Percent Frequency Effect Research Articles

A multiple parameter extraction and electromagnetic coupling correction technique for time domain induced polarisation full waveform data

ABSTRACTThe time domain induced polarisation (TDIP) method is characterised by a simple explanation and operational approach, and is used widely to prospect for various metal sulfide deposits. However, the existing TDIP method is parameterised with fewer interpretive output attributes than the frequency domain induced polarisation (FDIP) method, and there has been far less emphasis on electromagnetic (EM) coupling in the TDIP method. A new and simple method has been devised for the extraction of multiple parameters and the suppression of EM-coupling responses in full waveform TDIP data. The essence of this approach is the transformation of the TDIP rectangular wave into the fundamental wave and a series of odd harmonics using a discrete Fourier transform. By using real and imaginary components of the fundamental and odd harmonics, we can calculate various FDIP parameters, including the per cent frequency effect (PFE) and the phase. The decoupled PFE coefficients and decoupled phases can then be used to remove the EM-coupling response. This new method has been applied to a data set collected near a poly-metallic mine in the Tibetan Plateau. A comparison between the TDIP and FDIP methods with central gradient arrays was performed. The results indicate that the TDIP apparent chargeability results are affected greatly by the EM-coupling response. The PFEs and phases extracted from the TDIP full waveform data are in good agreement with those obtained from the FDIP data. Moreover, the locations of the high decoupled PFE coefficients and the large negative decoupled phases are consistent with the locations of known ore veins. This method can provide additional parameters for the subsurface media, improve the practical application effect and enrich the functionality of TDIP methods.

Distribution Analysis of Sulphide Mineral (Pyrite) Using Induced Polarization Method in Libureng, Bone, South Sulawesi

Sulphide minerals are any member of a group of compounds of sulphur with one or more metals. Some of these sulphide minerals are economically important. This study used induced polarization method to identify distribution and mineralized zone of sulphide mineral (Pyrite), in Libureng, Bone Regency, South Sulawesi. The data processing yielded resistivity value, percent frequency effect (PPE) value, and metal factor (MF) value which were then used to produce 2-D and 3-D section model. Based on the data interpretation, an anomaly linked to pyrite deposits was seen in four trajectories with resistivity value < = 50, PFE = > 3%, and MF > = 150, deposited in hydrothermal alteration zone, sericite zone.

Influence of petrophysical and geotechnical engineering properties on the electrical response of unconsolidated earth materials

We have investigated the influence of petrophysical and geotechnical engineering properties of unconsolidated near-surface geomaterials (soils) on their electrical responses or measurements. Complex resistivity measurements were performed at a constant effective stress on 32 samples of varying textures and compositions. Petrophysical and engineering properties that inherently affect the mechanical and strength behavior of the samples — that is, fines content, pore-size parameter, specific surface area, and fractal dimension of the grain-size distributions — were obtained from geotechnical analysis. The electrical parameters that describe the electrical response of the samples — that is, resistivity amplitude [Formula: see text], phase shift [Formula: see text], percent frequency effect (PFE), loss tangent [Formula: see text], and the normalized phase [Formula: see text] — are computed from the electrical measurements. Crossplots of the electrical and engineering parameters provide useful infor-mation on how the geotechnical properties of the soil material influence the electrical measurements. In particular, [Formula: see text] and [Formula: see text] values are strongly influenced by variations in the petrophysical and engineering properties [Formula: see text] in comparison with the other electrical parameters. Soils become more dissipative as their specific surface area increases. The PFE values are less sensitive [Formula: see text] to the petrophysical and engineering properties. Analysis of the correlations also indicates that characteristic or transitional values of the fines content (18%) and pore size [Formula: see text] exist, beyond which the phase and normalized phase values are insensitive to their respective increases. The characteristic value of 18% fines content is close to values reported in previous studies, which signifies transition in strength behavior of soils and, thus, such relations could be important in noninvasive strength assessment and monitoring of soils. Normalized phase values were used to assess the relative amount of fines in the studied soils.

Effectiveness of Natural Field Induced Polarization for Detecting Polymetallic Deposits

To validate the effect of Natural Field Induced Polarization (NFIP), a certain polymetallic deposit was chosen as the test site, where Induced Polarization (IP) using gradient array and the Magnetotelluric (MT) sounding were conducted simultaneously. Analysis and comparison of the data indicated that the anomaly of the Relative Percent Frequency Effect (RPFE) from the MT data and the anomaly of IP coincided well with each other in the extents of the anomalous site and anomaly magnitudes. The results showed that NFIP was effective in the exploration of polymetallic deposits, under certain conditions.

