SAAD BAKKALI.indd


123

ANALYSIS OF PHOSPHATE DEPOSIT “DISTURBANCES”  
USING THE HORIZONTAL-GRADIENT RESPONSES  

OF RESISTIVITY DATA (OULAD ABDOUN, MOROCCO)

Saad Bakkali

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

Email: saad.bakkali@menara.ma
 

RESUMEN

En 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 rocas fosfatadas 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 la roca fosfatada. Se llevó a cabo un proyecto de 
prospección eléctrica con un equipo Schlumberger sobre una extensión de 50 hectáreas. Se obtuvieron 
modelos del perfil geológico mediante análisis de gradiente horizontal del mapa de los “distorsiones” de 
fosfatos. Se logró localizar modelar la localizacion de las inclusiones de caliche y cubicar las reservas de 
fosfatos de manera más confiable.

Palabras clave: Prospección geofísica, resistividad, fosfatos, gradiente horizontal, Marruecos.

ABSTRACT

In 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 survey over an area of 50 hectares was carried out. 
Models of the geology were successfully obtained from the analysis of the horizontal-gradient map of 
“disturbances”. A new field procedure was tested to estimate the depth of disturbances. Phosphate reserves 
were better known.

Key words: Geophysical surveys, resistivity, phosphate, horizontal-gradient, Morocco. 

© 2005 ESRJ - Unibiblos.

Manuscript received August 2005
Paper accepted September 2005

EARTH SCIENCES 
RESEARCH JOURNAL

Earth Sci. Res. J. Vol. 9, No. 2 (Dec. 2005): 123-131



124

Saad Bakkali

INTRODUCTION

Morocco is a major producer of phosphate, with 
an annual output of 19 million tons and reserves in 
excess of 35 billion cubic meters. This represents 
more than 75% of world reserves. Resistivity surveys 
have been successfully used in the Oulad Abdoun 
phosphate basin in Khouribga Province (Figure 1), 
which is about 120 km south of Casablanca. The 
present survey was carried out in the Sidi Chennane 
deposit which a part of Oulad Abdoun basin, 
extending over some 800,000 hectares. 
The Sidi Chennane deposit is sedimentary stratiform 
and contains several distinct phosphate-bearing 
layers. These layers are found in contact with 
alternating layers of calcareous and argillaceous 
hardpan. In this field, extraction was begun after 
Grand Daoui deposit was exhausted. However, the 
new deposit contains many inclusions and lenses 
of extremely tough hardpan locally known as 
“dérangements” or disturbances, found throughout 
the phosphate-bearing sequence. The hardpan 
pockets are normally detected only at the time of 
drilling. They interfere with field operations and 
introduce a severe bias in the estimates of phosphate 
reserves.

Direct exploration methods such as well logging 
or surface geology are not particularly effective. 
However, the chemical changes which are detectable 
at the hardpan/phosphate rock interface produce 
an important resistivity contrast. Other factors 
such as changes in lithofacies and clay content and 
consistence appear to account for some additional 
resistivity difference. It was found that normal 
phosphate-bearing rock has a resistivity of 80 to 150 
Ω.m while the hardpan typically features resistivity 
values of between 200 and 1000 Ω.m.
A pilot resistivity survey was performed over an area 
of 50 hectares. The objective of this experiment was 
to try and map and constrain the anomalous regions 
corresponding to hardpan. A resistivity map was 
expected to allow the electrical resistivity signals 
to be imaged in 3D. We used a Schlumberger array 
with a span of 120 m designed to reach a depth of 
40 m. The so-called disturbances appear at random 
so that the apparent resistivity map may be used 
by the operating personnel as a kind of radar to 
plan the sequence of field operations. We use 
traditional analytic analysis by horizontal-gradient 
to circumscribe the extension in-depth of the effects 
of the random disturbances, or any other resistivity 
anomalies that may be present.

