MinEx CRC are developing a downhole swept frequency electro-magnetic (EM) tool. The technology is documented in a confidential patent application. The tool incorporates an electromagnetic sensor system, that facilitates rapid cycling across frequency ranges between ~1000 to 100,000Hz, with multiple advantages over single or dual frequency tools. The prototype tool fits within NQ diameter boreholes and can be deployed by wireline or fitted to the drill string for logging while-
tripping or logging-while-drilling applications.

Exploration value of downhole EM

Borehole electro-magnetic (EM) tools have proven value in mineral exploration due to their potential to ‘see’ variations in electrical conductivity in the rocks surrounding a borehole. Rocks with high conductivity include many orebody types, particularly those containing a high proportion of sulphide minerals. These include critical minerals that underpin modern life.

The distance that EM tools can ‘see’ around the borehole (known as the radius of investigation, ROI) has a dependence on the transmitted frequency and the conductivity distribution in the formation.

Conventional EM tools with single or dual frequency are limited to delivering averaged conductivity estimates of the material within relatively narrow
ROIs. Tools of specific frequency need to be carefully chosen to match target characteristics and ground conditions.

In contrast, the MinEx CRC broad bandwidth swept frequency tool delivers rich EM information content, with enhanced ROI at each frequency sweep. The MinEx CRC system has numerous advantages over single or dual frequency tools.

Advantages of the swept frequency EM

Swept frequency EM has a number of advantages:

• Wide range of penetration depths (ROI)
• Broad application across different target characteristics and ground conditions
• Enhanced potential to detect off-hole features
• Signal-to-noise is readily optimised for targeted geo-electrical settings
• Rich, multidimensional EM data with potential to apply ML techniques

Fit for purpose

The sensor system is small, light (less than 1kg), with independent power, independent data storage and a wireless communications system.When assembled in its housing, the prototype tool has a length of ~1.7m and weight <5kgs making the prototype Swept Frequency tool lightweight, independent and easy to transport.

The assembled prototype has an outerdiameter of 60mm. Significant further miniaturisation for future version of the tool  is readily achievable.

The tool will be driller deployable; either by wireline or by attachment to the drill string bottom hole assembly for logging-whiletripping applications.

Logging can be conducted in parallel with complimentary survey and geophysical logging techniques with little to no additional time penalty.

Reliable, quantified analyses

Test results and calibration of the MinEx CRC downhole swept frequency tool are underway. Several comparisons of the sensor response with conventional induction logging tool have been completed.

Ultimately standardised calibration procedures and processing methods will be provided to users to ensure accurate, precise analyse.

Project Industry Participants:

• IMDEX
• MRIWA
• Rio Tinto
• South 32

Project Research Participants:

• Curtin University
• UniSA
• CSIRO