New Methodology for Computation of the Earth?s Polarizability from Telluric Borehole Logging Measurements Taken at Different Frequencies

An important approach for computation of the earth’s polarizability is achieved by developing a novel method with new parameters called Natural Percent Frequency Effect (NPFE) and Natural Metal Factor (NMF) resulting from telluric logging data taken at different frequencies. The essential advantage of the developed approach relies on using natural electrical currents instead of an artificial electrical source usually needed in the standard frequency domain applied in the induced polarization method. Furthermore, a good qualitative correlation has been found between the new parameters and those of the traditional induced polarization method obtained in time and frequency domains. The new method has been tested in well 32 in the Rouez mine in France, where sulphide mineralization is well known to occur.

Electromagnetic coupling in frequency-domain induced polarization data: a method for removal

Summary Electromagnetic (EM) coupling is generally considered to be noise in induced polarization (IP) data and interpretation is difficult when its contribution is large compared to the IP signal. The effect is exacerbated by conductive environments and large-array survey geometries designed to explore deeper targets. In this paper we present a methodology to remove EM coupling from frequency-domain IP data. We first investigate the effect of EM coupling on the IP data and derive the necessary equations to represent the IP effect for both amplitude and phase responses of the signal. The separation of the inductive response from the total response in the low-frequency regime is derived using the electric field due to a horizontal electric dipole and it is assumed that at low frequencies the interaction of EM effects and IP effects is negligible. The total electric field is then expressed as a product of a scalar function, which is due to IP effects, and an electric field, which depends on the EM coupling response. It is this representation that enables us to obtain the IP response from EM-coupling-contaminated data. To compute the EM coupling response we recognize that conductivity information is necessary. We illustrate this with a synthetic example. The removal method developed in this work for the phase and the per cent frequency effect (PFE) data are applicable to 1-D, 2-D and 3-D structures. The practical utility of the method is illustrated on a 2-D field example that is typical of mineral exploration problems.

Influence of pore fluid chemistry on the complex conductivity and induced polarization responses of Berea sandstone

The spectral induced‐polarization (IP) response of rocks and soils is a complex function of pore solution chemistry, sample microgeometry, and surface chemical properties. We measure the complex conductivity and the time domain IP responses of Berea sandstone as a function of pore fluid ionic strength and pH. Complex conductivity is measured over the frequency range 10−3 to 106 Hz, and chargeability is computed using a time window of 0.16 to 1.74 s. The field IP parameters: phase, percent frequency effect, and chargeability are functions of both the surface and bulk electrical properties of the sample and are observed to decrease with increasing solution conductivity. Dividing these parameters by the sample resistivity yields normalized IP parameters (quadrature conductivity, metal factor, normalized chargeability) that are proportional to the imaginary component of the complex surface conductivity. Normalized IP parameters increase with ionic strength up to concentrations of 10−1 M NaCl and show a reduced response at pH 3, the point of zero charge for quartz‐dominated systems. For concentrations &gt;10−1 M NaCl, the normalized parameters decrease with increasing concentration. This decrease in surface polarization may indicate a decrease in the effective mobility of polarizing charges at high solution concentration. Our data indicate that normalized IP parameters are directly related to the physiochemical parameters that control the surface conductivity responses of rocks and soils. Normalization of IP measurements in environmental investigations should increase the effectiveness of IP surveys, especially in high‐conductivity environments.

A Method to Remove Electromagnetic Coupling from Induced Polarization Data for an “Exponential” Earth Model

—The electromagnetic (EM) coupling effect in induced polarization (IP) data is an important problem. In many works it has been computed only considering homogeneous or layered earth models with discretely uniform conductivity. In this study, an algorithm has been developed to compute the EM coupling effect in IP data measured on the earth, whose conductivity varies (increases or decreases) exponentially with depth. The EM coupling effects for Percent Frequency Effect (PFE) and phase data are computed for a dipole-dipole array with different separations, however the method can be applied to any electrode array. The results obtained for the cases of increasing and decreasing conductivity as a function of depth indicate that the EM coupling effect strongly depends on the subsurface resistivity and the dipole length. Here an “exponential” earth model is considered to remove EM coupling from the IP data in frequency and phase domain. For this purpose, first, the region of pseudo-section is divided into segments, and within each segment a typical average apparent resistivity (ρ a) curve is constructed. An exponential conductivity model is fitted to average ρ a data. The conductivity model is then used to compute EM responses. Next the data are corrected for the EM coupling contribution. This decoupling process is applied to field data from a galenite-pyrite mineralization area at the Dolluk site, in western Turkey. The results from the decoupling method developed here are compared with other techniques.

