

Xoff_correction nT/s Corrected Off-Time X component Channels 0-16 

Zon_correction nT/s Corrected On-Time Z component Channels 1-16 

Xon_correction nT/s Corrected On-Time X component Channels 1-16 

ResOff Ohm-m Offtime Apparent Resistivity 

DepOff m Offtime Depth channel 

ResXXXm Ohm-m Apparent Resistivity Depth Slice 

ResOn Ohm-m Ontime Apparent Resistivity 

DepOn m Ontime Depth channel 

pwrline powerline monitor data channel 

Grade Classification from 1-7 based on conductance of conductor pick 

Anom_labels Alphanumeric label of conductor pick 

On_AllCon S On-time conductance 

On_AllTau μs On-time decay constant 

Anom_ID Anomaly Character (K= thicK, N = thiN) 

Off_AllCon S Off-time conductance 

Off_AllTau μs Off-time decay constant 

TranOff μs Transmission turn-off time 

TranOn μs Transmission turn-on time 

TranPeak A Transmission peak current 

TranSwitch μs Transmission peak current time 

Off_Pick EM Off time anomaly pick 

Off_con S Off-time conductance 

Off_Tau μs Off-time decay constant 

/data/11-014_LeachBasin_CDI.gdb 

COLUMN UNITS DESCRIPTOR 

emfid AERODAS Fiducial 

x m UTM Easting (NAD83, Zone 11) 

y m UTM Northing (NAD83, Zone 11) 

bheight m Terrain clearance of EM bird 

ResOff Ohm-m Offtime Apparent Resistivity 

DepOff m Offtime Depth channel 

ResXXXm Ohm-m Apparent Resistivity Depth Slice 

ResOn Ohm-m Ontime Apparent Resistivity 

DepOn m Ontime Depth channel 

/Raw EM Data/Background Levelled Fight Databases/11-014-Flt#.gdb 

COLUMN UNITS DESCRIPTOR 

Line Line number 

Flight Flight # 

Emfid AERODAS Fiducial 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

21

Utctime hh:mm:ss.ss UTC time 

X m UTM Easting (NAD83, Zone 11) 

Y m UTM Northing (NAD83, Zone 11) 

Lat_wgs84 o Geographic Easting (NAD83, Zone 11) 

Long_wgs84 o Geographic Northing (NAD83, Zone 11) 

Galt m GPS altitude of Mag bird 

Bheight m Terrain clearance of EM bird 

Dtm_lag m Digital Terrain Model 

Ralt_lag m Radar Altimeter 

Magu_filt nT Raw Magnetic intensity 

Zon_lag nT/s Lagged Streaming On-Time Z component Channels 1-16 

Zoff_lag nT/s Lagged Streaming Off-Time Z component Channels 0-16 

Xon_lag nT/s Lagged Streaming On-Time X component Channels 1-16 

Xoff_lag nT/s Lagged Streaming Off-Time X component Channels 0-16 

Pwrline powerline monitor data channel 

TranOff μs Transmission turn-off time 

TranOn μs Transmission turn-on time 

TranPeak A Transmission peak current 

TranSwitch μs Transmission peak current time 

/Raw EM Data/Gated Compensated FlightDatabase/11-014-Flt#-gated_compensated_em.gdb 

COLUMN UNITS DESCRIPTOR 

emfid AERODAS Fiducial 

Zon_raw nT/s Raw Streaming On-Time Z component Channels 1-16 

Zoff_raw nT/s Raw Streaming Off-Time Z component Channels 0-16 

Xon_raw nT/s Raw Streaming On-Time X component Channels 1-16 

Xoff_raw nT/s Raw Streaming Off-Time X component Channels 0-16 

TranOff μs Transmission turn-off time 

TranOn μs Transmission turn-on time 

TranPeak A Transmission peak current 

TranSwitch μs Transmission peak current time 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

22

APPENDIX 3: AEROTEM DESIGN CONSIDERATIONS 

Helicopter-borne EM systems offer an advantage that cannot be matched from a fixed-wing platform. The ability 

to fly at slower speed and collect data with high spatial resolution, and with great accuracy, means the helicopter 

EM systems provide more detail than any other EM configuration, airborne or ground-based. Spatial resolution is 

especially important in areas of complex geology and in the search for discrete conductors. With the advent of 

helicopter-borne high-moment time domain EM systems the fixed wing platforms are losing their only advantage 

– depth penetration. 