OVERVIEW OF THE AREA OF STUDY

The Sidi Chennane phosphate deposit is within 
the Oulad Abdoun basin about 33 km south-east 
of Khouribga (Figure 2). Its boundaries are : West, 
meridian 372500 (Lambert), South, meridian 22800 
(Lambert), East, highway RP22, and North, the 
outcrops of the basement of the phosphate-rock 
sequence. The climate of the phosphate plateau 
is essentially arid. Rainfall is from November to 
May and is usually below 400 mm. Vegetation is of 
sparse dwarf palm trees. Rural population subsists 
on cattle ranching and seasonal agriculture in small 
villages, or douars. Ground water is increasingly 
scarce. Scattered wells depend on an aquifer in the 
Turonian limestones at depths of 100 m or more, 
which is sealed by the Senonian marls. 

Figure 1. Location of the area of study.



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Analysis of Phosphate Deposit “Disturbances” using the Horizontal-Gradient 
Responses of Resistivity Data (Oulad Abdoun, Morocco)

Figure 2. Main phosphate basins in Morocco.

GEOLOGY

The phosphate mineral was deposited over a long 
time window from Maestrichtian (late Cretaceous, 
about 80 ma), to Lutetian (early Eocene, 40 ma). 
However, deposition was irregular. Some layers 
are missing. Oulad Abdoun Basin occupies most of 
the phosphate plateau which is bounded toward the 
north by red outcrops of pre-Cenomanian sediments 
forming an extension of the south edge of the Central 
Massif. The Western boundary is the Rhamna Range, 
the Beni Amir plain is to the South and the Upper 
Atlas of Beni Mellal extends to the East. The geology 
of the study area is well understood (see Figure 3 
for stratigraphy). 

Figure 3. General outline of the stratigraphy.

T h e  g e o l o g i c  s e c t i o n 
rests unconformably on 
P a l e o z o i c  s c h i s t s  a n d 
quartzites. The basement 
is well located and the 
s e d i m e n t a r y  c o v e r  i s 
fairly thick (Kchikach 
and Hiyane, 1991). The 
u p p e r m o s t  f o r m a t i o n s 
o f  t h e  M a e s t r i c h t i a n 
and Eocene contain the 
phosphate-bearing strata 
which are 30 to 50 m thick. 
The earlier deposits, i.e. the 
lower 5 to 28 m, are clayey 
phosphates of Maestrichtian 
age. The upper 20 to 30 m 
are less homogeneous. 

They are layered phosphate 
marls and sandstones with 
some limestones of Eocene 
age. 
Below the phosphate-bearing 
strata one finds up to 70 m of 
Senonian marls and limestone 
marls; 20 to 60 m of Turonian 
limestones; a Cenomanian 
f o r m a t i o n  o f  a l t e r n a t e l y 
gypsum marls and limestone 
marls; and finally 10 to 60 m 
of red marls and mudstones of 
pre-Cenomanian age.
T h e  d i s t u r b a n c e s  m a y  b e 
differentiated by size of the 

pocket or inclusion, type of material, hardness, 
clay content, or type of contact with the phosphate 
rock. Two main types of disturbances are found. The 
first type is found throughout the mineral deposit : 
it appears to be a random mixture of limestones, 
marls, clays, cherts and low-grade phosphate with 
large amounts of cherty limestone. The second type 
is highly disturbed and lacks any dominant facies. It 
appears as an accumulation of low-grade phosphate 
limestone blocks with large nodules of chert, marl, 
some fragments of chert and phosphate rock. The 
latter type forms inclusions of 10 to more than 150 
m and is the most abundant during mining operations 
(Figure 4). 



