A resistivity survey of phosphate deposits containing hardpanpockets in Oulad Abdoun, Morocco

Saad BakkaliEarth Sciences Department, Faculty of Sciences and Techniques, Abdelmalek Essaadi University, Tangier, Morocco

Received: March 29, 2005; accepted: August 11, 2005

RESUMENEn la gran cuenca de Oulad Abdoun en Marruecos se inició la explotación de un nuevo depósito de fosfatos para reemplazar

al de Grand Daoui que se encuentra agotado. Existen inclusiones estériles de caliche que dificultan la extracción de las rocasfosfatadas y que son difíciles de detectar. La resistividad de los caliches excede un valor de 200 Ω-m contra 80 a 150 Ω-m para laroca fosfatada. Se llevó a cabo un proyecto de prospección eléctrica con un equipo Schlumberger sobre una extensión de 50hectáreas. Se obtuvieron modelos del perfil geológico mediante análisis de Fourier, y se logró localizar las inclusiones de calichey cubicar las reservas de fosfatos de manera más confiable.

PALABRAS CLAVE: Prospección geofísica, resistividad, fosfatos, análisis de Fourier, filtrado, Marruecos.

ABSTRACTIn the great Oulad Abdoun Basin exploitation of a new phosphate deposit was begun after the Grand Daoui horizon was

exhausted. Inclusions of sterile hardpan—so-called “disturbances” —are hard to detect and interfere with phosphate extraction.Their resistivity is above 200 Ω-m as against 80 to 150 Ω-m for the phosphate-rich mineral. A Schlumberger resistivity surveyover an area of 50 hectares was carried out. Models of the geology were successfully obtained from Fourier analysis. A new fieldprocedure was tested to deal with the presence of disturbances. Estimates of phosphate reserves were improved and better con-strained.

KEY WORDS: Geophysical surveys, resistivity, phosphate, Fourier analysis, filtering, Morocco.

INTRODUCTION

Morocco is a major producer of phosphate, with anannual output of 19 million tons and reserves in excess of 35billion cubic meters. This represents more than 75% of worldreserves. Resistivity surveys have been successfully used inthe Oulad Abdoun phosphate basin in Khouribga Province(Figure 1), which is about 120 km south of Casablanca. Thepresent survey was carried out in the Sidi Chennane depositwhich is a part of Oulad Abdoun basin, extending over some800 000 hectares.

The Sidi Chennane deposit is sedimentary and containsseveral distinct phosphate bearing layers. These layers arefound in contact with alternating layers of calcareous andargillaceous hardpan. In this field, extraction was begun af-ter Grand Daoui deposit was exhausted. However, the newdeposit contains many inclusions or lenses of extremely toughhardpan locally known as dérangements or disturbances,found throughout the phosphate bearing sequence. The hard-pan pockets are normally detected only at the time of drill-ing. They interfere with field operations and introduce a se-vere bias in the estimates of phosphate reserves.

Direct exploration methods such as well logging orsurface geology are not particularly effective. However, thechemical changes which are detectable at the hardpan/phos-

phate rock interface produce an important resistivity con-trast. Other factors such as changes in lithofacies and claycontent and consistency appear to account for some addi-tional resistivity difference. It was found that normal phos-phate-bearing rock has a resistivity of 80 to 150 Ω-m whilethe hardpan typically features resistivity values of between200 and 1000 Ω-m.

A pilot resistivity survey was performed over an areaof 50 hectares. The purpose of this experiment was to tryand map and constrain the anomalous regions correspond-ing to hardpan. A resistivity map was expected to allow theelectrical resistivity signals to be imaged in 3D. We used aSchlumberger array with a span of 120 m designed to reacha depth of 40 m. The so-called disturbances appear at ran-dom so that the apparent resistivity map may be used by theoperating personnel as a kind of radar to plan the sequenceof field operations. We use traditional Fourier analysis todetermine the amplitudes of the effects of the random distur-bances, or any other resistivity anomalies that may be present.

