GEM 3 Broadband Digital EM Sensor
EM Instrument

About the GEM-3

GEM-3 is a broadband digital electromagnetic sensor suitable for detecting buried metallic objects such as utilities, unexploded ordnance (UXO), and landmines. It is light, portable, and versatile. The basic unit weighs about 10 pounds (5kg). Since it is completely digital, the sensor has the potential to be programmed in many other operational modes.

Through many field trials and surveys since it debuted in 1996, the GEM-3 has demonstrated some unique capabilities as summarized below:

  • In demonstrations at controlled test sites prepared by the US government, the GEM-3 routinely surpassed all other sensors. Optional audio output can facilitate realtime detection in rough and heavily vegetated areas. 
Discrimination, Characterization, or Identification
  • Broadband EM data has demonstrated a strong potential for discriminating UXO or landmines from clutter based on Electromagnetic Induction Spectroscopy (EMIS). 
    • Electromagnetic Induction Spectroscopy (EMIS)
      An object partly or wholly made of metals manifests a unique spectral response under a broadband EM interrogation. An EMIS spectrum, which depends on geometrical shape and metal compositions of an object, may be able to identify an object based on its spectral fingerprints. EMIS is a newly discovered phenomenology for hidden object identification (US Patent No. 5,963,035). 
Metal Detection in Magnetic Geology
  • The GEM-3 quadrature components are relatively immune to magnetic soils or rocks. A sum of all quadrature components (the optional audio is based on this sum), therefore, is an excellent detection indicator. 
The GEM-3 sensor head contains three concentric coils. Two outer coils are used as a transmitter and the innermost one is used as a receiver coil. The two transmit coils creates a central magnetic cavity that produces zero output to the inner receiver coil. For a frequency-domain operation, the GEM-3 prompts for a set of desired transmitter frequencies. Built-in software converts this into a digital “bit-stream,” which is used to construct the desired transmitter waveform. This bit-stream represents the instruction on how to generate a complex waveform that contains all frequencies specified by the operator.

Sensor Specifications

The basic GEM-3 Package consists of: a 64-cm diameter sensing head, handle boom, console and display unit, and battery charger. Standard software includes WinGEM, Windows-based operation software. The optional 96-cm head, due to its size, must be mounted on a cart.
  • Programmable Operation 
    • Bandwidth -  30 Hz to 24 kHz 
    • Frequency domain  - Single, multiple, or stepping frequencies 
    • Maximum sampling rate -  Approx. 15 Hz at one frequency or 8 Hz at 10 frequencies 
  • Hardware
    • Removable sensor head - Sealed fiberglass, 64cm in diameter -standard; Options, 40cm or 96cm in diameter 
    • Console (CPU, display, etc) - 11cm x 17cm x 5cm anodized aluminum box (2.5 kg) 
    • Internal rechargeable battery - 13.2 VDC 
    • Coils configuration - Concentric 
  • Output
    • Inphase and quadrature in ppm stored in the console; capacity is approx. 50,000 data points 
    • The data are also available in realtime thru an RS232 port (“remote mode”) 
    • Realtime digital display 
    • Detection audio, representing the sum of all quadrature components - optional 
  • Software and Operation Manual
    • WinGEM – Windows-based upload and data download through a PC 
    • PPM to conductivity or permeability conversion programs - optional 
  • GEM-3 Hardware Options
    • 40cm or 96cm diameter sensing heads 
    • 96-cm diameter head mounted on a 2-wheel cart with GPS 
    • Realtime detection audio output 
    • GPS antenna mount assembly
  • GEM-3 Software Options
    • Conversion of ppm data to conductivity and permeability 
    • EMIS discrimination software 
Simple Handheld Survey
The GEM-3 can be handheld for the 40cm and 64cm diameter heads. The 96cm diameter head must be mounted on a cart. The GEM-3 collects data at a constant interval in time. For a simple survey, the GEM-3 is self-sufficient for dead reckoning navigation and can store about 50,000 data points before it requires downloading to a PC, using a Windows-based software called WinGEM. 

Handheld 40-cm head

In "remote mode",the raw ASCII data are available to be exported through an RS-232 port. This feature enables integrating the GEM-3 with a PC that can accept GPS or other navigation aids for precision surveys. In the remote mode, the GEM-3 can run continuously by downloading the data directly into a PC.

Handheld Survey with Audio Output

The GEM-3 can be used in a way similar to “mag-and-flag” so that the operator can flag targets in realtime. This approach can be useful in rough and vegetated areas where a systematic, or “georeferenced,” survey is difficult. The GEM-3 sensing head is only 0.5-inch (1.3cm) thick and can be easily slipped under shrubs. An optional audio produces a sound that represents the sum of quadrature responses at all frequencies. The quadrature component is relatively immune to magnetic geology and to variations in the sensor height. 

Handheld Survey with GPS Navigation
The handheld GEM-3 GPS mount consists of two custom-made fiberglass tubes equipped with nylon screws so that they can be inserted and tightened along the GEM-3 handle boom. Two photos below show how the mount is assembled onto the GEM-3. An optional handle bar is provided to help carry the sensor. Customers may specify extra features.

GEM-3 Survey on a Cart

The GEM-3 Carts are built for large sensing heads, the 96-cm diameter head and possibly for an array of the GEM-3. The cart contains no metal parts and is built only with synthetic materials – fiberglass, nylon, and PVC. In fact, the cart is built in modules so that any other sensor platforms may be mounted between the two detachable wheel assemblies. The wheels are 50-cm in diameter with a foam-filled tire along with custom-built fiberglass axles. The cart includes a GPS antenna mount. Upon order, customers may specify extra features and functions. 

Pushcart for a 96-cm dia sensor head with a
GPS antenna

Underwater GEM-3 Survey
With proper modifications for mounting and waterproofing, the GEM-3 also can be used for underwater surveys as shown below.

Ordering Information

Order Number
GEM-3 530-GEM3-2002