Induced polarization field surveys on deposits of pisolitic bauxite

Geoexploration, 12 (1974) : 235-250 o Elsevier Scientific Publishing Company, Amsterdam -

INDUCED POLARIZATION LITIC BAUXITE

Printed in The Netherlands

FIELD SURVEYS ON DEPOSITS OF PISO-

R. CANZIANI and D. PATELLA Institute of Geodesy and Geophysics, University of Bari (Italy). Observatory for Geophysics and Cosmical Physics, University of Bari (Italy)

(Accepted for publication September 29, 1973)

ABSTRACT Canziani, R. and Patella, D., 1974. Induced polarization field surveys on deposits of pisolitic bauxite. Ceoexploration, 12: 235-250. This paper describes the results of a series of Induced Polarization (I.P.) experimental soundings on deposits of pisolitic bauxites. The results prove the presence of this mineral and differentiate the bauxitic levels from the sterile ones. The quantitative interpretation of the soundings has been made and the data obtained correspond very well with those from holes previously drilled in the area under investigation. INTRODUCTION

The geoelectrical method of resistivity measurement, applied to the search for bauxites, has not proved sufficiently discriminating up to now (Mosetti, 1953, 1955; Benvenuti and Norinelli, 1968). Tournier and Vajdea (1969) used the Induced Polarization (I.P.) method for the first time on deposits of various types of non-pisolitic bauxite and the results they obtained must be judged very encouraging, since the polarizability of such types of bauxite turned out to be higher than that of the surrounding non-mineralized formations. In principle there was nothing that could assure the applicability of the I.P. method also to the case of the pisolitic bauxites of the Salentine peninsula (South Italy), since the I.P. phenomenon may vary notably from one rock type to another. To this end, two surveys were carried out: (1) on a deposit of pisolitic bauxite near Giuggianello (zone I of Fig.l), where the geological situation is the simplest that may be found; and (2) on a deposit near Otranto (zone II of Fig.l), where the situation is much more complicated. The preliminary results of such a research are reported in a previous paper (Canziani et al., 1972), in which after analysis of the experimental data, the following conclusions are drawn: (a) The pisolitic layers are always distinguishable from the sterile layers

23ti

427.

40. 0c14 ?e.*14’

ql3’14’

Fig.1. Index map showing the location of the surveyed bauxitic deposits.

because

of their

higher polarizability; this is concluded from the study of the of soundings (in the Schlumberger sense) grouped according to typical situations. (b) There is a satisfactory correspondence between the pseudo-sections of the lines of equal apparent chargeability and the true sections constructed from data obtained from boreholes. (c) Finally, the suitability of the field techniques employed is confirmed by an attempt at quantitative interpretation using a semi-empirical method previously reported (Mongelli and Patella, 1971). On the basis of such encouraging results, we now propose to show in the following sections the results of an analytical procedure for quantitative interpretation of the same soundings using a method, for whose theoretical foundations the reader is referred to previous papers (Patella, 1972 a, b, 1973). whole

set

GEOLOGICAL

OUTLINE

The bauxitic deposits of the Salentine peninsula are found generally in the form of pockets situated on the strata of a calcareous-dolomitic formation of Cretaceous age, which constitutes the framework of the whole peninsula. In such Cretaceous limestones, some subcircular or extended erosional basin-shaped depressions, probably caused by karst-phenomena, are observed. These depressions are filled by sediments of residual and colluvial type which