126

Saad Bakkali

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

The study area was selected for its representativity 
and the resistivity profiles were designed to contain 
both disturbed and enriched areas (Kchikach and 
Hiyane, 1991). The sections were also calibrated by 
using vertical electrical soundings. 
High values of apparent resistivity were encountered 
due to the presence of near-vertical faulting between 
areas of contrasting resistivity, and fault zones which 
may contain more or less highly conducting fault 
gouge. The gouge may contain gravel pockets or 
alluvial material in a clay matrix (Kchikach et al., 
2002). Such anomalous sections are also classified 
as disturbances. Apparent resistivity values in these 
profiles locally exceeded 200 Ω.m. 
In order to locate and define the anomalous areas 
or disturbances, an electric current of intensity I 
was passed between electrodes A and B, and the 
voltage drop ∆V was measured between the potential 
electrodes M and N. The apparent resistivity is found 
from ρ

app 
= K (∆V/I), where K is the geometric 

constant of the instrument which depends only on 
the distance between electrodes (Gasmi et al., 2004). 
Thus the ratio between I and ∆V yields the resistivity 
of the terrain. Our Schlumberger profiles set required 
the electrodes to be aligned and equidistant from 
their central point O so that MN<<AB (Bakkali and 
Bouyalaoui, 2004). 
The lateral inhomogeneities of the ground can be 
investigated by means of the apparent resistivity 
obtained from the survey. As the surface extension 
of the layers is displayed we may infer the presence 
or absence of any disturbances as well as any facies 
variations. Our resistivity measurements were 
performed by means of a Syscal2 resistivity meter by 

These pockets are found both in the underlying 
formation and in the upper members of the phosphate 
sequence, and as a result there are strong resistivity 
contrasts 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 on prior evidence 
from field data.

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

FIELD PROCEDURES

Resistivity is an excellent parameter and marker for 
distinguishing between different types and degree of 
alteration of rocks. Resistivity surveys have long been 
successfully used by geophysicists and engineering 
geologists and the procedures are well established. 



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Analysis of Phosphate Deposit “Disturbances” using the Horizontal-Gradient 
Responses of Resistivity Data (Oulad Abdoun, Morocco)

BRGM Instruments using a rectangular array of 20 m 
x 5 m. In order to reach a mean depth of exploration 
of 40 m we carried out 51 traverses at a spacing of 
20 m. There were 101 stations at 5 m distance for 
every traverse, which makes 5151 stations altogether 
in the survey (Bakkali and Bahi, 2005).

METHODOLOGY

Interpretation of resistivity anomalies is the 
process of inferring information on the position 
and composition of a target mineral body in the 
underground. In the present case the targets were 
essentially the inclusions called perturbations. The 
amplitude of an anomaly may be assumed to be 
proportional to the volume of a target body and to 
the resistivity contrast with the mother lode. If the 
body has the same resistivity as the mother lode 
no anomaly will be detected. Apparent resistivity 
measurements are obtained from a harmonic 
potential V which fulfils Laplace’s equation ∆V=0 
in the surrounding space external to the body. The 
potential gradient falls off as 1/r2 (Blakely, 1995). 
As a first approximation, the scalar potential in the 
neighborhood of the perturbations and within the 
sources complies with the equation ∆V=-2ρI δ(r), 
where δ(.) is the Dirac delta, ρ is the resistivity in 
the anomalous region and I is the current at a point 

electrode in an elastic halfspace, i.e. the topographic 
surface of the study area (Dey and Morrinson, 1979). 
The apparent resistivity map which one obtains from 
such a survey is actually a map of discrete potentials 
on the free surface, and any major singularity in 
the apparent resistivities due to the presence 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 scalar potential differences 
assumed to be harmonic everywhere except over the 
perturbed areas. Thus one assumes that the potential 
difference data may be processed by Fourier analysis 
without any terrain correction (Cooper, 2004). 

HORIZONTAL-GRADIENT AND 
DOWNWARD PROLONGATION PROCESS

Figure 6 shows the map of resistivity anomalies 
corresponding to “disturbed” phosphate zones. 
Figure 7 is the corresponding map of the “disturbed” 
zones. This procedure enables us to define the 
sources. The standard procedure was basically 
Fourier filters which enhance some features of the 
anomaly map at the expense of other features, thus 
enabling us to connect the mapped surface anomalies 
to depth (Bakkali, 2006). 

Figure 6. A map of apparent resistivity anomalies of the area of study.



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Saad Bakkali

Figure 7. A map of the corresponding “disturbed” zones of the resistivity anomalies map.

