Downhole Magnetometric Resistivity Research Articles

Down-hole magnetometric resistivity inversion for zinc and lead lenses localization at tobermalug, county Limerick, Ireland

The magnetometric resistivity (MMR) method consists of measuring the variation of magnetostatic field associated to current density variations. It is based on the principle that direct current flow between two electrodes takes the path of least resistance through the earth. Consequently, conductive or relatively conductive ore bodies such as massive, semi-massive to disseminated sulphides are preferentially energized, which increases the current density in their vicinity. MMR is more efficient than electromagnetic methods in the case of weak conductivity contrasts.To improve the signal to noise ratio the magnetic field can be measured in boreholes rather than on the surface. Down-hole magnetometric resistivity measurements have been conducted in Tobermalug prospect in County Limerick, Ireland. The survey was used as an alternative to down-hole electromagnetic to delineate subhorizontal zinc/lead mineralization lenses that can be poorly conductive. Two survey areas were investigated, DHMMR1 and DHMMR2.Interpretation is based on the regularized least-squares inversion of MMR data, in which the MMR forward problem is solved with a finite volume discretization of the electrostatic and magnetostatic equations. Inversion of synthetic data generated over two conductive horizontal discs model shows that conductive structures are well positioned but their extensions is biased along current electrodes orientation.Inversion of the field data allowed localizing a few conductive elongated targets. At DHMMR1, the conductivity is weaker and seems to be associated to disseminated mineralization. DHMMR2 contains a higher conductivity and more elongated target. It appears to be associated to semi-massive sulphides.

Looking left, right and centre with DHMMR

The Down-hole Magnetometric Resistivity (DHMMR) technique uses a grounded dipole to inject current into the ground and a downhole sensor to measure the resultant magnetic fields. Until recently, the sensor was usually a single component induction coil probe. Recently a 12-hole DHMMR program conducted on the North Mine, Broken Hill, used a 3-component B-field `Atlantis' probe. The results from this survey were spectacular, particularly when considering the target and location: The target was the narrow discontinuous ribbons of low-conductivity Zinc Lodes, located 20-50m above the highly-conductive main North Mine orebody, and underneath/next to the North Mine infrastructure and development. One risk facing the survey was that the main orebody would act as a short-circuit, causing the impressed current to avoid the Zinc Lodes entirely. To mitigate this, one transmitting dipole electrode was placed down a deep drill hole in a Zinc Lodes intersection and the other was dug into a surface expression of the Zinc Lodes, ~1.5km south of the drill hole electrode. This layout very effectively isolated and energized the Zinc Lodes mineralisation. The 3-component B-field probe has a noise level, at the frequencies used in the Broken Hill survey, which is significantly below that of sensors used previously for DHMMR. This resulted in better data and faster acquisition times. Additionally, there is relatively little processing required after the survey to present data in a meaningful manner for interpretation. One final product of the survey was vector geomagnetic data (not utilized to date). The success and accuracy of this survey using new equipment in difficult conditions is expected to lead to a wider and better appreciation of DHMMR's capabilities.

A comparison of receiver technologies in borehole MMR and EM surveys

ABSTRACTA series of downhole magnetometric resistivity (DHMMR) and downhole electromagnetic (DHEM) surveys were conducted near Broken Hill, New South Wales, Australia, and at Zinkgruvan, Sweden, to determine how probe and receiver equipment choices affect the amount of noise visible in borehole MMR and EM data. Noise analyses performed on the data, using the standard deviation to gauge the relative noise levels between different probes and receiver systems, indicate that high noise levels in MMR data result primarily from the use of a three‐component EM probe, which has a reduced effective area and hence a higher noise floor compared with a single‐component EM probe. High noise, attributable to cultural sources such as nearby power lines, in either MMR or EM data can be reduced through the use of full‐waveform, multipurpose receiver systems. These systems allow for the use of tapered stacking, which is a more effective method of eliminating coherent noise associated with power‐line transients than the boxcar‐stacking method used by traditional receiver systems.

B-Field Probes for Downhole Magnetometric Resistivity Surveys

The downhole magnetometric resistivity (DHMMR) method relies on the measurement of low-amplitude (~100 pT), low- frequency (1–4 Hz) magnetic fields arising from galvanic current flow in the Earth. Normal magnetic sensors have been used extensively in surface magnetometric resistivity (MMR) surveys, but equivalent B-field probes have not been generally available for use in standard mineral exploration drillholes. However, recent advances in magnetic sensor design have led to the manufacture of B-field probes, which provide significant advantages over the alternative TEM probes for DHMMR surveys. Synthetic and laboratory examples highlight the differences between data acquired from B-field and TEM probes. Field examples from the Flying Doctor Prospect, near Broken Hill, New South Wales, demonstrate the improved resolution achieved by using a B-field probe for DHMMR surveys.