Advantage 1 – Spatial Resolution 

The AeroTEM system is specifically designed to have a small footprint. This is accomplished through the use of 

concentric transmitter-receiver coils and a relatively small diameter transmitter coil (5 m). The result is a highly 

focused exploration footprint, which allows for more accurate “mapping” of discrete conductors. Consider the 

transmitter primary field images shown in Figure 1, for AeroTEM versus a fixed-wing transmitter. 

The footprint of AeroTEM at the earth’s surface is 

roughly 50m on either side of transmitter 

Figure 1. A comparison of the footprint between AeroTEM and a fixed-wing system, highlights the 

greater resolution that is achievable with a transmitter located closer to the earth’s surface. The 

AeroTEM footprint is one third that of a fixed-wing system and is symmetric, while the fixed-wing system 

has even lower spatial resolution along the flight line because of the separated transmitter and receiver 

configuration. 

At first glance one may want to believe that a transmitter footprint that is distributed more evenly over a larger 

area is of benefit in mineral exploration. In fact, the opposite is true; by energizing a larger surface area, the 

ability to energize and detect discrete conductors is reduced. Consider, for example, a comparison between 

AeroTEM and a fixed-wing system over the Mesamax Deposit (1,450,000 tonnes of 2.1% Ni, 2.7% Cu, 5.2 g/t 

Pt/Pd). In a test survey over three flight lines spaced 100 m apart, AeroTEM detected the Deposit on all three 

flight lines. The fixed-wing system detected the Deposit only on two flight lines. In exploration programs that seek 

to expand the flight line spacing in an effort to reduce the cost of the airborne survey, discrete conductors such 

as the Mesamax Deposit can go undetected. The argument often put forward in favour of using fixed-wing 

systems is that because of their larger footprint, the flight line spacing can indeed be widened. Many fixed-wing 

surveys are flown at 200 m or 400 m. Much of the survey work performed by Aeroquest has been to survey in 

areas that were previously flown at these wider line spacings. One of the reasons for AeroTEM’s impressive 

discovery record has been the strategy of flying closely spaced lines and finding all the discrete near-surface 

conductors. These higher resolution surveys are being flown within existing mining camps, areas that improve 

the chances of discovery. 

The footprint of a fixed-wing system is roughly 

150 m on either side of the transmitter
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

23

Figure 2. Fixed-wing (upper) and AeroTEM (lower) comparison over the eastern limit of the Mesamax 

Deposit, a Ni-Cu-PGE zone located in the Raglan nickel belt and owned by Canadian Royalties. Both 

systems detected the Deposit further to the west where it is closer to surface. 

The small footprint of AeroTEM combined with the high signal to noise ratio (S/N) makes the system more 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

24

suitable to surveying in areas where local infrastructure produces electromagnetic noise, such as power lines 

and railways. In 2002 Aeroquest flew four exploration properties in the Sudbury Basin that were under option by 

FNX Mining Company Inc. from Inco Limited. One such property, the Victoria Property, contained three major 

power line corridors. 

The resulting AeroTEM survey identified all the known zones of Ni-Cu-PGE mineralization, and detected a 

response between two of the major power line corridors but in an area of favorable geology. Three boreholes 

were drilled to test the anomaly, and all three intersected sulphide. The third borehole encountered 1.3% Ni, 

6.7% Cu, and 13.3 g/t TPMs over 42.3 ft. The mineralization was subsequently named the Powerline Deposit. 

The success of AeroTEM in Sudbury highlights the advantage of having a system with a small footprint, but also 

one with a high S/N. This latter advantage is achieved through a combination of a high-moment (high signal) 

transmitter and a rigid geometry (low noise). Figure 3 shows the Powerline Deposit response and the response 

from the power line corridor at full scale. The width of power line response is less than 75 m. 

