The PEM method uses precisely controlled surface transmitters which send out a time domain electromagnetic signal deep into the earth. The signal interacts with the changes in electrical conductivity associated with mineral deposits, structural features or pore water. A receiver sensor detects and measures the secondary field in three orthogonal (X,Y,Z) directions as it is moved along the surface or in exploration drill holes. The changes are recorded digitally at the receiver console and displayed in real time at various time intervals after the transmitter signal is abruptly turned off. Interpretation based on theory, modelling and field geological data and experience, results in realistic predictions of the geology below.
All survey components are man-portable, easily deployed and proven under extreme conditions around the world.
The field methods developed for PEM by Crone address all situations likely to be encountered in mining and exploration. For example, DEEPEM uses a large fixed transmitter loop, up to 1000m x 2000m and a moving receiver outside the loop. This approach helps detect and resolve deep multiple zones while maintaining high daily production rates. In fact, a recent modification of this method, using two receivers and a combination of DEEPEM and large in-loop surveys, has been used as a reconnaissance tool to cover large areas very quickly, efficiently and economically. This method can be used instead of the more typical moving loop survey for reconnaissance exploration.
Other methods include:
- Moving In-Loop – (an excellent reconnaissance exploration tool)
- Moving Loop – (or “Sling Ram”, often used in conjunction with above)
- Large In-Loop – (for horizontal bodies) Moving loop surveys are used as a reconnaissance tool when little is known of the attitude (dip, strike and plunge) of the mineralisation being sort.
- Fixed loop surveys are used to better define the mineralisation when there is some indication of the dip and strike of the target and readings can be taken either inside or outside (or both) to maximise coupling.
Recently, Outer-Rim Exploration Services has developed a large fixed loop reconnaissance survey method which competes favourably with moving loop surveys on a cost and time basis. This method allows for the problems associated with poor coupling and can cover large areas very quickly using multiple receivers. The method has the advantage over moving loop of “looking” deeper and providing valuable structural and lithological information using current channelling.
The LANDTEM can be used, in place of the standard coil, in all of the above modes of operation.
Downhole EM surveys, utilising the Crone 3 Component technology, are a valuable tool for target definition of conductive mineralisation – the use of DHEM can significantly reduce the drilling costs associated with ore discovery and delineation. The transmitter loop layout can be configured to obtain specific information depending on the target and site characteristics.
Modelling of three component borehole EM data shows what we can expect to see in the axial and cross components for a particular survey. The following diagram (Fig #1) shows the modelled response of a flat lying, 250m x 250m, 50 mhos conductive plate at 550m depth. The vertical drill holes all missing the conductor by 75m from its edge. Diagram (Fig #2) shows actual example where the cross components indicate the direction of the conductor from the hole.
Both figures courtesy of Crone Geophysics & Exploration Ltd
In Figure 2, the first box shows the drill hole and loop geometry.
The second box (going down the page) shows the plots of the three components from the PEM survey. The axial component indicates an off-hole conductor at a depth of 430m. The X and Y components cross over from positive to negative at the same depth.
The next box shows the primary field in a section through the hole. The X component crosses over from positive to negative at approximately 240m, the Y component is zero and the axial component is positive throughout the hole.
The last box on the left hand side shows the correlation between the theoretical and measured primary field. The slight discrepancy is due to the use of a straight line segment to approximate the hole. The X component cross over is a critical point and this matches perfectly. This information can be used for probe rotation.
The first box on the right hand side shows eight thin plate models at various directions from the hole. The size, conductance and distance were determined by a best fit to the axial component data.
The next box shows the best fit for each of the plates in the previous box. It is very difficult to get a reliable direction to the conductor from these models.
The next shows the modelled X-Y data. This gives a very clear direction to the conductor as the ‘below – right’ model is the only one where both the X and Y components cross from positive to negative.
The last box shows the results of drilling to the northwest (as the interpretation indicates). Between 481.4m and 482.3m, thinly bedded silty argillite was intersected with 10cm seams of massive pyrrhotite and trace chalcopyrite. The X-Y data can still be used for these in-hole conductors – the centre of the conductor lies down dip and slightly west of the intersection point.
- Measured Parameters: Secondary transient EM field in nanoTeslas/second in three orthogonal directions. Borehole data is corrected for probe rotation.
- Display: Decay curve and profile channel responses in high resolution graphic mode.
- Time Channels: Programmable channel position and width. Synchronisation: Cable, radio and matched crystal clocks.
- Transmitter Power: 24VDC battery or 4.8kW generator.
- Transmitter Loop Sizes: User selected single or multi-turn loops up to 2000 x 2000m.
- Borehole Probes: Axial probe – 31.5mm diam., 1.6m long. X-Y Probe – 31.5mm diam., 2.0m long. O’Tool – 31.5mm diam., 1.2m long. (All pressure tested to 3000m depth.)
- Power Winches: Capacities to 3000m with high strength, kevlar reinforced cable.
Outer-Rim Exploration Services is committed to securing the health and safety of all its employees and the safety of the client’s representatives and others in the work place.
Outer-Rim believes that injuries are preventable and that all employees have an obligation to work in a manner which adheres to the established safe work practices, ensuring the health and safety of the individual and that of others in the vicinity. All practical steps must be taken to prevent injury to employees and other persons working or visiting the work place.
The management is constantly reviewing the Company’s safety procedures, in consultation with its employees, in an effort to improve and refine the procedures. The management believes that the training of all employees in safe work practices is essential and that this will be an ongoing programme.
A full copy of our Safety Policy is available upon request.