237

are represented by terra rossa, clay and bauxite in the form of pisoiites or nodules variously d~stribu~d within the clayey matrix (Co~cchia and Dell’Anna, 1959; Crescenti and Vighi, 1964). The bottom of such pockets is more ore less irregular, depending on the intensity of the superficial weathering processes undergone by the limestone. The clayey-bauxitic deposits can be found outcropping or beneath a cover characterized by Neogene or Pleistocene c&tic sediments. The ~st~bution of pisolites and bauxitic nodules is markedly discontinuous so that, strictly speaking, it is more correct to regard them as pisolitic levels or cores, variously distributed within the clayey matrix, rather than as thick layers. The pisolites are found with a variety of shapes, but a rough spherical form may be recognized whose diameter can vary from fractions of a millimeter to about 40 mm. Their weight percentage can reach maximum values of 50-55%; in practice for a value less than 5%, the clayey-bauxitic level is considered sterile from an economic viewpoint. The bauxitic deposit near Otranto, where we have carried out a survey, is situated inside a depression probably of karst-origin and roughly elliptical, with axes of about 200 and 300 m, the major one being in the E-W direction. The calcareous basement of Cretaceous Age reaches a maximum depth of about 40 m (starting from the field level) and almost completely surrounds the pocket. The same clayey-bauxitic pocket occurs as outcrops for the most part; only on the eastern and western sides is the mineralization covered by two Quaterna~ layers of grey marls and yellowish detail limestones respectively, with a maximum thickness of 5 m but generally 2 m or less. Alternations are often encountered of sterile and productive levels with weight percentages up to about 30%. An analogous situation can be found in the deposit of Giuggianello, but the covering layers are absent. The mineralization is made up of a series of pockets, in places inter~onneeted and irregularly shaped. The bottom is flat, and maximum depth is 13 m, but more generally about 5 m. The weight percentage of pisolites and nodules reaches the maximum value of 45% FIELD PROCEDURE

The instrument used in field surveys was the “Seigel Induced-Polarization Unit, Mark V” built up for measuring I.P. effects in the so-called “time domain”. Current is sent into the ground by means of two electrodes A and 23, and the I.P. secondary voltage is measured between two other electrodes M and N. With this instrument, the I.P. measurement is given by the time-integral of the I.P. transient voltage, A U,(t), starting from the interception of the pri-

238

mary current (flowing in the earth for 1.5 set) up to 0.5 sec. Since measurements of the primary current 1 and of the primary voltage S U can be made, it is possible to obtain at once: (1) The dimensionless apparent chargeability m, (Patella, 1972 a, b), given by: t2 A Q(t)dt --- 1 (& -+I) s t, ma= ---au

(1)

generally expressed in mV JV or directly in parts per thousand. (2) The ~ppurent res~~tivity pa, given by: Pa = K (A u/0 K being the geometrical factor. (3) The apparent fictitious resistivity

(2) p: (Patella 1973), given by:

(3) expressed in CZm * 1P3 . The electrode a~angement used was Schlumberger’s, and the field technique that of vertical soundings along profiles choosen in such a way that increasingly more complicated stratigraphic situations could be investigated. Location

of the soundings

Sounding have been carried out near Giug~anello (1st survey) and near Otranto (2nd survey) (see Fig.1). In both zones, the centers of the electrode arrays were located near holes drilled by the “Montevergine S.p.A.“, so that a comparison with the geophysical results has been possible. Near GiuggianeIlo, the soundings were aligned along two profiles corresponding to two simple strati~aphi~ situations. Near Otranto four parallel and equidistant profiles were choosen in order to investigate the unstoped part of the deposit.

239

FIELD RESULTS

Linearity of the I.P. response Before considering the results of the quantitative interpretation of the I.P. soundings with the method proposed by one of us (Patella, 1972 a,b, 1973), it is necessary to check the linearity of the I.P. response, in order to apply correctly the interpretation procedures, whose such theoretical foundations are based on the assumption of linearity. To this end it is common practice to study the I.P. response (i.e. the timeintegral of the secondary voltage) as a function of some measurable quantity during the charging period, as for instance the energizing current I and the primary voltage AU. Fig.2A shows the linear behaviour of the I.P. response as a function of the current I for every spacing between the current electrodes, during a sounding carried out near a hole whose stratigraphic column is shown on the right-hand side of the figure. Fig.2B shows the linear behaviour of the I.P. response as a function of the primary voltage AU for the same sounding.

Fig.2. Linear behaviour of the I.P. response. Time-integral of the I.P. voltage. A. As a function of the energizing current. B. As a function of the primary voltage for various current electrode spacings.

