Induced polarization effect on grounded-wire transient electromagnetic data from transverse electric and magnetic fields

Transient electromagnetic (TEM) data can be seriously distorted by induced polarization (IP) phenomena when a polarizable body is present. The TEM field generated by a grounded-wire source contains transverse electric (TE) and transverse magnetic (TM) modes. The IP effect is most commonly studied with the TEM total field, rather than considering the difference between TE and TM fields. To investigate the effect of IP phenomena on the TE and TM fields, we have performed a detailed analysis on IP-distorted TEM data based on numerical and field examples. We first compare the IP effect on the TE and TM fields when polarizable bodies with different polarizable parameters are present. The TM field is more severely affected by the IP effect than the TE field. Compared to a single grounded-wire source, a double-line grounded-wire source can generate a larger TM field in the horizontal electric field. We compare the IP effect on TEM data from single- and double-line grounded-wire TEM configurations, and find that the data from the double-line configuration have a higher TM/TE ratio and are more severely affected by IP phenomena than in the single-line case. Thus, it would be easier to identify and extract the IP response from field data acquired with a double-line grounded-wire source configuration. These results have been verified by a field survey of the Kalatongke copper-nickel ore district, which has predominantly layered geology, in Xinjiang, China.

Excitation process under the ramp-step waveform of inductive source-induced polarization method

Most previous studies explain the induced polarization (IP) effects in transient electromagnetic (TEM) data using an idealized but unrealizable step-waveform transmitter current. However, the ramp-step waveform, which is commonly applied in TEM measurement, has been given less attention. To explore the effects of the switch-off time, we have compared the IP responses induced by two waveforms: the step and the ramp step. We apply a wire-filament circuit composed of a Debye model and an inductor to identify the differences in the aspect of the energy transfer process. Furthermore, we extend the analysis to illustrate the IP effects in a frozen-soil zone, metallic sulfide ore, and graphite ore and to analyze the relationship between the switch-off time, IP effects, and the polarization parameters. The results indicate that the primary and secondary fields act as excitation sources of the polarization field. In the step waveform case, the excitation source of the polarization field is the secondary field. As the switch-off time increases, the contribution rate of the primary field gradually increases, especially in the high-resistivity media. The finding provides a new understanding of the excitation process of the IP effects and indicates that source contributions are variable in different situations. Moreover, a longer switch-off time weakens the IP effects severely, and in the high-resistivity, high-polarizable media, the IP effects are more sensitive to the switch-off time. Therefore, a suitable switch-off time should be chosen based on the properties of the polarizable media, such as resistivity and time constant. To detect a relatively high-resistivity, high-polarizable body, the switch-off time should be as short as possible. Nevertheless, to detect a relatively low-resistivity polarizable body, the IP effects are fairly insensitive to the switch-off time, so the transmitter waveform can easily meet the requirements.

Transient electromagnetic responses of 3D polarizable body

The transient voltage response of a central loop electromagnetic system above a buried polarizable parallelepiped target (such as pyrites, sulfides, clays, etc.) is studied. The voltage response, which includes a positive part and a negative part (related to the polarization properties), is studied by varying the body characteristics, including the Cole‐Cole parameters, body dimensions, depth, and thickness. By varying the loop radius and by using profiling, the lateral body dimensions can be identified, particularly using the negative response for polarizable bodies. There is a clear difference in the dependencies of the positive and negative voltage responses on the body characteristics, which means that the negative voltage is not just redundant data, but we may use it, together with the positive response, to have better understanding for the body dimensions and polarization parameters effects.

On: “Mineral discrimination and removal of inductive coupling with multifrequency IP” by W. H. Pelton, S. H. Ward, P. G. Hallof, W. R. Sill, and P. H. Nelson (GEOPHYSICS, 43, 588–609).

