It is well known that there is a trade-off between depth of exploration and target definition in terms of conductivity, extent and orientation. Greatest depth is obtained with large fixed loop Turam-type systems which generate large half space responses and along with current gathering makes target detection difficult. Better spatial resolution is obtained with a moving transmitter configuration with a short intercoil spacing but is limited to a shallower depth of exploration. These variations in survey requirements make system flexibility an important design consideration.
Time Domain systems are also now routinely employed for general geological exploration such as for freshwater aquifers in bedrock fractures, and mapping groundwater contaminant plumes. Mapping to the shallow depths required in these applications requires a very wide bandwidth and many narrow sampling gates.
Recognition of these diverse requirements led GEONICS to develop the extremely flexible PROTEM time domain system. The digital, 3 channel receiver is used with any of the 3 TEM transmitters and choice of receiver coil to cover all applications. With its 23 bit resolution, system bandwidth of 500kHz, microsecond sampling gates and simultaneous XYZ component measurements, the PROTEM receiver provides the ultimate in time domain capability. Used with the GEONICS 3-component coil, mineral surveys are quickly completed with more data in either the fixed loop or slingram mode. The three component measurement also allows a quick and accurate check on geoelectric sounding data for lateral variations in conductivity which could invalidate a layered-earth interpretation.
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Three interchangeable transmitters - TEM47, TEM57 and TEM37 - are used with the PROTEM receiver and the appropriate receiver coil to make up different PROTEM systems for various applications such as mineral exploration, structural mapping, resistivity sounding and contaminant plume mapping.
The TEM47 is the smallest, lightest battery operated transmitter with a very fast turn-off time to enable the near surface response to be measured. The PROTEM 47 (PROTEM receiver and TEM47 transmitter) is most often used for shallow geoelectric sounding looking for conductive contaminant plumes, saline intrusion or general stratigraphy mapping. In this mode single turn transmitter loops from 5 m up to 100 m on a side with turn-off times as short as half a microsecond can be used to give maximum near surface resolution.
The maximum transmitter output of 3 A into a 100 m x 100 m loop gives a good response and resolution to depths down to 150 m making this the ideal instrument for resistivity sounding over a large area.
The TEM47 uses a reference cable to achieve the high synchronization accuracy required for shallow sounding. Regardless of application, the high frequency receiver coil is used in PROTEM 47 systems. This receiver coil has the bandwidth necessary to capture the earliest portion of the transient decay.
When the TEM47 is used in a PROTEM 47 system for profiling, it supplies 3 A to an 8-turn 5 x 5 m moving transmitter loop to provide a dipole moment of 600 ampere square metres. With base frequency of 75 Hz and 20 gates from 49 microseconds to 2.9 milliseconds this configuration is optimal for slingram (horizontal loop) surveys for mineral exploration to shallow depths, and for groundwater exploration in bedrock fractures. Electrical sounding is performed simultaneously with the search for fault or dike-like targets.
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The TEM57 and PROTEM receiver are the principal components of PROTEM 57 systems. The design and performance of the PROTEM 57 make it a highly portable, powerful and versatile time domain system.
The TEM57 is powered by either a 600 W generator or by rechargeable batteries in a backpack. PROTEM 57 systems are synchronized by either a reference cable or quartz crystal, usually determined by the size of the loop and whether they are being used for large loop soundings or profiling.
The PROTEM 57 is used for a wide variety of applications. The system can sound the depth, thickness and conductivity of layers down to 300 m below surface, for applications such as mapping the thickness of aquifers, clay layers and assessing water quality. In coastal areas, the PROTEM 57 has defined the depth to saline intrusion as accurately as chemical samples from wells.
The PROTEM 57, with a short reference cable, portable transmitter and 3-D receiver coil can delineate complex ore bodies within 200 m of surface. Deeper conductors can be characterized by profiling with a crystal synchronized receiver and a large, fixed transmitter loop. Modelling provides conductivity thickness, dip and extent of the ore body,
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The TEM37 is the most powerful transmitter used with the PROTEM receiver forming the basis of the PROTEM 37 (formerly the EM37-3) system. The high powered PROTEM 37 can be used to sound to depths greater than 1 kilometre, and in the large fixed loop profiling mode, detect and delineate ore bodies at over 500 metres.
The transmitter and receiver in a PROTEM 37 system are equipped with quartz crystals for synchronization, although a reference cable can be used if transmitter-receiver separation is less than 200 m. The TEM37 is powered by a specialized motor-generator, which can energize loops up to 2000 x 2000 m in size.
The PROTEM 37 is used to profile deeply buried conductors, which makes the system a principal tool of mineral exploration. Both the 3-D receiver coil and BH43 borehole probe are used routinely with the system. The PROTEM 37 also enjoys widespread application for deep soundings for groundwater exploration, mapping saline intrusion, and for geothermal exploration as well as general regional geological research where structures and layers can be detected at depths greater than 1000 m.
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The BH43 provides 3-dimensional time-domain EM exploration from boreholes, in conjunction with a PROTEM system. Boreholes as deep as 2 km can be surveyed using a PROTEM system with a 500 x 500 m transmitter loop; at developed mines, the transmitter loop can be laid out in underground workings.
The probe has 3 sensors, which simultaneously measure orthogonal components of decay. Along the hole, spatial resolution as fine as 1 m can be obtained; the actual measurement interval depends on the desired resolution of the response.
The wide bandwidth of the probe, coupled with the excellent temporal resolution and large dynamic range of the PROTEM system provides maximum diagnostic information and a high degree of rejection of powerline and other noise sources.
Computer programs for editing, displaying and interpreting BH43 responses are supplied with the probe, including a program for calculating all field-components in a conductive earth. The BH43 probe is available separately, or as a complete borehole system with cable, main winch, dummy probe, test cable and winch, and retrieval tools.
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The EM42 is the deepest penetrating time-domain EM system made by GEONICS. It uses long grounded-wire or large-loop transmitters to generate primary fields that can exceed 3 x 10^8 A-m^2. The EM42 uses very low repetition rates to measure the EM transient over a very broad spectrum. Specialized hardware and software for digital signal processing enhance the signal-to-noise ratio.
The EM42 is used for deep geological sounding, with applications in petroleum, mineral, groundwater and geothermal exploration. Extensive software for data reduction and interpretation is supplied with the system.
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