The most important conclusion that can be drawn from such an analysis is that a linear response is obtained when a significant portion of the energizing current passes through a discontinuity in which the known different resistivities of adjacent layers distort the underground distribution of current densities.

240

Quantitative

of some

interpretatiorr

typical fieid curves

After checking the linearity of the I.P. response, the next step in the quantitative interpretation is to apply the complete or partial curve-matching procedure of the field curves in the representations m, against AB/2 and Pa against AB/2. The general mathematical expression of the apparent chargeability ma as a function of half the distance between the current electrodes (AB/Z) of a Schlumberger configuration in the case of a horizontally layered earth, was found to be (Patella, 1972 a, b): K’(h) J, (hAW2)hdh 1 + 2 (AB,2j2 7 _Q ._

ma = m,

(4)

1 + 2 (A~~2~* W K(h)J, (~A~/2)~d~ J where ml is the chargeability of the first layer, K(h) is the so-called kernel function in the resistivity D-C sounding theory, and K’(X) is a new kernel function which depends also on the chargeability of each layer. This type of representation involves the use of appropriate sets of standard graphs; these do not exist in published form, although a method of easy construction of m, master curves has been developed (Patella, 1972 b). Thus there is a considerable loss of time in choosing the proper set of parameters to yield a theoretical curve comparable with the experimental one. In order to overcome such a serious disadvantage, the second type of representation (pi against AB/2) has been proposed by one of us (Patella, 1973). The theoretics foundations are the same basic hypotheses as for the first type. This last representation has the considerable advantage that resistivity master curves can be adopted entirely to fit pi field curves. In fact, it is easy to show how the apparent fictitious resistivity (defined in the section “Field Procedure”) is also equal to the product of the apparent ch~geability with the apparent resistivity (Patella, 1973), i.e.: Pi=

maPa

(5)

Since the general expression

J:: K(h)J, (XABI2)XdX )

Pa = p1 I1 -t- 2 ( AB/2)2 After taking the prodult tain the expression: P: =

P:

(1 + 2 (AB/2)2

for pa is:

with the above expression 7

K’(X)J,

(hABI2)XdXj

(6) for ma, we easily ob-

(7)

0

where p ; = m, p 1 is the fictitious resistivity of the first layer. Thus, the general expression for p: is mathematicalIy identical to that for ~~ and therefore it is justifiable to use the usual resistivity standard graphs in

241

the interpretation-matching procedure. Attention must be paid to the fact that the fictitious resistivity of each layer then plays the role of the true resistivity. If an I.P. sounding is interpreted according to this type of representation, we obtain only the fictitious resistivity of each layer and the depths of the discontinuities. The results obtained in this way may not be significant as regards the investigation of the underground structure, but the chargeability is certainly a more selective physical parameter. In order to obtain the chargeability of each layer, it is necessary to interpret the associated resistivity field graph. From the known values of the true and fictitious resistivities of the layers obtained from the interpretation of the two field curves, it is possible to obtain at once the chargeability of each layer. It is sufficient to take a ratio since the fictitious resistivity of a rock is defined as the product of its chargeability by its resistivity (Patella, 1973), i.e., p ‘= mp. The combination of the two methods (I.P. and resistivity) may in places supplement the information yielded by each separately, apart from the main objective of this research which is to show which method is more discriminating in the search for pisolitic bauxites. In the following we shall analyse some typical soundings taken from the set of soundings performed in the two surveys in order to represent the more common situations. Each of the following figures will show the resistivity and the fictitious resistivity field curve of a sounding. The results of the interpretation of both curves are reported in the lower part of the figures and directly compared with the indications furnished by the borehole located in the center of the sounding. The interpretation of both resistivity and fictitious resistivity field curves has been carried out on the assumption that the underground structure behaves as though it were horizontally layered. A preliminary inspection of the data obtained from boreholes - previously drilled - assured the validity of such an assumption over an extensive part of the surveyed area. A rough approximation has been introduced in places, especially in those cases in which the extent of the bauxitic levels is not sufficiently great compared with the electrode layout or when the dip of the discontinuities on the flanks of the calcareous depression hardly exceeds some tolerance level. In the worst situations, the soundings have been completely disregarded as far as the quantitative interpretation is concerned. In each figure the resistivity data are represented by small circles and the fictitious resistivity data by small squares. The theoretical fits are represented by dashed lines. We point out the fact that all our field curves show in the left-hand branch the presence of a superficial thin layer (humus) which the corresponding boreholes do not indicate and therefore its actual thickness cannot be verified. Let US examine first some field curves obtained during the survey new

