INTRODUCTION

The response of a typical porphyry deposit buried under overburden in a resistivity/induced polarization survey is investigated. Forward modeling is performed in order to determine the response of the porphyry in various geological models. Responses are calculated under various depths of both resistive and conductive overburden. The survey is modeled using a pole-dipole array with 100 m dipoles and n=1 to 6, and a 2000 m long survey line extending from 6400N to 8400N. The porphyry is a two-dimensional target 500 m wide and more than 500 m depth extent centered at 7450N. Models are constructed using the Matlab software package and forward modeling is carried out using UBC-GIF DCIPF2D software. The forward results are subsequently inverted using the DCIPINV2D package. Inversion gives an idea of what structure can be recovered from "real" data. The model resistivities and chargeabilities have additive random noise of 50% to 80% in order to make the models more geologically realistic.

The response of a buried conductive/chargeable porphyry is calculated at overburden depths of 100 m, 200 m and 312.5 m. The wallrock has a fixed resistivity of 1000 Ohm m and chargeability 5 ms. The resistive overburden has resistivity 300 Ohm m and chargeability 3 ms. The conductive overburden has resistivity 30 Ohm m and chargeability 3 ms. Porphyry resistivity is 100 Ohm m with chargeability 30 ms. The resistivities and chargeabilities in each model, along with the random variations, are listed in Table 1.

DISCUSSION

With no overburden present (Line 1000E), the porphyry is clearly visible in both the apparent resistivity and chargeability pseudosections. As well, the inverted results easily recover the porphyry. Resistive overburden of resistivity 300 Ohm m and chargeability 3 ms is added to the models in thickness' of 100m, 200m and 312.5 m. When the porphyry is buried at 100 m (Line 2100E), there is only a weakly conductive zone on the apparent resistivity pseudosection. There is still a strong chargeable zone at 100 m depth in the IP pseudosection. The inverted results clearly show both the depth and location of the porphyry. For greater depth of burial, there is little indication of the porphyry on the apparent resistivity pseudosection. However, there is still a strong chargeability response in the pseudosection at the greater depths. The inverted results at 200 m depth (Line 2200E) recover the porphyry quite well, with the porphyry top near the expected depth of 200 m. At 312.5 m depth (Line 2300E), the inversion results only indicate the top of the porphyry.

When the porphyry is buried under conductive overburden (resistivity 30 Ohm m, chargeability 3 ms), the overburden dominates all of the apparent resistivity pseudosections. At 100 m depth (Line 3100E) there is only a faint indication of the porphyry on the resistivity pseudosection and inverted results. However, the chargeability shows the porphyry quite clearly. At 200 m depth (Line 3200E), the porphyry is obvious in the chargeability pseudosection, though the response is weak. Inverting the data recovers the chargeable zone at 200 m depth. When the porphyry is covered by 312.5 m of conductive overburden(Line 3300E), there is still a weak indication of a chargeable zone at depth in the pseudosection. The inversion confirms that there is a chargeable zone at 300 m depth. Thus the porphyry is a detectable target, even under 312.5m of conductive overburden. The reason for this is that the porphyry is more conductive than the wallrock, thus channeling the current into the porphyry and producing a chargeability response.

To test this theory, the resistivity of the porphyry is set equal to that of the wallrock (1000 Ohm m) in the final model. Again, the porphyry is placed under 312.5 m of conductive overburden (Line 3400E). In this case, both the apparent resistivity and chargeability pseudosections appear non-anomalous, although there are indicators of an extremely weak chargeable zone at depth. However, these "anomalous" chargeabilities are less than the random variation of the wallrock chargeability. The inversion results recover a weak chargeable zone, though the contrast in chargeability is so minor that it can't be considered a reliable target. Hence, the porphyry is no longer detectable buried beneath 312.5 m of conductive overburden, if its resistivity is the same as the wallrock.

CONCLUSION

Resistivity and chargeability forward modeling of a buried porphyry deposit provides insight into what can be detected from a field survey. Various geological models were constructed in order to obtain a range of responses for a resistivity/induced polarization survey. The predicted data obtained from forward modeling was inverted to see if the original models could be recovered. It was found that a strongly chargeable and conductive porphyry, buried by up to 312.5 m of overburden can be detected with a resistivity/induced polarization survey, even if buried by fairly conductive (30 Ohm m) overburden. A more resistive porphyry will be more difficult to detect. A porphyry buried by more resistive overburden is easily detected.