Magnetic Induced Polarization Research Articles

Magnetic measurements in electrical prospecting by resistivity methods

The electrical resistivity and induced polarization (IP) methods are widely used in geological mapping, prospecting and exploration of mineral deposits, engineering geology, hydrogeology, archaeology, and geotechnical and environmental applications. Historically, these methods have formed the basis of the electrical prospecting technique. In these methods, a DC or low-frequency AC electrical current is introduced into the earth through a grounded transmitter line. The measured quantity is the electric field. However, if the earth’s resistivity or chargeability changes horizontally, this change gives rise to an anomalous magnetic field, which is studied by the magnetometric resistivity (MMR) and magnetic induced polarization (MIP) methods, respectively. Along with advantages, some shortcomings are inherent in the MMR and MIP techniques. Apparently, the main drawback of these methods is that the magnetic fields of both the transmitter line wire and ground electrodes on the surface are several orders of magnitude greater than the anomalous magnetic field response. This introduces a significant “noise” to magnetic-resistivity data. We investigate the potential of using a circular electric dipole (CED) in magnetometric resistivity techniques. It has been found that the application of a CED, instead of a conventional transmitter line, dramatically enhances the signal-to-noise ratio.

Switching the magnetically induced polarization of Co4Nb2O9 by dielectric-relaxation-related internal electric field

The possibility of switching the magnetically induced polarization (MIP) in magnetoelectric antiferromagnet Co4Nb2O9 by a dielectric-relaxation-related internal electric field was investigated. The MIP at a lower temperature is more difficult to be reversed, and this can be explained based on the Landau-Ginzburg-Devonshire theory as well as the domain nucleation and growth theory.

Magnetic Induced Polarization - using new technology for greater detection capability of deep and elusive mineralization

The Magnetic Induced Polarization (MIP) method uses the measurement of magnetic fields to directly detect internal and external current flow from IP-generating targets, rather than the resultant surface currents as with conventional Electric Induced Polarization (EIP). Magnetometric Resistivity (MMR) measures the magnetic field produced by galvanic current flow to detect horizontal variations in resistivity. We focus primarily on the MIP method but since MIP and MMR data are collected simultaneously, we will treat them together where appropriate. MIP/MMR is insensitive to horizontal layering, and is especially suitable for regions with highly conductive cover where EIP and resistivity responses are sharply attenuated. Magnetic fields easily propagate through such conditions; therefore MIP/MMR is minimally impacted by conductive cover. The other major advantage of MIP/MMR, over traditional electrical IP and resistivity, is that it completely eliminates the need for measurement electrodes. Hence, it is effective in difficult ground contact conditions such as dry sandy soils, frozen ground, and rocky scree slopes. For inversion purposes, MIP has an additional benefit that magnetic fields can be measured in all three axes simultaneously, which provides significantly more information about target position and attitude. By using SQUID technology and remote referencing, we are able to improve the data quality and extract useful three component MIP and MMR data. We present a number of field trials using both frequency and time domain methods to analyze the MIP and MMR responses from porphyry copper, and unconformity uranium ore bodies.

Inductive source induced polarization

Induced polarization (IP) surveys are commonly performed to map the distribution of electrical chargeability that is a diagnostic physical property in mineral exploration and in many environmental problems. Although these surveys have been successful in the past, the galvanic sources required for traditional IP and magnetic IP (MIP) surveys prevent them from being applied in some geological settings. We develop a new methodology for processing frequency domain EM data to identify the presence of IP effects in observations of the magnetic fields arising from an inductive source. The method makes use of the asymptotic behaviour of the secondary magnetic fields at low frequency. A new quantity, referred to as the ISIP datum, is defined so that it equals zero at low frequencies for any frequency-independent (non-chargeable) conductivity distribution. Thus, any non-zero response in the ISIP data indicates the presence of chargeable material. Numerical simulations demonstrate that the method can be applied even in complicated geological situations. A 3-D inversion algorithm is developed to recover the chargeability from the ISIP data and the inversion is demonstrated on synthetic examples.