Fig.3. Interpretation

of a typical sounding on the deposit of Giuggianelfo.

/

8

b)

L

243

Giuggianello, where the geological situation is the simplest that may be found. Fig.3 deals with a situation often encountered in this zone wherein, directly under the thin layer of humus, we found a comparatively thick bauxitic layer. The interpretation of the resistivity curve (a three-layer sequence) does not show any differentiation between the bauxitic clay and the sterile clay, but delineates very well the presence of the calcareous basement. The pi field curve may be interpreted as a three or a four-layer sequence. In the first case, the data, partially matched with two-layer master-curves lead to a set of results quite divergent from the borehole indications. In fact, it can be seen from the chargeability values, that the I.P. method also does not differentiate the sterile from the bauxitic clay. Besides, a layer appears within the calcareous substratum which the borehole does not indicate. The fictitious resistivity data have then been interpreted as a four-layer sequence (see the right-hand side of the figure), and a distinction then appears between the sterile and the bauxitic clays. In addition, there is good agreement between the interpreted depths and those from the borehole. Fig.4 shows a situation in which the position of the sterile and bauxitic layers is reversed with respect to Fig.3. In this case, the resistivity field curve (of which many examples were obtained in this survey) may be interpreted as a three or a four-layer sequence. In the first case, there is no differentiation between the sterile and the bauxitic clay and the depth of the discontinuity associated with the calcareous basement definitely does not correspond to reality. The corresponding fictitious resistivity field curve has been interpreted with a two-layer master-curve and the sequence of the chargeabilities again shows a net contrast between the sterile clay and the bauxitic clay. In the second case, the interpretation of the resistivity field graph shows a differentiation between the sterile and the bauxitic clay and the interpreted depths now agree very well with those from the borehole. The results of the interpretation of the pg field curve are the same as before and the resulting chargeabilities again show a net contrast between the bauxitic level and the surrounding formations. Let us now examine some field soundings carried out in the zone of Otranto. Fig. 5 refers to the simplest stratigraphic situation, but one which is less important in this type of mining exploration. Nevertheless, the interpretation of such soundings. may be considered as a guide to the more complex ones. The apparent resistivity data have been interpreted as a three-layersequence by the auxiliary point method using two-layer master-curves. The apparent fictitious resistivity data also represent a three-layer sequence, and have been completely matched by a three-layer master. As can be seen, there is a good agreement between the depth values ob_ tained with the two interpretations and between these last and the borehole indications,

Cl.77

40

Fig,4. Interpretation

a.?5

1BOO

of a typical

31

sounding

2.D

i

on the deposit

I:/ i10”3

p’ Kbw%Pi

of Ciuggianello.

245

Fig.5. Interpretation

of a typical sounding an the deposit of Qtranto.

Fig.6 refers to a common situation wherein a bauxitic level appears covered by a marly layer. The pa data have first been interpreted as a four-layer sequence with the auxi~i~y-puint method, while the pa data have been completely matched by a two-layer mastercurve. Comparing the results of this first interpretation with the data from the borehole, we note good agreement in the depth of the discontinuity “sterile clay-marl”‘. No distinction appears between the subsequent layers down to the cakareous basement whose interpreted depth cannot actually be verified, since the borehole stops at a depth of 13 m. Thus neither resistivity nor I.P. differentiates the bauxitic level from the adjacent non-mineralized upper layer. On the right-hand side in this figure a second interpretation is reported, resulting from the possibility of interpreting the pa data as a five-layer sequence, The five-fayer interpretation has been made by the auxiliary-point method (Homifius, 1962). We see that the obtained sequence of layers now agrees with that from the borehole, apart from the suppression of the thin