Figure 3. The Powerline Deposit is located between two major power line corridors, which make EM 

surveying problematic. Despite the strong response from the power line, the anomaly from the Deposit is 

clearly detected. Note the thin formational conductor located to the south. The only way to distinguish 

this response from that of two closely spaced conductors is by interpreting the X-axis coil response. 

Advantage 2 – Conductance Discrimination 

The AeroTEM system features full waveform recording and as such is able to measure the on-time response due 

to high conductance targets. Due to the processing method (primary field removal), there is attenuation of the 

response with increasing conductance, but the AeroTEM on-time measurement is still superior to systems that 

rely on lower base frequencies to detect high conductance targets, but do not measure in the on-time. 

The peak response of a conductive target to an EM system is a function of the target conductance and the EM 

system base frequency. For time domain EM systems that measure only in the off-time, there is a drop in the 

peak response of a target as the base frequency is lowered for all conductance values below the peak system 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

25

response. For example, the AeroTEM peak response occurs for a 10 S conductor in the early off-time and 100 S 

in the late off-time for a 150 Hz base frequency. Because base frequency and conductance form a linear 

relationship when considering the peak response of any EM system, a drop in base frequency of 50% will double 

the conductance at which an EM system shows its peak response. If the base frequency were lowered from 150 

Hz to 30 Hz there would be a fivefold increase in conductance at which the peak response of an EM occurred. 

However, in the search for highly conductive targets, such as pyrrhotite-related Ni-Cu-PGM deposits, a fivefold 

increase in conductance range is a high price to pay because the signal level to lower conductance targets is 

reduced by the same factor of five. For this reason, EM systems that operate with low base frequencies are not 

suitable for general exploration unless the target conductance is more than 100 S, or the target is covered by 

conductive overburden. 

Despite the excellent progress that has been made in modeling software over the past two decades, there has 

been little work done on determining the optimum form of an EM system for mineral exploration. For example, 

the optimum configuration in terms of geometry, base frequency and so remain unknown. Many geophysicists 

would argue that there is no single ideal configuration, and that each system has its advantages and 

disadvantages. We disagree. 

When it comes to detecting and discriminating high-conductance targets, it is necessary to measure the pure in 

phase response of the target conductor. This measurement requires that the measured primary field from the 

transmitter be subtracted from the total measured response such that the secondary field from the target 

conductor can be determined. Because this secondary field is in-phase with the transmitter primary field, it must 

be made while the transmitter is turned on and the transmitter current is changing. The transmitted primary field 

is several orders of magnitude larger than the secondary field. AeroTEM uses a bucking coil to reduce the 

primary field at the receiver coils. The only practical way of removing the primary field is to maintain a rigid 

geometry between the transmitter, bucking and receiver coils. This is the main design consideration of the 

AeroTEM airframe and it is the only time domain airborne system to have this configuration. 

Figure 4. The off-time and on-time response nomogram of AeroTEM for a base frequency of 150 Hz. The 

on-time response is much stronger for higher conductance targets and this is why on-time 

measurements are more important than lower frequencies when considering high conductance targets 

in a resistive environment. 

Advantage 3 – Multiple Receiver Coils 

AeroTEM employs two receiver coil orientations. The Z-axis coil is oriented parallel to the transmitter coil and 

both are horizontal to the ground. This is known as a maximum coupled configuration and is optimal for 

detection. The X-axis coil is oriented at right angles to the transmitter coil and is oriented along the line-of-flight. 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

26

This is known as a minimum coupled configuration, and provides information on conductor orientation and 

thickness. These two coil configurations combined provide important information on the position, orientation, 

depth, and thickness of a conductor that cannot be matched by the traditional geometries of the HEM or fixedwing systems. The responses are free from a system geometric effect and can be easily compared to model type 

curves in most cases. In other words, AeroTEM data is very easy to interpret. Consider, for example, the 

following modeled profile: 

Figure 5. Measured (lower) and modeled (upper) AeroTEM responses are compared for a thin steeply 

dipping conductor. The response is characterized by two peaks in the Z-axis coil, and a cross-over in the 

X-axis coil that is centered between the two Z-axis peaks. The conductor dips toward the higher 

amplitude Z-axis peak. Using the X-axis cross-over is the only way of differentiating the Z-axis response 

from being two closely spaced conductors. 