OVERVIEW OF THE AREA OF STUDY

The Sidi Chennane phosphate deposit is within theOulad Abdoun basin about 33 km south east of Khouribgaand 24 km SSW of Oued Zem (Figure 2). Its boundaries are:West, meridian 372500 (Lambert), South, meridian 228000

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(Lambert), East, highway RP22, and North, the outcrops ofthe basement of the phosphate-rock sequence. The climateof the phosphate plateau is essentially arid. Rainfall is fromNovember to May and is usually below 400 mm. Vegetationis of sparse dwarf palm trees. Rural population subsists oncattle ranching and seasonal agriculture in small villages, ordouars. Ground water is increasingly scarce. Scattered wellsdepend on an aquifer in the Turonian limestones at depths of100 m or more, which is sealed by the Senonian marls. Thisaquifier is also the sole water supply for the various miningoperations.

GEOLOGY

The phosphate mineral was deposited over a long timewindow from Maestrichtian (late Cretaceous, about 80 ma),to Lutetian (early Eocene, 40 ma). However, deposition wasirregular. Some layers are missing. Oulad Abdoun Basin oc-cupies most of the phosphate plateau which is bounded to-ward the north by red outcrops of pre-Cenomanian sedimentsforming anÄextension of the south edge of the Central Mas-sif. The Western boundary is the Rhamna Range, the BeniAmir plain is to the South and the Upper Atlas of Beni Mellalextends to the East. The geology of the study area is wellunderstood (see Figure 3 for stratigraphy).

The geologic section rests unconformably on Paleozoicschists and quartzites. The basement is well located and the

sedimentary cover is fairly thick (Michard, 1975). The up-permost formations of the Maestrichtian and Eocene containthe phosphate-bearing strata which are 30 to 50 m thick. Theearlier deposits, i.e. the lower 5 to 28 m, are clayey phos-phates of Maestrichtian age. The upper 20 to 30 m are lesshomogeneous. They are layered phosphate marls and sand-stones with some limestones of Eocene age.

Below the phosphate-bearing strata one finds up to 70m of Senonian marls and limestone marls; 20 to 60 m ofTuronian limestones; a Cenomanian formation of alternat-ing gypsum marls and limestone marls; and finally 10 to 60m of red marls and mudstones of pre-Cenomanian age.

The disturbances may be differentiated by size of thepocket or inclusion, type of material, hardness, clay content,or type of contact with the phosphate rock. Two main typesof disturbances are found. The first type is found throughoutthe mineral deposit: it appears to be a random mixture oflimestones, marls, clays, cherts and low-grade phosphate withlarge amounts of cherty limestone. The second type is highlydisturbed and lacks any dominant facies. It appears as anaccumulation of low-grade phosphate limestone blocks withlarge nodules of chert, marl, some fragments of chert andphosphate rock. The latter type forms inclusions of 10 tomore than 150 m and is the most abundant during miningoperations (Figure 4). These pockets are found both in theunderlying formation and in the upper members of the phos-

Fig. 1. Location of the study area.

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Fig. 2. Main phosphate basins in Morocco.

Fig. 3. General outline of the stratigraphy.

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by geophysicists and engineering geologists and the proce-dures are well established. The study area was selected forits representativity and the resistivity profiles were designedto contain both disturbed and enriched areas (Kchikach andHiyane, 1991). The sections were also calibrated by usingvertical electrical soundings.

High values of apparent resistivity were encountereddue to the presence of near vertical faulting between areas ofcontrasting resistivity, and fault zones which may containmore or less highly conducting fault gouge. The gouge may

phate sequence, and as a result there are strong resistivitycontrasts between the disturbances and the normal phosphate-bearing rock. These contrasts were confirmed in the test runs(Figure 5). Geophysical prospection could thus be based onprior evidence from field data.

FIELD PROCEDURES

Resistivity is an excellent parameter and marker for dis-tinguishing between different types and degree of alterationof rocks. Resistivity surveys have long been successfully used

Fig. 4. Adverse effects of « disturbances » on mining operations

Fig. 5. A resistivity traverse over three « disturbances ».

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contain gravel pockets or alluvial material in a clay matrix(Kchikach et al., 2002). Such anomalous sections are alsoclassified as disturbances. Apparent resistivity values in theseprofiles locally exceeded 200 Ω-m.