246

+ N1

247

layer of sterile clay between the marl and the bauxitic clay. On the basis of the resistivity values, we cannot anyhow distin~ish the bauxitic clay from the sterile clay. On the other hand, with the same interpretation of the P,’ curve, the chargeability shows a net contrast between the bauxitic clay, the sterile clay and the marl. Finally let us examine Fig.?. The resistivity data show the trend of a threelayer sequence whose interpretation delineates very well the first discontinuity between the detrital limestone and the sterile clay whereas it once again does not differentiate the bauxitic clay from the sterile clay. The last discontinuity must be assigned to the calcareous basement, according to the last resistivity value. n&G?Cm3 a,,,I

?D

t

‘ a1181

J

Fig.7. Interpretation

of a typical sounding on the deposit of Otranto.

The trend of the apparent fictitious resistivity data of this example (a fourlayer sequence) has been by far the most frequent one during the whole survey in the zone of Otranto. The interpretation of this curve shows a first discontinuity which may be related to the contrast humus-detrital limestone, and a second discontinuity which may be ascribed again to the contrast detrital limestone--sterile clay. The last discontinuity, which differs notably from that outlined by the resistivity curve, may be assigned to the contrast sterile clay-bat&tic clay, The ch~geability values reproduce the complete strati~aphic succession in which the detrital limestone and the bauxitic clay have the same higher polarizability,

248

In this example, there is no good agreement between the interpreted depths and those from the borehole. This may be due to the fact that the sounding has been carried out in a zone where both the bauxitic level and the trend of the calcareous basement are somewhat irregular; this was checked from other neighbouring boreholes. As a consequence, the results of the interpretation agree much better with the indications of the neighbouring boreholes than with the data of the borehole located in the center of the sounding. The interpretation of the whole set of soundings performed in the two zones did not show any other result of particular interest, and none disagreeing with those just outlined. In Table I we report for each rock type the range of interpreted values of both resistivity and chargeability, obtained from the analysis of the soundings carried out in the zone of Otranto, since the number of soundings performed near Giuggianello was notably smaller and therefore not sufficient to furnish general indications on this deposit. TABLE I Ranges of resistivity and chargeability of the typical rocks in the deposit of Otranto Rock type

Resistivity (am)

Chargeability ~___

Humus

50-

300

0.06-

1.3

Sterile clay

15-

28

0.32-

2.6

Bauxitic clay (with different weight percentages of pisolites)

17-

28

6.4 -120

Marls

45-

54

7.4 -

10.2

Detrital limestones

24-

135

0.9 -

8.3

0.4 -

2.2

Cretaceous limestones

60+1500

(X

10e3)

As can be seen from Table I, the sterile clay is indistinguishable from the bauxitic clay in the resistivity sense, while the ranges of chargeability are quite different. Therefore, at least in the zone of Otranto, the I.P. method seems to be more discriminating. Some uncertainty might derive from the presence of such covering formations as marls and detrital limestones, whose chargeabilities are in places comparable with the smallest values pertinent to the bauxitic clays (see for instance Fig.7). These smallest values in the chargeability of the bauxite have been always found, in the zone of Otranto, to correspond with the smallest weight percentages of pisolites. Finally let us point out some comparative considerations between the two surveys.