3D Inversion of Magnetic Induced Polarization Data

The magnetic induced polarization (MIP) is an exploration technique used to obtain information relating to the induced polarization characteristics of the subsurface through measurements of the primary magnetic field associated with steady-state current flow in the earth. According to Seigel, the polarization magnetic field due to polarization current can be expressed as a sum of the products of chargeability and the derivative of primary magnetic field, due to ohmic current, with respect to the logarithmic conductivity (or sensitivity). The magnetic field and the sensitivity matrix can be computed by subsequently solving Poisson's equation and a magneto static problem in terms of potentials using a finite-volume algorithm. The MIP response is a function of chargeability difference ii-rf0) and relative conductivity (o/ff,,), where t}0 and on are constants. When solving the inverse problem we need to impose positivity of the solution but the fact that MIP responses depend only upon the difference in chargeability, means we have options regarding how we set up the inversion. We can: (1) invert for t\\ without constraints and add a constant to the final result, (2) invert for rj while imposing positivity, or (3) work with log^. We compare all three methods here. Our inversion problem is formulated as an optimisation problem where the objective function of the model is minimised subject to the constraints that the model adequately reproduces the data. We use a Gauss-Newton method to obtain the model perturbation at each iteration. The system of equations is solved using conjugate gradient least squares method. In order to make the inversion produce depth or distance information, a depth weighting or sensitivity-based weighting is required. Through synthetic model studies, we have shown that the conductivity ratio between a target and its host has a large effect on the MIP response. Ratios greater than two orders of magnitude difference will eventually make the MIP response undetectable. However, if the ratio is in the range of 0.1 to 10, the effect on the recovered chargeability is limited. The inversion algorithm is demonstrated by inverting the data set from Binduli, Australia.

A Sub-Audio Magnetics case study: Flying Doctor Pb-Zn-Ag Deposit, Broken Hill, Australia

The Flying Doctor Deposit is located approximately 5 km north-east of Broken Hill, NSW. The area lies along strike from the main Broken Hill ore bodies and is considered highly prospective for the development of base-metal sulphide mineralisation. The deposit was originally detected by induced polarization profiling techniques. Subsequent drilling in 1965 led to the discovery of a massive sulphide body. Over the last 20 years, comprehensive geological and geophysical data have been acquired at the site and consequently, the Flying Doctor Deposit has become an important test site for the evaluation of new geophysical techniques in the Broken Hill region.One of the first Sub-Audio Magnetics (SAM) trials was conducted at the Flying Doctor deposit in late 1992 to test the feasibility of acquiring total field magnetometric resistivity (TFMMR) data simultaneously with high definition total magnetic intensity (TMI) da a. Subsequent trials in 1995 focussed on acquiring total field magnetometric induced polarization (TFMMIP) and total field electromagnetic (TFEM) parameters. It was found that the Flying Doctor Deposit exhibited no magnetic susceptibility contrast but that the mineralisation was readily detected by both the TFMMR and TFMMIP parameters. This paper describes the results of the TFMMR/TFMMIP surveys and compares them with results obtained with prior surveys including gradient-array induced polarization (IP), dipole-dipole array induced polarization and magnetic induced polarization (MIP) surveys.The SAM trials demonstrated that it is possible to map several geophysical properties simultaneously with a single instrument, and confirmed that SAM is capable of acquiring data cost-effectively at spatial resolutions equivalent to high definition magnetics. An important conclusion from the surveys was that much greater interpretative power can be derived from having multiple data sets which reflect independent physical properties.