In order to locate and define the anomalous areas ordisturbances, an electric current of intensity I was passedbetween electrodes A and B, and the voltage drop ΔV wasmeasured between the potential electrodes M and N. Theapparent resistivity is found from ρ

app= K (ΔV/I), where K is

the geometric constant of the instrument which depends onlyon the distance between electrodes (Gasmi et al., 2004). Thusthe ratio between I and ΔV yields the resistivity of the ter-rain. Our Schlumberger set required the electrodes to bealigned and equidistant from the central point O so thatMN<<AB (Figures 6 and 7). The longer the section, the deeperis the sensitivitπ of the survey (Bakkali and Bouyalaoui,2004).

The lateral inhomogeneities of the ground can be in-vestigated by means of the apparent resistivity obtained fromthe survey. As the surface extension of the layers is displayedwe may infer the presence or absence of any disturbances aswell as any facies variations. Our resistivity measurementswere performed by means of a Syscal2 resistivity meter byBRGM Instruments using a rectangular array of 20 m x 5 m.In order to reach a mean depth of exploration of 40 m wecarried out 51 traverses at a spacing of 20 m. There were 101stations at 5 m distance for every traverse, which makes 5151stations all together in the survey (Bakkali and Bahi, 2005).

FOURIER ANALYSIS

Interpretation of resistivity anomalies is the process ofextracting information on the position and composition of atarget mineral body in the ground. In the present case thetargets were essentially the inclusions called perturbations.

The amplitude of an anomaly may be assumed to be propor-tional to the volume of a target body and to the resistivitycontrast with the mother lode. If the body has the same resis-tivity as the country rock no anomaly will be detected.

Apparent resistivity measurements are obtained from aharmonic potential V which fulfils Laplace’s equation ΔV=0in the surrounding space external to the body. The potentialgradient falls off as 1/r2 (Blakely, 1995). As a first approxi-mation, the scalar potential in the neighborhood of the per-turbations and within the sources complies with the equa-

Fig. 6. Diagram of a Schlumberger resistivity survey.

Fig. 7. A centered Schlumberger device.

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tion ΔV= -2ρI δ(r), where δ(.) is the Dirac delta, ρ is theresistivity in the anomalous region and I is the current at apoint electrode in an elastic halfspace, i.e. the topographicsurface of the study area (Telford et al., 1995). The apparentresistivity map which one obtains from such a survey is ac-tually a map of discrete potentials on the free surface, andany major singularity in the apparent resistivities due to thepresence of a perturbation will be due to the crossing from a“normal” into a “perturbed” area or vice versa. In other words,the apparent resistivity map may be considered a map of scalarpotential differences assumed to be harmonic everywhereexcept over the perturbed areas. Thus one assumes that thepotential difference data may be processed by Fourier analy-sis (Cooper, 2004).

Figure 8 shows the map of resistivity anomalies ob-tained by the EasyMapping3 routine from our apparent re-sistivity survey (Monnereau, 1997). This procedure enablesus to define the sources, after a sequence of filtering opera-tions by analytic prolongation and vertical second derivativeprojection. These standard procedures are basically Fourierfilters which enhance some features of the anomaly map atthe expense of other features, thus enabling us to connect thesurface anomalies to the sources at depth.

Downward analytic prolongation will also amplify theeffect of the deep structures, and enables us to separate theeffect of adjacent sources (Rakotoniaina, 1999). To someextent it enables us to obtain an outline of the anomalousstructures and, in any case, to find the depth to the roof ofthe body. Of course there is a possibility to confuse severalstructures with a single structure (Lutz, 1999).

The second-derivative operation is equivalent to high-pass filtering. It is useful for interpretation as it refines theresponse of the geophysical instrument, and it producessharper contours. As derivation is sensitive to noise (Cooperand Cowan, 2003), this procedure enhances the expressionof any structures and discontinuities.

EXPRESSION IN THE FREQUENCY DOMAIN

Let f,g,h be the data, the transformed data, and a filter;and let F, G, H be the respective spectral transforms. Weprocess the data as follows.

Space domain Frequency domain Space domain → →

Data FFT G(u,v)=F(u,v) . H(u,v) Inverse FFT Transformed data

The data are imaged symmetrically in the space do-main in order to minimize edge effects on the transformedmap. Otherwise some spurious high frequency effects mayappear around the edges.