Dual-frequency and multi-parameter IP instrument and its application research

Variable IP method mainly measures the Percent Frequency Effect (PFE), and phase IP method mainly measures phase. They both aim at finding IP anomaly. However, only using PFE anomaly or single phase anomaly can’t distinguish IP body’s property. The authors put forward a new method—dual-frequency and multi-parameter IP method, which can not only find anomaly of PFE and phase, but also provide the property information of IP anomaly resource. The authors also have invented the dual-frequency and multi-parameter IP instrument. A lot of experiments and field work have been done. The results show that the instrument is valuable in distinguishing IP anomalies.

Induced‐polarization and electromagnetic modeling of a three‐dimensional body buried in a two‐layer anisotropic earth

The integral equation method is used for induced‐polarization (IP) and electromagnetic (EM) modeling of a finite inhomogeneity in a two‐layer anisotropic earth. An integral equation relates the exciting electric field and the scattering currents in the homogeneity through the electric tensor Green’s function deduced from the vector potentials in the lower layer of the earth. Digital linear filtering and three‐point parabolic Lagrangian interpolation with two variables speed up the numerical evaluation of the Hankel transforms in the tensor Green’s function. The results of this integral equation method for isotropic media are checked by direct comparisons with results by other workers. The results for anisotropic media are indirectly verified, mainly by checking the tensor Green’s function. The calculated results show that the effects of anisotropy on apparent resistivity and percent frequency effect are to reduce the size of the anomalies, shift the anomaly region downward toward the lower centers of the pseudosections, and enhance the effect of overburden; in other words, to shade the target from detection. This is due to the increase of currents flowing horizontally through the earth over the target. The effects of anisotropy on horizontal‐loop EM responses are to reduce the amplitude and lower the critical frequency of the maximum of the quadrature component.

Theory of induced‐polarization logging in a borehole

Currently, there is interest by the petroleum well‐logging industry in the potential use of induced polarization (IP) measurements to improve formation evaluation in shaly sands. Shell Development Company has constructed an experimental four‐electrode IP and resistivity logging tool to obtain downhole measurements in shaly sands. This study contributes to the theoretical understanding and interpretation of the dynamic (i.e., time‐dependent) response of this type of downhole IP logging device. A low‐frequency (e.g., 32 Hz or less) electric current oscillating at a single fixed frequency is applied between a pair of current electrodes in a borehole. The resulting voltages induced between pairs of potential measuring electrodes in the borehole are calculated by solving the time‐dependent Maxwell’s equations. Inductive electromagnetic (EM) coupling contributions to apparent (e.g., measured) IP phase angles are automatically taken into account. The model is applied to the study of normal logging arrays for which the voltage measuring electrodes are interior to the current electrodes. The model responses are calculated for normal arrays in both infinitely thick noninvaded formations and infinitely thick invaded formations. EM coupling contributions to apparent IP phase angles have an approximately universal dependence on a scaling parameter defined here. The scaling relationship permits the quantitative estimate of EM coupling effects for specific tool parameters (i.e., electrode spacings and frequencies) and formation characteristics (i.e., apparent conductivities). Therefore, scaling relationships of this type should be useful in the design of IP tools. An inverse method, developed for determining true formation IP phase angles and resistivities from apparent values measured by an IP tool, utilizes data from multiple pairs of voltage‐measuring electrodes and exploits the fact that, for the systems of interest, the inverse resistivity and IP problems can be “decoupled.” The assumption that IP phase angles have a logarithmic dependence on frequency over a decade frequency interval leads to a nonlinear relationship between percent frequency effect (PFE) and IP phase angle. This nonlinear relationship agrees well with experimental data.

Changes in complex resistivity during creep in granite

A sample of Westerly granite was deformed under constant stress conditions: a pore pressure of 5 MPa, a confining pressure of 10 MPa, and an axial load of 170 MPa. Pore volume changes were determined by measuring the volume of pore fluid (0.01M KClaq) injected into the sample. After 6 days of creep, characterized by accelerating volumetric stain, the sample failed along a macroscopic fault. Measurements of complex resistivity over the frequency range 0.001–300 Hz, taken at various times during creep, showed a gradual increase in both conductivity and permittivity. When analysed in terms of standard induced polarization (IP) techniques, the changing complex resistivity resulted in systematic changes in such parameters as percent frequency effect and chargeability. These results suggest that it may be possible to monitor the development of dilatancy in the source region of an impending earthquake through standard IP techniques.

