Comparison of different airborne TEM systems

Helicopter airborne SkyTEM, AeroTEM, VTEM and HELITEM systems reach penetration depths of 500 m and ensure quick high-quality surveys over hundreds and thousands of square kilometers for geological mapping, mineral exploration, and groundwater applications.

See Table 1 to compare the features and specifications of helicopter-borne SkyTEM, AeroTEM, VTEM and HELITEM systems.

Table 1.

Comparison of different airborne TEM systems

Parameter/System SkyTEM AeroTEM VTEM HELITEM
Number of turns in transmitter loop
Pulse shape



Peak current, A

Peak dipole moment, x1000 NIA

First gate, µs

1-16
square wave,
dual transmitter mode


120 А

3, 4, 32, 150, 500, 1000

4

8
triangular



250 А

40, 340, 1000



4
polygonal



310 А

240, 625, 1300

13

2
half-sine



1412 А

2000

175

The systems available at the market differ in their peak dipole moment. Small moments are designed to discriminate shallow (to 50-100 m) targets against highly resistive background.

Clients most often choose Geotech airborne versatile time-domain electromagnetic (VTEM) systems which have over two million line-kms flown. VTEM systems are available in three configurations, with dipole moments 240, 625, and 1300 x1000 NIA. VTEMEarly time collects data in 45 channels; measurements begin in very early times, with the first gate at 18 µs, thus delivering accurate data from the very top of the section.

SkyTEM is advantageous by operating in a dual transmitter mode with about ten times different peak currents (10 and 120 A) and first gates of a few microseconds to tens of milliseconds to map the near surface and look as deep as 500 m concurrently.

HELITEM with a peak dipole moment as large as 2000,000 NIA and the first gate as late as 175 µs is engineered mainly for deep soundings. The newly launched innovative MULTIPULSE system is dual-mode, with a high-power pulse (half-sine wave) for large exploration depths and a lower-power pulse (square wave) for near-surface (50-100 m) resolution.

TDEM Geomodel web app is designed to process airborne TEM data acquired at any pulse shape (square, triangular, half-sine, polygonal), gate, and fly height.


VTEM

Geotech (www.geotech.ca), with its filials and headquarters in Canada, Ghana, South Africa, Australia, China, Malta, and Brazil, is the world leader in airborne geophysical surveys. It has almost 30 years of experience in designing and using TEM systems. Geotech developed its first fully digital time-domain electromagnetic system (VTEM) in 2002 and had performed over 2000,000 km of geophysical surveys flown worldwide by 2011.

Geotech VTEM systems are available in three configurations for different applications: VTEM, VTEM Plus and VTEM MAX (Fig. 1). They differ in peak dipole moments and penetration depths: the dipole moments of VTEM and VTEM MAX reach 240,000 NIA and 1300,000 NIA, respectively. The features and specifications of VTEM, VTEM Plus and VTEM MAX are compared in Table 2.

Table 2.

Specifications of VTEM systems

Parameter/System VTEM VTEM plus VTEM MAX
Transmitter area per turn, m2
Number of transmitter turns
Pulse shape
Peak current, A
Peak dipole moment, м2
Rep. frequency, Hz
The measured field components
240 m2
4
polygonal
250 А
240000
25/30
Z
540 m2
4
polygonal
до 310 А
до 625000
25/30
Z, X, Y (option)
960 m2
4
polygonal

1300000
25/30
Z, X, Y (option)

airborne system VTEM
а
airborne system VTEM Plus
b
airborne system VTEM MAX
c

Fig. 1. Geotech VTEM systems: VTEM (а), VTEM plus (b) и VTEM MAX (c)

The system generates polygonal pulses and measures off-time earth responses (after transmitter turn-off).

VTEM Plus, one of most largely used systems, is easy to operate and easily disassembled to be shipped using standard containers max. 2.5 m. About 30 such systems were available worldwide in the beginning of 2010.

In 2008, three kimberlite pipes (Trio, Sparky, Gemeni) were discovered with VTEM data. Note that the data included a sign reversal at late times (a negative response corresponding to an IP effect), which was attributed to a shale conductor in the apical part of the DO27 kimberlite.

Cunion (2009) compared ground Protem and ZeroTEM (100 x100 m transmitter loop) and airborne VTEM results from ten kimberlites in the Kokong field (Botswana). VTEM was as efficient as the ground surveys at nine out of ten pipes (Fig. 2).

сравнение ProTEM, ZeroTEM и VTEM
Fig.2. Comparison of ground Protem and ZeroTEM (100 x100 m transmitter) and airborne VTEM data, after (Cunion, 2009).


SkyTEM

airborne SkyTEM

SkyTEM is remarkable for its dual-mode transmitter operation with more than ten-fold difference of peak currents (10 A and 120 A) and micro- to milli-second first gates of the two pulses. The system maps depths to 500 m and the near surface concurrently.

The vertical (Z) and horizontal (X) receivers are fixed on one end of an elongate hexagonal frame.

SkyTEM systems are available in five configurations (SkyTEM301, SkyTEM304, SkyTEM312, SkyTEM508, SkyTEM516) that differ in transmitter loop size, from 341 m2 in SkyTEM101 to 536 m2 in SkyTEM508. See Fig. 3 for photographs and Table 3 for specifications of the systems.

Table 3.

Specifications of SkyTEM systems

Parameter/System SkyTEM301 SkyTEM304 SkyTEM508

Transmitter area per turn, m2
Number of turns
Peak current, A
Peak dipole moment, NIA

Frequency, Hz
Gate center time

Total weight
LM mode

341
1
6
2000

325
4.2 - 346 µs

500 kg
HM mode

341
1
95
32000

25
44 - 3525 µs


LM mode

341
1
9
3000

275
5 - 876 µs

550 kg
HM mode

341
4
110
150000

25
70 - 8904 µs


LM mode

536
1
7
5000

275
7-874 µs

720 kg
HM mode

536
4
120
500000

25
65 - 13352 µs



airbone system SkyTEM301
a
airbone system SkyTEM304
b
airbone system SkyTEM508
c

Fig.3. SkyTEM systems: SkyTEM301 (a), SkyTEM304 (b), SkyTEM508 (c).


HELITEM

HELITEM was designed by Fugro Airborne Surveys (formerly, till summer 2013, affiliated with Fugro Group; currently with CGG).

See Fig. 4 for a photograph and Table 4 for specifications of the HELITEM system.

Table 4.

Specification of HELITEM system

Parameter/System HELITEM
Transmitter area per turn, m2
Number of turns
Pulse shape
Peak current, A
Peak dipole moment, NIA
Frequency, Hz
The measured field components
First gate, µs
708 m2
2
half-sine
1412
2000000
25/30
Z, X, Y
175

airborne EM HELITEM

Fig.4. HELITEM system.

HELITEM transmitter generates half-sine pulses at a peak current as high as 1412 A and a peak dipole moment reaching 2000,000 NIA, the largest among existing airborne systems.

The system measures three components (X, Y, Z) of the secondary anomalous response; on- and off-time recordings are in 4 and 26 channels, respectively. On-time and off-time measurements begin at first gates 155 µs and 175 µs, respectively, and fail to cover the top 100 m of the section.

The innovative MULTIPULSE system launched by Fugro Airborne Surveys on 10 September 2013 is operated at a dual mode, with a high-power pulse (half-sine wave) for large exploration depths and a lower-power pulse (square wave) for near-surface (50-100 m) targets; the new system provides improved near-surface resolution. Before MULTIPULSE, it was only SkyTEM that used a square pulse of a small peak current (about 10 A) to collect high-quality data from the near-surface.