Simultaneous modelling of the phase and amplitude components of downhole magnetometric resistivity data

The downhole magnetometric resistivity (DHMMR) technique is an effective method for follow-up exploration of massive and disseminated sulphide deposits. The method comprises the “in-hole” measurement of low-amplitude, low-frequency magnetic fields associated with galvanic current flow between two current electrodes. This paper presents methods for the simultaneous modelling of both the magnetometric resistivity (MMR) amplitude and phase (or magnetic-induced polarisation [MIP]) response for DHMMR data. Analyses of the MIP response and the interactions between conductive and polarisable bodies and their host are calculated using variations of the Cole–Cole model for complex impedance. The inphase and quadrature components provide symmetric signatures at the target depth, but some results are counter-intuitive when expressed as the MIP (phase) response. These methods are used to provide an interpretation of DHMMR data obtained from the Flying Doctor Prospect near Broken Hill, New South Wales.

Direct detection of gold bearing structures at St Ives, WA – DHEM vs DHMMR

The St Ives terrain is covered by a thick regolith that is saturated with hyper-saline groundwater. The regolith forms a major barrier for electrical and EM surveys designed to detect gold-bearing structures in bedrock.The down-hole magnetometric resistivity (DHMMR) method was trialled at St Ives and detected only small responses from known structures. DHMMR does not detect strong anomalies because the structures are often less than 10 m thick, and have small cross-sectional areas for channelling current. The resistive bedrock does not support large current densities for channelling into targets.Down-hole electromagnetic (DHEM) surveys were trialled at St Ives and detected a strong well-defined anomaly at the Junction gold mine. The anomaly was modelled as a plate in layered earth, and the plate corresponded to a major gold-bearing shear. Petrophysical tests suggest that the shear is conductive because of brine-saturated porosity.DHEM surveys did not detect strong anomalies from a gold-bearing shear at the Argo gold mine, despite the shear having low resistivity relative to host rock in downhole resistivity logging. The absence of DHEM response may be because the shear is not conductive enough to support an anomalous current system, or because the conductivity is not connected on a large scale.

Modelling the Magnetic Induced Polarisation (MIP) Response from the Down-Hole Magnetometric Resistivity (DHMMR) Method

The Down-Hole Magnetometric Resistivity (DHMMR) method, an extension to the surface Magnetometric Resistivity (MMR) method, is an effective technique used for the exploration of massive sulphide deposits. The method is based on the ``in-hole' measurement of low-amplitude, low-frequency magnetic fields associated with galvanic current flow through the earth. To date, the interpretation of DHMMR data has focused on modelling the amplitude component (which reflects changes in resistivity), neglecting the phase component (which reflects changes in induced polarization). This paper proposes to address the numerical interpretation of decoupled axial component phase data using DHMMR data acquired from the Flying Doctor prospect (near Broken Hill, New South Wales) as a case study. 3D electromagnetic modelling of polarisable tabular bodies in a layered, non-polarisable, half space has provided useful results. In addition to demonstrating a systematic decrease in the size of phase anomalies with increasing distance from the receiver, we have shown that the phase anomaly due to a polarisable body is also dependent on the body's position relative to the transmitter and receiver.

Geophysical surveys at the Nkomati Mine, Mpumalanga, South Africa

Nkomati Mine, a joint venture between Anglovaal Mining and Anglo American, exploits a magmatic Ni-Cu-Co-PGE massivesulphide deposit associated with the Uitkomst Complex, inMpumalanga, South Africa. The mineralisation consists of flatlyingmassive sulphides, which are magnetic, dense andconductive. Geophysics has been used to assist the explorationprogram for further resources. Methods that were tried includesurface magnetic, gravity and controlled source audiomagnetotelluric (CSAMT) surveys, plus downhole electromagnetic(DHEM) and downhole magnetometric resistivity (DHMMR)surveys.The downhole methods were the most effective for deep targets. Because of the highly conductive nature of the sulphides, standardimpulse type time-domain DHEM did not give good results untiltransformed into step-type data. However, strong responses wereobtained from frequency-domain measurements. Since all of theholes were vertical, no useful cross component data could becollected and the location of the sources suffered from radialsymmetry. In contrast, the DHMMR was able to resolve on whichside of the hole the conductors lay.CSAMT successfully defined the disseminated and massivemineralisation at depth. Surface magnetic and gravity surveys wereuseful for outlining relatively shallow mineralisation outside of themain complex. Qualitative interpretation, and modelling of thegravity and magnetic data, showed a good correlation with theCSAMT results.