Geophysical methods as mapping tools in a strata-bound gold deposit; Haile Mine, South Carolina slate belt

Very low frequency electromagnetic, magnetic, and induced polarization methods. The very low frequency electromagnetic resistivity data show high resistivity zones that correlate with siliceous units known to host the gold. The magnetic data clearly identify the mafic and ultramafic dikes which cut metasediments and also fill faults that offset them. The induced polarization method allows examination of the mineralized metasediments in two dimensions by giving vertical sections of resistivity and percent frequency effect, parameters which correlate with silica and pyrite content, respectively. Lithologic units as well as ore zones can thus be mapped in this terrain which is deeply weathered and partially covered by coastal plain sediments.--Modified journal abstract.

Electromagnetic coupling in ip measurements using some common electrode arrays over a uniform half-space

Electromagnetic (EM) coupling between grounded wires gives rise to frequency dependence of resistivity and hence, spurious anomalies in induced polarisation (IP) surveys. A general expression for the EM Coupling thus involved is derived for gradient array over a homogeneous earth, and on assigning special conditions, it is reduced to give the coupling in other commonly used arrays. Computations are made to evaluate the percent frequency effect (PFE) as defined in the frequency domain method owing to EM coupling with gradient, Schlumberger, Wenner, collinear dipole—dipole, pole—dipole, three— and pole—pole arrays. A comparison of the EM coupling of various arrays is drawn from practical considerations of IP surveys. In arrays where the potential electrodes are placed in a space enclosed by the current electrodes, the EM coupling is negative in sign and where the potential electrodes fall outside the space covered by the current electrodes, the EM coupling is positive. On the central traverse in gradient, Schlumberger and Wenner arrays, the EM coupling remains indeterminate or tends to be very high. Gradient array can still be used in IP surveys by confining observations to traverses away from the central one. Dipole—dipole, pole—dipole, three— and pole—pole arrays introduce even less coupling compared to dipole—dipole array. Notwithstanding that the studies are confined to a homogeneous earth, the comparison does provide useful criteria to avoid EM coupling, or to keep it within limits while exploring under more complex conditions.

A PARAMETRIC STUDY OF INDUCED‐POLARIZATION MODELS

The transmission surface or network technique was used to study the induced‐polarization response of a model having infinite strike length. The model consisted of a horizontal cylinder (the target) in a uniform half‐space overlain by an overburden. Responses to the dipole‐dipole, pole‐dipole, and gradient electrode arrays were compared as model parameters were systematically varied. Target conductivity, target depth, and overburden thickness and conductivity were each changed in turn. For different electrode spacings and locations, the peak response amplitudes and positions were plotted. It was found that the peak response amplitude did not decrease when large electrode spacings were used. The maximum percent frequency effect (PFE) response remains relatively constant for mildly conductive to very conductive targets. At the same time, the electrode separation required to achieve this maximum response increases by about a factor of two. The position of the peak PFE and the amplitude of the peak metal factor (MF) response were insensitive to overburden conductivity. There is a sharp drop of both PFE and MF when a thin overburden is introduced, and a gradual further reduction as its thickness increases. Responses to the pole‐dipole and dipole‐dipole arrays were comparable in magnitude, whereas results for the gradient arrays were at best equivalent and sometimes much smaller.

COMPLEX RESISTIVITY SPECTRA OF PORPHYRY COPPER MINERALIZATION

Complex resistivity spectra, in the frequency range 0.001 to 10 hz, have been obtained through computer analysis of waveforms tape recorded at porphyry copper deposits. When care is taken to avoid distortions caused by the geometric effects of electrical inhomogeneities, the spectra of typical porphyry copper mineralization are remarkably uniform in character. The geometric effects of veins, which generally complicate the response of hand samples, can be reduced through in‐situ measurements using electrode separations of a few meters. In the frequency range of interest for inducedpolarization (IP) exploration, the observed spectra are accurately described by [Formula: see text] where K is a constant and b is a positive fraction. The fraction b has a value less than 0.1 and is a complete measure of IP. In the frequency range of interest, Laplace transformation gives the step response to be approximated by [Formula: see text] Typical current waveforms used in IP prospecting can be synthesized by superposition of steps. Using these frequency and time response functions, percent frequency effect, phase, and pulse‐transient parameters are compared as measures of IP. By using a volume distribution function, it is shown that a distribution of lossy capacitors will explain the observed response. Physically, this might correspond to the double‐layer capacitance of metallic particles in mineralized rock.

On: “Evaluation of the Measurement of Induced Electrical Polarization with an Inductive System,” by G. W. Hohmann, P. R. Kintzinger, G. D. Van Voorhis, and S. H. Ward, (GEOPHYSICS, October 1970, p. 901–915)

In the above paper, it was clearly established that the induced electrical polarization (IP) phenomenon could not be measured inductively in a basin and range terrain where the apparent resistivities ranged from 3 ohm-m to 600 ohm-m and where the apparent percent frequency effect (PFE) ranged from 1 to 25.