The downward prolongation operator may be expressedin the frequency domain by a factor e2πνz, where z is the tar-

geted depth. The frequency ν may be defined as ν=√(u2+v2),where u and v are the frequencies in the East and North di-rections (Gunn, 1975).

Once the resistivity map was filtered in the frequencydomain the downward prolonged map at a depth of 50 mwas restored back to the space domain by inverse Fouriertransformation. Downward prolongation was carried out insteps of 10 meters which enabled us to sample the entiresequence of the phosphate formation. Numerical filtering wascarried out with the SIGNAL2.1 software (Monnereau, 1997;see Figure 9). The vertical derivative operator in the frequencydomain may be expressed as (2πν)2 (Gunn, 1975).

RESULTS

The resistivity map as obtained by the above proce-dure in the study area provided a direct image for an inter-pretation of the resistivity survey. We were able to identifythe anomalies which turned out to be strongly correlated withthe disturbances. We found that the disturbances as detectedfrom surface measurements were distributed apparently atrandom with uniform density (Figure 8).

The map of resistivity anomalies was high-pass filteredwith the second-derivative algorithm. The anomalous areaswere neatly outlined. A possible area of coalescing resistiv-ity anomalies was detected (Figure 9). As mentioned above,the disturbances were outlined in depth down to 50 m. Fig-ures 10 and 11 show two anomalies which appear to coa-lesce at 50 m depth and practically grow together near thesurface. The corresponding resistivity profile (Figure 12) fitsthe calibration tests (Figure 13). The high resistivities overanomalous areas are interpreted as reflecting the fact that thepockets of disturbed material tend to become more consoli-dated at depth. We find that the analytic prolongation proce-dure helps to better constrain the location of anomalous ar-eas on the surface. When a second derivative procedure isalso used, some additional improvement may be obtainedand visualization of the anomalies in depth was found to beuseful. The overall effect is that of scanning the anomalousbodies.

CONCLUSIONS

We have described a pilot study to test a geophysicalapplication to a specific problem in the phosphate miningindustry. The results proved satisfying. Resistivity surveysmay be adapted to the specific field conditions in WesternMorocco.

Values of resistivity in excess of 200 Ω-m correlatedconsistently with disturbances. Standard data processing pro-cedures were found to be consistently useful and the filteredmaps may be used as auxiliary tools for decision makingunder field conditions.

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Fig. 9. Second-derivative map for Figure 8. Note the good correlation between « disturbances » and steep anomalies.

Fig. 8. A map of resistivity anomalies for AB=120 m (distances in meters).

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Fig. 10. As in Figure 9 : downward extrapolation of anomalies to a depth of 50 m.

Fig. 11. Localization map from the preceding figures, showing coalescence (stars) at 50 m depth.

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Fig. 12. As in Figure 11. The traverse is indicated by the solid line.

Fig. 13. The original resistivity profile over the coalescing anomalies (solid triangles).

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It was found that the maps of vertical prolongation atdifferent depths were particular useful to the surveyors forimproving and constraining their estimates of phosphate re-serves in the deposit. The set of resistivity maps may be usedas a kind of radar image to plan the sequence of mining op-erations.

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BAKKALI, S. and L. BAHI, 2006. Cartographie des «dérangements » de séries phosphatées par mesures derésistivités électriques. Journal des Sciences Pourl’Ingénieur, J.S.P.I., 6, (under press).

BLAKELY, R. J., 1995. Potential Theory in Gravity andMagnetic Applications, Cambridge University Press,441.

COOPER, G., 2004. The stable downward continuation ofpotential field data. Expl. Geophys., 35, 260-265.

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KCHIKACH, A. and M. HIYANE, 1991. Apport de lagéophysique à la détermination des dérangements à SidiChennane Nord, Publications École Mohammadiad’Ingénieurs, Rabat, Maroc.

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Saad BakkaliEarth Sciences Department, Faculty of Sciences and Tech-niques, Abdelmalek Essaadi University, Tangier, MoroccoEmail: saad.bakkali@menara.ma