Electromagnetics

Based on the design principles of inductive electromagnetics, Ground Conductivity Meters provide a non-invasive method for measurement of subsurface conductivity and magnetic susceptibility. Without any requirement for soil-to-instrument contact, surveys can be performed quickly – facilitating dense data collection and, consequently, excellent spatial resolution – and over most geologic environments, including conditions of highly resistive surface materials such as sand and gravel.

VLF TRANSMITTER/ RECEIVER

EM16 | EM16R | TX27

 

The EM16 VLF Receiver is the most widely used electromagnetic geophysical instrument of all time. Local tilt and ellipticity of VLF broadcasts are measured and resolved into inphase and quadrature components of VLF response. The EM16 has discovered several base and precious-metal ore bodies and many water-bearing fractures and faults.

The EM16R Resistivity Attachment uses a pair of electrodes to measure the apparent resistivity of the earth. The combined EM16/16R instrument can detect a second earth layer if the layer occurs within the VLF skin-depth. In addition, the EM16/16R can map resistive alteration for gold exploration.

The TX27 is a portable VLF transmitter supplying a VLF field for surveying with either the EM16 or EM16/16R if remote broadcasts are weak, intermittent or poorly coupled with the target. For EM16 surveys, the TX27 antenna consists of a long (typically 1 km) grounded wire.

MEASURED QUANTITIES

EM16: inphase and quadrature components of the secondary VLF field, as percentages of the primary field

EM16R: apparent resistivity in ohm-metres, and phase angle between Ex and Hy               

PRIMARY FIELD SOURCE

EM16: ferrite-core coil

EM16R: Stainless-steel electrodes, separated by 10 m: impedence of sensor is 100 MΩ in parallel with 0.5 pf             

SENSOR

9.8 kHz

OPERATING FREQUENCY

15 to 25 kHz (optionally to 30kHz) depending on VLF broadcasting station

MEASURING RANGES

EM16: inphase: ±150%; quadrature: ±40%

EM16R: 300,3K,30KΩ-m; phase: 0 – 90°

POWER SUPPLY

EM16/EM16R: 6 alkaline “AA” cells

DIMENSIONS

EM16/EM16R: 53x30x22 cm

WEIGHTS

EM16:1.8 kg;shipping:6.2 kg

EM16R:1.5 kg;shipping:6 kg

Borehole Probes

Borehole geophysics provides a means of evaluating various properties of subsurface rocks, soils and fluids. Probes measuring various, specific properties are lowered into boreholes collecting either continuous or point data. During logging, data is both presented graphically in real-time, and recorded digitally for subsequent review and analysis.

Providing critical information not available through other means of investigation, the use of borehole probes is common for many applications including groundwater and natural resource exploration, and environmental characterization.

EM39 Conductivity

The EM39 provides measurement of the electrical conductivity of the soil water matrix surrounding a borehole or monitoring well using the inductive electromagnetic technique. The unit employs coaxial coil geometry with an intercoil spacing of 50 cm to provide a substantial radius of exploration into the formation while maintaining excellent vertical resolution. Measurement is unaffected by conductive borehole fluid or the presence of plastic casing. The instrument operates to a maximum depth of 500 metres.

The combination of a large conductivity range, high sensitivity and very low noise and drift, allows accurate measurement of subsurface conditions. Typical applications include groundwater contamination monitoring, groundwater and mineral exploration, and general geotechnical investigations.

The 4-conductor EM39 probe can be used with many commercially available borehole logging systems, or with a dedicated winch and console system from Geonics.

Measurements can either be recorded with the DL600 digital data logger, or viewed in real-time using the EM39RT program with field computer.

GAMMA39 Natural Gamma

The EM39 Borehole Conductivity Probe has been designed for rapid, accurate measurements of groundwater contamination in the earth and rock surrounding monitoring wells. Since clays also impact the electrical measured conductivity, Geonics introduced the GAMMA39 Natural Gamma Ray Probe to resolve this ambiguity. Whereas conductivity highs with coincident gamma ray highs often indicate enhanced clay content, conductivity highs not associated with a gamma ray high can be expected to be due to enhanced TDS in the groundwater.

Like the conductivity probe, the gamma ray probe is unaffected by plastic casing in the well. It requires no special licences, can be used anywhere, is relatively fast to operate and can, of course, also be employed to detect radioactive wastes in the ground.

EM39S Magnetic Susceptibility

The EM39S probe provides measurement of the magnetic susceptibility of the formation in the vicinity of a borehole or monitoring well. The EM39S, with intercoil spacing of 50 cm, provides good vertical resolution while still achieving a reasonable range of investigation into the surrounding medium. High sensitivity and low noise performance characteristics ensure an excellent range of measurement for most geological applications.