Recent base metal discovery at the Hill 800 Prospect

Hill 800 Prospect is located northwest of Licola, near the Jamieson River in the eastern highlands of Victoria. It was discovered in 1994 and is currently held under EL 3592 by Mount Wellington Gold N.L. (Mount Wellington Gold). The exploration model is for volcanic-hosted massive sulphides in the Cambrian Jamieson Volcanics. Mount Wellington Gold's interest in the area was sparked by new regional magnetic and radiometric data that became available in 1993 under the Victorian Initiative for Minerals and Petroleum. Investigation of potassium anomalies resulted in the discovery of gossans at Hill 800. Soil geochemical data show high gold values over Hill 800 and high copper values surrounding this, to the south and west. The best assay values in drillholes are 37 m @ 3.16 g/t gold from 0.5 m, and 6 m @ 1.1% copper from 253 m in HED1. The high gold values occur in an oxidised and altered cap near the top of Hill 800. The high copper values occur separately from the gold values, in altered volcanics. The geophysical surveys carried out by Mount Wellington Gold include: airborne radar (AIRSAR), airborne frequency-domain electromagnetics (DIGHEM), ground magnetics, time-domain electromagnetics (SIROTEM), induced polarization, downhole electromagnetics, downhole magnetometric resistivity (DHMMR), spontaneous potential and controlled source audio magnetotellurics (CSAMT). The gold and copper mineralisation currently detected at Hill 800 does not have a significant geophysical response. Responses recorded to date are generally from geological rather than mineralised sources. The latest induced polarization results accurately predicted pyrite enrichment (+- gold). A drilling target was defined from a 3-D model of the inverted 100 m line-spaced data. The induced polarization chargeability anomaly was associated with high resistivity, and when drilled, was found to be caused by disseminated sulphides (pyrite). The DHMMR survey on diamond drillhole HED4A indicates a conductive source, possibly massive sulphides, above the hole at about 460 m depth. The SIROTEM data also show subtle high conductivity at late times in this area. These data have many anomalies, which have not yet been effectively tested by the drilling program. Magnetic susceptibility measurements on rock chips from the drilling program indicate that the weathered and altered volcanics have lower susceptibility than the fresh volcanics - perhaps due to magnetite destruction associated with the alteration/weathering. The regional magnetics show a highly magnetic source beneath Hill 800, probably an intrusion. Modelling shows that this intrusion may come to within a hundred metres of the surface, so it is a likely source of the alteration fluids.

A downhole magnetometric resistivity technique for electrical sounding beneath a conductive surface layer

The magnetometric resistivity (MMR) method has not been used systematically for vertical electrical sounding because surficial measurements of the magnetic field caused by static current flow from a point source of current at the surface are independent of the form of the variation of earth resistivity with depth. However, data obtained from an adaptation of the MMR method in which the ratio of the magnetic fields below and above a known conductive surface layer is measured as a function of the horizontal range from the current source can be inverted to obtain the unknown resistivity of deeper material. The practical limitation is that the integrated conductivity of the surface layer must not be much smaller than the integrated conductivity of the unknown zone in order to observe diagnostic ratios significantly different from unity. The expressions derived for the magnetic‐field ratio and for the sensitivity of the ratio to changes in the resistivity of a plane‐layered unknown zone are closed‐form inverse Hankel transforms. The transforms are evaluated analytically for a model of constant, uniform resistivity. One possible geotechnical application of the method is the detection of offshore, resistive permafrost beneath the shallow Beaufort Sea. A theoretical design study in which ratios of the magnetic fields at the bottom and top of the sea are synthesized and the sensitivity of those ratios to changes in sea‐floor resistivity is determined as a function of range and depth, respectively, reveals that the resistivity of a permafrost zone 50 m thick buried 20 m below the sea floor can be identified. The maximum range needed is of the order of 200 m, which appears to be smaller than the maximum spacing required for a Schlumberger sounding to yield similar information.

The Down-hole Magnetometric Resistivity (DHMMR) Method

Summary The down-hole magnetometric resistivity method (DHMMR) is a pseudo-DC grounded dipole geophysical survey technique which allows absolute direction to a conductor from a borehole to be established. Anomalies produced by elongated block conductors are similar in shape and can be modelled in a similar way to gravity anomalies produced by block-shaped bodies having a density contrast. The anomalies are therefore simpler in shape than TEM anomalies and are easier to recognize in areas of geological complexity. The DHMMR technique is faster and more directional than down-hole TEM but has poorer resolution of conductor size and shape, and hence its ideal role is as a complement to down-hole TEM surveys. Case histories from two volcanogenic base metal prospects, near Charters Towers, Queensland, demonstrate the directivity of the method in the search for off-hole massive sulphides.

Downhole magnetometric resistivity response of a half‐plane conductor

The magnetometric resistivity (MMR) methods measure the magnetic field of the galvanic current flow introduced by current electrodes in the ground. The downhole MMR version overcomes some of the limitations imposed by a conductive overburden on the surface MMR methods. This paper is concerned with downhole MMR responses of buried conductors. The model considered is an infinitely conductive half‐plane conductor in a uniform whole‐space host, as an ideal representation of a plate‐like conductive orebody buried deep underground. Exact analytical downhole MMR responses are derived for arbitrary locations of current electrode sources and magnetic field sensors. Type curves are shown for various source‐sensor‐target orientations to illustrate some characteristics of the responses.