249

Our experience is that in the zone of Giuggianello a higher resistivity value very often appears in conjunction with the complex sterile and bauxitic clays (see Fig.3), but not so frequently in the zone of Otranto. This is probably due to the higher resistivity of the clayey matrix. The considerably reduced value of the chargeability of the bauxite (see again Fig.3) if compared with the range of Table I must be considered to be directly related to the increase in its resistivity. Such a result may be considered as a particular case of the so-called “mirror-image effect” (Parasnis, 1970). Moreover in places a distinction appears in the resistivity sense between the sterile and the bauxitic clay in the deposit of Giuggianello (see Fig.41,. whereas in the zone of Otranto no such result was ever obtained. Such an event would be a confirmation of what Benvenuti and Norinelli (1968) stated during their surveys on bauxitic formations of the Salentine peninsula. The conclusion was put forward by these authors that the resistivity values obtained in the presence of mineralized levels with the same weight percentage of pisolites may in places vary within a quite wide range and a distinction from the sterile clay may or may not appear depending on reasons so far not well understood. CONCLUDING REMARKS

From the above results we may conclude that the Induced Polarization method must be judged to have been successful in the search for pisolitic bauxites, while the galvanic resistivity method confirmed its weakness in delineating the bauxitic levels. As regards the quantitative interpretation of the I.P. soundings, we must consider it under the double aspect of (1) giving the necessary data in order to obtain an idea of the extent and depth of the bauxitic levels and (2) being the first systematic confirmation of the validity of the theoretical foundations of the interpretative procedure. The support for such confirmation is furnished by the positive comparison with the borehole indications. ACKNOWLEDGEMENTS

We wish to thank the “Societh Mineraria Montevergine” (Italy) for permission to carry out the soundings and for providing the data from their boreholes. The study was performed with financial aid from the National Research Council (C.N.R. - Comitato per le Scienze Geologiche e Minerarie). REFERENCES Benvenuti, G. and Norinelli, A., 1968. Correlazioni tra resistivita e contenuto pisolitico di mineralizzazioni a bauxite. Boll. Geofis. Teor. Appl., 10: 183-194. Canziani, R., Mongelli, F. and Patella, D., 1972. Misure sperimentali di Polarizzazione Indotta su giacimenti di bauxite pisolitica nel Salento. Riv. Min. Sicil., 23: 227-233.

250

Cotecchia, V. and Dell’Anna, L., 1959. Contributo alla conoscenza delle bauxiti e terre rosse de1 Salento. Mem. Note 1st. Geol. Appl. Univ. Napoli, 7: 3-20. Crescenti, V. and Vighi, L., 1964. Caratteristiche, genesi e stratigrafia dei depositi bauxitici cretacei de1 Gargano e delle Murge; centii sulle argille con pisoliti bauxitiche de1 Salento (Puglia). Boll. Sot. Geol. Ital., 83(7 ): 5-58. Homilius, J., 1961. Uber die Auswertung geoelektrischer Sondierungskurven im Falle eines vielfach geschichteten Untergrundes. Z. Geophys., 27: 282-300. Mongelli, F. and Patella, D., 1971. About a method of representation of IP-values obtained over sedimentary formations (two-layer cases). Geophys. Prospect., 19: 652661. Mosetti, F., 1953. Su alcune richerche geoelettriche di zone mineralizzate. Ind. Min., 4 (12): 1-4. Mosetti, F., 1955. A proposito delle possibilita di prospezione geoelettrica della bauxite. Ind. Min., 6(7): 373-380. Parasnis, D.S., 1970. Some recent geoelectrical measurements in the Swedish sulphide ore fields illustrating scope and limitations of the methods concerned. In: L.W. Morley (Editor), Mining and Groundwater Geophysics. Queen’s Printer, Ottawa, pp. 290-301. Patella, D., 1972a. An interpretation theory for Induced Polarization vertical soundings (time-domain). Geophys. Prosp., 20: 561-579. Patella, D., 1972b. Easy construction of master curves for the quantitative interpretation of Induced Polarization vertical soundings over layered structures (time-domain). Riv. Ital. Geofis., 21: 123-131. Patella, D., 1973. A new parameter for the interpretation of Induced Polarization field prospecting (time-domain). Geophys. Prospect., 21. Tournier, D. and Vajdea, V., 1969. Recherches experimentales et theoriques par les mkthodes de polarisation provoquee et de resistivite appliquees a la prospection de gisements de bauxite. Bull. Bur. Rech. Geol. Minibres, Ser.2, Sect.11 (4): 63-79.