Geophysical responses over the Mount Ararate Prospect

Mount Ararat Prospect is hosted in Cambrian greenstones in western Victoria. Early exploration showed excellent correlation between the soil geochemical copper peaks and electromagnetic anomalies over a gossan. Follow-up drilling resulted in discovery of the Mount Ararat Prospect by Pennzoil in 1975. Massive banded and disseminated chalcopyrite-pyrrhotite-sphalerite mineralisation occurs in mafic and pelitic schists. The deposit is uneconomic, with an inferred resource of approximately 1 million tonnes with grades of 2.7% copper, 10 g/t silver and 0.6 g/t Chalcocite is common in a zone of supergene enrichment. Geophysical surveys, including airborne electromagnetic (INPUT) survey, vertical-loop electromagnetics, ground magnetic surveys, magnetic induced polarization, induced polarization and rapid reconnaissance magnetic induced polarization RRMIP, were used to delineate the extent of mineralisation. The INPUT survey and gradient-array induced polarization results were disappointing due to a very conductive surface layer. Magnetic induced polarization surveys were effective in delineating an anomalous conductive zone corresponding to the mineralisation. Downhole resistivity and induced polarization logs show the mineralised zone has coincident chargeability and resisitivity responses but the chargeability response is broader and more extensive indicating a possible disseminated pyrite halo in a broader zone around the massive sulphides. Experimental time domain electromagnetic surveys using the UTEM and SIROTEM systems recorded responses over the mineralisation but these techniques have not been routinely used for exploration. The early-time electromagnetic data is affected by a highly conductive near surface layer. The magnetic responses associated with the deposit are caused by the host rocks, and are not a direct response from the mineralisation. These units remain prospective along strike for base metals and gold.

Geophysics and exploration of the volcanic hosted massive sulphide prospect near Wickliffe, western Victoria

The Wickliffe Prospect (also known as Victor 2 Prospect) is located on the eastern edge of the Cambrian Mount Stavely Volcanic Complex near the town of Wickliffe in western Victoria. The Mount Stavely Volcanic Complex comprises volcanic breccia/ sandstones thought to be time-equivalent to the Mt Read Volcanics in western Tasmania.North Ltd regarded the prospect as an alteration system to a lead-poor, copper-zinc rich volcanic hosted massive sulphide system. Disseminated and stringer sulphide mineralisation and alteration has been discovered in the volcanic units of the Towanway Tuff, towards its stratigraphic top. This horizon has been sheared off in the Wickliffe Prospect area, but may still be present in unexplored areas to the south.Regional geophysical datasets, including airborne time domain electromagnetics, magnetics and radiometrics, together with detailed ground magnetic and gravity surveys, have been used to define lithological variations and structure but do not directly detect the mineralisation.Western Mining Corporation Ltd, Pennzoil of Australia Ltd, CRA Exploration Pty Ltd and North Ltd, have explored the prospect area. In 1977, Pennzoil discovered the stringer-type pyrite-chalcopyrite, which led to definition of the prospect. Best drilling intersections were 7 m @ 0.13 ppm gold, 1 m @ 0.93% copper and 24 m @ 0.29% copper.The mineralisation has coincident soil geochemistry copper, lead and zinc highs and geophysical responses. Magnetic induced polarization anomalies of -3 to -2 mV/V chargeability and 80–110% HN normalised horizontal field conductivity are coincident with the mineralisation. A gradient-array induced polarization chargeability anomaly of 15 mV/V was recorded, and a rapid reconnaissance magnetic induced polarization broad chargeable zone and relative phase shift anomaly of 0.3° to 0.4° are also coincident with the mineralisation.

MIP Test Survey Over the HYC Deposit in McArthur River Area, NT.