The authors discussed the behavior of the resistivity spectra by means of the Cole‐Cole dispersion model. They also discussed the corrections with which the petrophysical resistivity spectrum can be reduced into an apparent resistivity spectrum caused by a polarizable body embedded in an unpolarizable environment. The application of the Cole‐Cole dispersion model is a marked step forward in spectral IP analysis. However, closer attention must be paid to the assumptions and approaches on which the authors base the relations between the petrophysical and apparent spectra. The authors based their relations between the true and apparent spectra on the use of the dilution factor [Formula: see text]. In accordance with the definition by Seigel (1959), they assumed that [Formula: see text] is a real constant (independent of frequency) over the whole frequency range under consideration. First consider the justification for the assumption of the existence of a constant factor [Formula: see text] in the light of an example calculated for phase spectra. Similar considerations could also be made with the aid of amplitude spectra.

The behavior of the apparent resistivity phase spectrum in the case of a polarizable prism in an unpolarizable half‐space

In the application of the broadband induced polarization method, it is necessary to know how a petrophysical resistivity spectrum is transformed into an apparent spectrum measured in the field. Investigated in the present work was the forming of an apparent spectrum in the case of a polarizable three‐dimensional prism embedded in an unpolarizable half‐space for gradient and dipole‐dipole arrays. The computations were done numerically using the integral equation technique. The frequency dependence of the resistivity of the prism was depicted by means of the Cole‐Cole dispersion model. With this simple model geometry, the phase spectra of apparent resistivity resemble quite closely in functional form the original petrophysical phase spectrum of the Cole‐Cole dispersion model. The apparent spectra have shifted on the log‐log scale downward, owing to geometric attenuation, and toward lower frequencies. The apparent Cole‐Cole parameters have been inverted from the apparent spectra. The apparent chargeability is generally noticeably smaller, owing to the geometric attenuation, than the chargeability of the original petrophysical spectrum. The apparent frequency dependence, on the other hand, is very close to the value of the original frequency dependence. The shift of the apparent phase spectrum toward lower frequencies partly compensates for the decrease in the apparent time constant caused by attenuation of the spectrum. The apparent time constant is thus close to the true time constant of the petrophysical spectrum. It is therefore possible in principle to obtain by direct inversion from an apparent spectrum measured in the field a reasonable estimate of the frequency dependence and time constant of the true spectrum of a polarizable body.

Geophysical investigation for Copper-Sulphide ore at Dhusa, Nepal

Self-Potential, Induced Polarisation and Magnetic Surveys conducted in an anomalous zone delineated by preliminary to advanced stages of geochemical exploration showed good correlation with the geochemical anomaly. SP and IP survey conducted have a very limited depth of investigation. Qualitative interpretation of SP curves clearly indicated the existence of two faults, whereas combined results of SP and IP showed weathered and "deep­ seated" (30m) polarizable body. The magnetic survey did not give any significant result. Three bore holes drilled in the best geophysical, geochemical anomaly zone intersected mineralization of chalcopyrite chalcocite and pyrite with an average grade of 0.17% Cu over 38m true thickness. The post drilling IP and resistivity sounding conducted at 10 points along a line passing through one of the drill holes indicated the possibility of better mineralization further down dip. The mineralization is expected to be present in the form of lenses.

Topographic effects in resistivity and induced‐polarization surveys

We have made a systematic study of dipole‐dipole apparent resistivity anomalies due to topography and of the effect of irregular terrain on induced‐polarization (IP) anomalies, using a two‐dimensional (2-D), finite‐element computer program. A valley produces a central apparent resistivity low in the resistivity pseudosection, flanked by zones of higher apparent resistivity. A ridge produces just the opposite anomaly pattern—a central high flanked by lows. A slope generates an apparent resistivity low at its base and a high at its top. Topographic effects are important for slope angles of 10 degrees or more and for slope lengths of one dipole‐length or greater. The IP response of a homogeneous earth is not affected by topography. However, irregular terrain does affect the observed IP response of a polarizable body due to variations in the distance between the electrodes and the body. These terrain‐induced anomalies can lead to erroneous interpretations unless topography is included in numerical modeling. A field case demonstrates the importance of including topography, where it is significant, in interpretation models. A technique for correcting apparent resistivity for topographic effects uses the finite‐element program to compute correction factors.