Electrical methods case studies

Pine point mineralization, Northwest Territories Strata-bound sulphide deposit (lead-zinc-silver in a limestone host rock) Background effects in limestone are very low Resistivity pseudo-section relatively featureless FE and MFE sections show persistent anomaly, even at very large spacing. Pine Point type mineralization, Northwest Territories Figure 4.19 shows the apparent resistivity, frequency eect and metal factor pseudo-sections at Pine Point. Pine Point mineralization is an example of strata-bound sulde deposition. Although there are only weak concentrations of metallic minerals, the surrounding limestones are an ideal host rock within which to work, because the background eects are very low. The deposits do not show up on the resistivity pseudo-section, which shows a uniformly high level of apparent resistivity. But careful IP work with a wide spacing of electrodes (250 ft) can locate relatively small zones at relatively large depths.

Detection of clay lens in an aquifer Map of PFE detects a clay-rich lens in a sandy gravel Clay reduces the fluid permeability Hence the IP survey aided in the assessment of the potential of the aquifer as a reservoir Detection of clay lens in an aquifer Figure 4.20 shows a contour map of the percent frequency eect. Relatively high values near the centre of the map were interpreted as a clay-rich lens in sandy gravel. The clay-rich zone reduces the uid permeability of the local aquifer. Hence the IP survey aided in the assessment of the potential of the aquifer as a reservoir.

Disseminated porphyry copper Disseminated porphyry copper Figure 4.21 shows both the apparent resistivity and metal factor pseudosections over the copper-molybdenum Brenda ore body, in British Columbia. The apparent resistivity section is nearly (but not quite) featureless; the discovery was made on the basis of the IP results, which were successful largely because of the low level of background IP eects in this area. Although the metallic mineral concentration is low, it is ore-grade because only copper and molybdenum mineral are present. Relatively featureless resistivity, discovery of mineralization made on the basis of the MF pseudo-section. Discovery is ore-grade, in spite of low grade due to copper and molybdenum.

Disseminated sulphide Disseminated sulphide The disseminated suldes in the Cactus area, Arizona underlie 200-400 ft of rock. The deposits fail to generate any anomaly on the apparent resistivity survey. Nevertheless, they show up clearly on the metal factor pseudo-section (Figure 4.22). The depth of burial may be inferred from the fact that the IP survey was carried out with a large electrode spacing (200 ft), and the anomaly only shows up at and below the n=2 level. Deeper deposits inferred from psuedo-section: note large spacing required to detect MF anomaly

Gortdrum copper-silver, Ireland Limited data (one profile only, no pseudo-section) Chargeability anomaly well defined, centered over ore body. Gortdrum copper-silver, Ireland Figure 4.23 shows the chargeability and apparent resistivity proles across a low grade copper-silver ore body in Ireland. Although the deposit contains less than 2% conducting minerals, the chargeability anomaly is well dened and centred over the ore body. In contrast, the corresponding apparent resistivity prole reects the contrast between the sandstones and limestones.

Lornex copper porphyry, British Columbia Resistivity shows only a mild increase to the east – probably due to thinning of conductive overburden Short spacing IP shows the mineralization, relatively low grade at the east. Long spacing IP indicates mineralization continues at depth to the west. Lornex copper porphyry, British Columbia Figure 4.24 shows three dierent pole-dipole proles, carried out at dierent times. The short spacing, a = 61 m survey was carried out rst and drilling of the anomaly at the east (right end) of the line conrmed the presence of low grade pyrite and copper. Later, with the measurements made with the larger spacing it was realized that, at depth, the mineralization continued to the west. Later drilling conrmed that the concentrations in this region were ore-grade. None of the resistivity proles show any appreciable anomaly, other than a mild increase in apparent resistivity, corresponding to the thinning of the conductive overburden.

Krain sulfide deposit, British Columbia Krain sulde deposit, British Columbia Figure 4.25 shows a further example in which the resistivity prole is only weakly indicative of the mineralization, whereas the metal factor pseudo-section shows a very clear, inverted-V shaped anomaly associated with the sulde ore body.

Model study of “inversion” Model study of inversion As pointed out in the notes above, in many cases the pseudo-section is only a very approximate representation of the subsurface. This is illustrated in Figure 4.26, in which a computer model is used to generate resistivity data. The pseudo-section shows the typical inverted-V appearance related to the location of the conductive anomaly. The inverse-modelling, or \inversion" approach is to iteratively update a model of the subsurface until the predicted data match the observations. Inversion will recover a much more accurate section (sometimes, inaccurately, referred to as a \true section"). The inversion result reects the inherently poor resolution of the resistivity method; note that the resolution decreases with increasing depth. The lack of correspondence between the pseudo-section and the true model is even more dramatic in Figure 4.27, in which the near surface model contains a number of small, highly conductive anomalies. These have a dramatic eect on the pseudo-sections, masking the deeper anomaly, but the inversion succeeds in separating the various targets. See the UBC Geophysical Inversion Facility website for more information (www.geop.ubc.ca/ubcgif) Inversion result, or “real” section Psuedo section

Model study of “inversion” Model study of inversion As pointed out in the notes above, in many cases the pseudo-section is only a very approximate representation of the subsurface. This is illustrated in Figure 4.26, in which a computer model is used to generate resistivity data. The pseudo-section shows the typical inverted-V appearance related to the location of the conductive anomaly. The inverse-modelling, or \inversion" approach is to iteratively update a model of the subsurface until the predicted data match the observations. Inversion will recover a much more accurate section (sometimes, inaccurately, referred to as a \true section"). The inversion result reects the inherently poor resolution of the resistivity method; note that the resolution decreases with increasing depth. The lack of correspondence between the pseudo-section and the true model is even more dramatic in Figure 4.27, in which the near surface model contains a number of small, highly conductive anomalies. These have a dramatic eect on the pseudo-sections, masking the deeper anomaly, but the inversion succeeds in separating the various targets. See the UBC Geophysical Inversion Facility website for more information (www.geop.ubc.ca/ubcgif) Inversion result, or “real” section Psuedo section

Example of “inversion”, Century deposit, Australia Pseudo-section Inversion result, with interpretation Example of inversion, Century deposit, Australia The Century deposit in NW Queensland consists of ne grained sphalerite and gelena, with minor pyrite. The discovery of the deposit was based on a zinc geochemical anomaly; early geophysical failed to nd a gravity or an electromagnetic anomaly associated with the deposit. Resistivity and IP were more successful. Figure 4.28 shows the IP pseudo-section and the inversion of the IP data (with superimposed drilling information). The IP inversion indicates a major fault that displaces the ore sequence. From the UBC Geophysical Inversion Facility website. Predicted IP data

Dam leakage, Mexico Detection and repair of Dam leakage, Mexico The geological foundation of the Amistad dam was surveyed using three dierent resistivity arrays (Figure 4.29) in order to locate leakages through the karstic limestones at the bottom of the dam. The anomalies detected around coordinates 565 and 620 were drilled and grouted, successfully repairing the leakage. The contractor concluded from the study that the Wenner array was insensitive to lateral structures.