The susceptibility response is unaffected by plastic casing, and unlike conventional magnetometers, is unaffected by either variations in remanent magnetization of the surrounding soil or rock, or the strength of the earth’s magnetic field.

Ground Conductivity Meters

EM31-MK2

The EM31-MK2 maps geologic variations, groundwater contaminants, or any subsurface feature associated with changes in ground conductivity. Using a patented electromagnetic inductive technique that allows measurements without electrodes or ground contact. With this inductive method, surveys can be carried out under most geologic conditions including those of high surface resistivity such as sand, gravel, and asphalt.

Ground conductivity (quad-phase) and magnetic susceptibility (in-phase) measurements are read directly from an integrated DL600 data logger (which can be easily removed from the console for data transfer). Real Time (RT) graphical presentation of the data during collection is possible by connecting a computer directly to the RS232 output port on the front panel with an optional RS232 interconnect cable.

The effective depth of exploration is about six metres, making it ideal for geotechnical and environmental site characterization. Important advantages of the EM31-MK2 over conventional resistivity methods are the speed with which surveys can be performed, the precision with which small changes in conductivity can be measured and the continuous readout and data collection while traversing the survey area. Additionally, the in-phase component is particularly useful for the detection of buried metallic structure and waste material.

EM31-SH

The EM31-SH is a “short” version of the EM31-MK2 providing an effective depth of exploration of about four metres. With a smaller coil separation (2 m) and lighter weight, the EM31-SH offers improvements in sensitivity to smaller near-surface targets, lateral resolution and portability, while maintaining the high levels of accuracy and stability provided by the standard EM31-MK2. A “trailer-mount” (see photo) is available for either instrument, offering greater convenience in some field operations.

EM34-3

The EM34-3 is a simple-to-operate, cost-effective instrument for the geologist and hydrogeologist alike; applications have been particularly successful for the mapping of deeper groundwater contaminant plumes and for the exploration of potable groundwater resources.

Using the same inductive method as the EM31-MK2, the EM34-3 uses three intercoil spacings – 10, 20 and 40 m – to provide variable depths of exploration down to 60 metres. With three intercoil spacings and two dipole modes (horizontal as shown, and vertical) at each spacing, vertical soundings can be obtained. In the vertical dipole (horizontal coplanar) mode, the EM34-3 is very sensitive to vertical geologic anomalies and, as a consequence, is widely used for groundwater exploration in fractured and faulted bedrock.

The EM34-3 includes connectors for an analog signal output, as well as an input which can be used with a rechargeable battery option. Digital signal output, required for data collection with the DL600/DAS70 system, is available as an option for all models of the EM34-3.

EM34-3XL

In regions of particularly high cultural and/or atmospheric noise, the EM34-3XL – including increased transmitter power and a larger transmitter coil – improves the signal-to-noise ratio by a factor of 10 at the 40 m spacing, and by a factor of 4 at the 10 and 20 m spacings.

EM38-MK2

The EM38-MK2 provides measurement of both the quad-phase (conductivity) and in-phase (magnetic susceptibility) components within two distinct depth ranges, all simultaneously, without any requirement for soil-to-instrument contact. With a maximum effective depth of exploration of 1.5 m, applications in agriculture, archaeology and general soil sciences are common.

The standard EM38-MK2 includes two receiver coils, separated by 1 m and 0.5 m from the transmitter, providing data from effective depth ranges of 1.5 m and 0.75 m respectively when positioned in the vertical dipole orientation, and 0.75 m and 0.375 m respectively when in the horizontal dipole orientation. The EM38-MK2-1 model includes one receiver coil only, at 1 m from the transmitter.

New coil technology, supported by temperature compensation circuitry, markedly improves temperature-related drift characteristics as compared with the preceding generation of EM38 instruments.

An optional, collapsible calibration stand supports automation of the instrument calibration procedure. Once positioned within the stand, the instrument can be calibrated within seconds, without any requirement for iterative adjustments.

Supporting both walking and trailer-mounted survey methods, external power sources can be connected to the instrument for extended field operations; an optional, lightweight, rechargeable battery pack provides for 25 hours of continuous operation. Also for trailer- mounted operations, a protective capsule, constructed of durable plastic materials, is available as an option.

Data collection is supported by the DAS70-AR Data Acquisition System, or other appropriate field computer, connected by either RS-232 serial cable or Bluetooth wireless technology.

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