HEM versus AeroTEM 

Traditional helicopter EM systems operate in the frequency domain and benefit from the fact that they use 

narrowband as opposed to wide-band transmitters. Thus all of the energy from the transmitter is concentrated in 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

27

a few discrete frequencies. This allows the systems to achieve excellent depth penetration (up to 100 m) from a 

transmitter of modest power. The Aeroquest Impulse system is one implementation of this technology. 

The AeroTEM system uses a wide-band transmitter and delivers more power over a wide frequency range. This 

frequency range is then captured into 16 time channels, the early channels containing the high frequency 

information and the late time channels containing the low frequency information down to the system base 

frequency. Because frequency domain HEM systems employ two coil configurations (coplanar and coaxial) there 

are only a maximum of three comparable frequencies per configuration, compared to 16 AeroTEM off-time and 

12 AeroTEM on-time channels. 

Figure 6 shows a comparison between the Dighem HEM system (900 Hz and 7200 Hz coplanar) and AeroTEM 

(Z-axis) from surveys flown in Raglan, in search of highly conductive Ni-Cu-PGM sulphide. In general, the 

AeroTEM peaks are sharper and better defined, in part due to the greater S/N ratio of the AeroTEM system over 

HEM, and also due to the modestly filtered AeroTEM data compared to HEM. The base levels are also better 

defined in the AeroTEM data. AeroTEM filtering is limited to spike removal and a 5-point smoothing filter. Clients 

are also given copies of the raw, unfiltered data. 

Figure 6. Comparison between Dighem HEM (upper) and AeroTEM (lower) surveys flown in the Raglan 

area. The AeroTEM responses appear to be more discrete, suggesting that the data is not as heavily 

filtered as the HEM data. The S/N advantage of AeroTEM over HEM is about 5:1. 

Aeroquest Surveys is grateful to the following companies for permission to publish some of the data 

from their respective surveys: Wolfden Resources, FNX Mining Company Inc, Canadian Royalties, Nova 

West Resources, Aurogin Resources, Spectrem Air. Permission does not imply an endorsement of the 

AeroTEM system by these companies. 
 Job# 11014

Aeroquest International - Report on a Helicopter-Borne AeroTEM IV Electromagnetic & Magnetic Systems Survey 

28

APPENDIX 4: AEROTEM INSTRUMENTATION SPECIFICATION SHEET 

AEROTEM Helicopter Electromagnetic System 

System Characteristics 

• Transmitter: Triangular Pulse Shape Base Frequency 90 Hz 

• Tx On Time – 1,900 (90 Hz) μs 

• Tx Off Time – 3,600 (90 Hz) μs 

• Loop Diameter - 12 m 

• Peak Current - 410 A 

• Peak Moment – 220,000 NIA 

• Typical Z Axis Noise at Survey Speed = 10 nT/s peak to peak 

• Sling Weight: 1200 lb 

• Length of Tow Cable: 50.1 m 

• Bird Survey Height: 30 m nominal 

Receiver 

• Two Axis Receiver Coils (x, z) positioned at a horizontal offset of 1m and 4.8 m from 

the centre of transmitter loop, respectively. 

• Selectable Time Delay to start of first channel 28 , 55, or 83 ms 

Display & Acquisition 

• AERODAS Digital recording at 36000 samples per second (27.778 μs channel width) 

• Recording & Display Rate = 10 readings per second. 

• On-board display - six channels Z-component and 1 X-component 

System Considerations 

Comparing a fixed-wing time domain transmitter with a typical moment of 500,000 NIA 

flying at an altitude of 120 m with a Helicopter TDEM at 30 m, notwithstanding the 

substantial moment loss in the airframe of the fixed wing, the same penetration by the lower 

flying helicopter system would only require a sixty-fourth of the moment. Clearly the 

AeroTEM system with 220,000 NIA has more than sufficient moment. The airframe of the 

fixed wing presents a response to the towed bird, which requires dynamic compensation. 

This problem is non-existent for AeroTEM since transmitter and receiver positions are fixed. 

The AeroTEM system is completely portable, and can be assembled at the survey site within 

half a day.