A test survey using the magnetic induced polarization (MIP) method was executed over the HYC sulfide deposit in the McArthur River area of the Northern Territory, Australia in September, 1992. The survey conducted by the Metal Mining Agency of Japan (MMAJ) using Scintrex equipment and personnel, was planned to define the MIP response over the huge stratified sulphide deposit and to compare the results with other geophysical data (including conventional IP) compiled since discovery in 1955. For current flow parallel to strike of the HYC deposit, a strong MIP anomaly was observed, providing accurate interpretation of depth and dip direction of the ore horizon and the upper pyritic shale-siltstone member. The MIP anomaly was in good agreement with the conventional electrical IP data collected with pole-dipole array. On the other hand, current flow perpendicular to strike produced no MIP anomaly. This result is consistent with the theory of MIP, indicating that the exploration programme must take account of regional geology and structures. Petrophysical study of drillcores showed the strongest IP source to be the ore horizon containing abundant sulphides. The upper pyritic shale-siltstone member also contains a considerable amount of sulphide and gives a significant IP effect. The MIP test survey verified the HYC orebody as a target for both electrical and magnetic IP methods, and MIP is suggested as a viable prospecting technique for similar base-metal targets in the region.

Extensions to the Phenomenological Theory of Induced Polarization

In the induced polarization method of geophysical exploration, the frequency dependence or dispersion of the complex resistivity of the medium is exploited. We present here a general formulation of the method for the idealized situation where two well-defined regions are both polarizable. The objective is to express the measured or apparent complex resistivity in terms of the complex resistivities of each region. We show that the resultant frequency dependence is not simply a weighted sum of the two constitutive parts except in the case where both media are weakly polarizable. In general, the higher order interaction causes distortion of the resultant frequency response in addition to the first-order dilution effect that is well known. Some comments about the related time domain or transient responses are also included. Finally, we show the connection with the magnetic induced polarization (MIP) method where an analogous development of the dilution and distortion factors follows rather easily.

An approximate Model of the Magnetometric Resistivity (MMR) and Magnetic Induced Polarization (MIP) Responses of Dipping Dikes Beneath a Conductive Overburden

Much of Australia is covered by a weathered, conductive surface layer. Conductive, dike-like, mineral deposits, embedded in a resistive basement material, exist beneath it and often contact it. A suitable mathematical model of this geology is a set of intersecting half planes. The MMR and MIP anomalies of the model are calculated analytically. A limited set of type curves for the simple model of a single dipping dike beneath a conductive overburden is presented. The curves reveal that the shape of the MMR anomaly is a strong function of dip. A computer program is provided to generate additional curves and to stimulate further research.

Some recent magnetic induced‐polarization developments—Part II: Survey results

Because magnetic induced‐polarization (MIP) field measurements reflect polarization current both in the interior of polarizable bodies and in the external medium (return current), an interesting variety of response types, depending upon the distribution of ground conductivities and polarization, has been observed in field surveys. Most of the survey results obtained to date have been amenable to interpretation on the basis of the theoretical and model studies, although some unexplained aspects exist. One interesting type of response is that associated with massive, highly conducting sulfide bodies. These often show reversed polarity anomalies because they provide a channel for the return polarization current due to disseminated sulfides which are adjacent to them. The massive sulfide bodies themselves do not produce appreciable induced‐polarization (IP) effects due to their high‐conductivity contrast with surrounding rocks. The results of ten MIP surveys over known metallic mineral deposits are presented from Australia, and one is presented from Canada. Both time‐domain (transient) and frequency‐domain (complex conductivity) measurements have been made on different deposits.

A progress report on the magnetic induced polarization (MIP) method

A progress report on the magnetic induced polarization (MIP) method

THE MAGNETIC INDUCED POLARIZATION (MIP) METHOD

The magnetic induced polarization (MIP) method derives information relating to the induced polarization characteristics of the earth through measurements of the magnetic fields associated with galvanic current flow. In the time domain, transient magnetic fields due to polarization current flow are measured. In the frequency domain, either the change in magnetic field with frequency or the phase shift of the magnetic field at a single frequency may be measured. The MIP method responds to regions of anomalous polarization, rather than providing physical property information. It tends to emphasize induced polarization effects in highly conducting bodies. It has special merit in certain problem areas, for example, where highly conducting overburden exists, or where the surface conditions render ground contact difficult. The results of field tests are presented where surveys using the MIP method have been carried out over “massive” and disseminated types of sulfide bodies or graphitic zones.