(Figure 8) was calculated starting from a plan 
obtained by an adjustment by the method of least 
squares of a grid of 5 cells out of 5 cells centered 
on the point to determine. 
We note the good correlation between “disturbed” 
phosphate zones and the horizontal-gradient 
intensity)
The downward prolongation of the horizontal-
gradient apparent resistivity map reveals the extension 
in-depth of the contrasting densities. The operator of 
prolongation in the spectral domain is e z u v2

2 2+r  where 
is the depth of prolongation and o the frequency 
defined as u v2 2= +o ] g, where u and v are the 
frequencies in the eastern and northern directions 
(Gunn, 1975). We have applied this process to the 
map of the horizontal-gradient apparent resistivity 
to estimate the extension in-depth of the contrasting 
of density between the disturbances and the normal 
phosphate-bearing rock. This singular procedure will 
enable us to map the extension of anomalic zones 
in-depth (Bakkali, 2006).
The data are imaged symmetrically in the space 
domain in order to minimize edge effects on the 
transformed map. The second-derivative operation 
is equivalent to high-pass filtering. It is useful 
for interpretation as it refines the response of the 
geophysical instrument, and it produces sharper 
contours. As derivation is sensitive to noise (Cooper 
and Cowan, 2003), this procedure enhances the 
expression of any structures and discontinuities. 

The magnitude of the horizontal-gradient is usually 
estimated by finite difference methods from values 
measured at gridded points on the apparent resistivity 
anomaly map. 
The magnitude is determined using the following 
equations:

,
, ,

x y
x

x y

y

x y
app

app app

2

2

2

2

2
2

2
2

= +t
t t

l ^
^ ^

h
h h

with    
,

x

x y

ax2
, ,app i j i j1 1

2
2

=
-t t t+ -
p

^ h

and     
,

y

x y

ay2
, ,app i j i j1 1

2
2

=
-t t t+ -
p

^ h

where x is the eastern coordinate and y the 
northern coordinate. ,i jt  is the pseudo-apparent 
resistivity defined at grid point (i,j). Grid intervals 
in the x-direction and y-direction are axp  and ayp   
respectively. Maxima horizontal-gradient may be 
occur immediately over steep or vertical boundaries 
separating rock masses of contrasting densities. On 
the horizontal-gradient apparent resistivity map, 
lines drawn along ridges formed by enclosed high 
horizontal-gradient magnitudes correspond to these 
boundaries. The horizontal-gradient represents in 
this case an indicator of the level of variation of the 
contrast of density between the disturbances and 
the normal phosphate-bearing rock. The value of 
the intensity of the horizontal-gradient anomalies 



129

Analysis of Phosphate Deposit “Disturbances” using the Horizontal-Gradient 
Responses of Resistivity Data (Oulad Abdoun, Morocco)

Otherwise some spurious high frequency effects 
may appear around the edges. 
Once the horizontal-gradient resistivity map was 
filtered in the frequency domain the downward-
prolonged corresponding map at a depth 5 to 50 m 
(Figure 9) was restored back to the space domain 
by inverse Fourier transformation. Downward 
prolongation was carried out in steps of 10 meters 
which enabled us to sample the entire sequence 
of level of the contrasting of density between the 
disturbances and the normal phosphate-bearing 
rock.

RESULTS AND CONCLUSIONS

The resistivity map as obtained by the above 
procedure in the study area provided a direct image 
for an interpretation of the resistivity survey. We 
were able to identify the anomalies which turned 
out to be strongly correlated with the disturbances. 
We found that the disturbances as detected from 
surface measurements were distributed apparently 
at random as confirmed by the various maps of 
the downward prolongation of horizontal-gradient 
apparent resistivity intensity. These maps represent 
an effective indicator of the intensity level of 
“disturbance” in depth.

The anomalous areas were neatly outlined. As 
mentioned above, the disturbances were outlined 
in depth down to 50 m. The contrasting of density 
between the disturbances and the normal phosphate-
bearing rock from 5 to 50 m depth attest that the 
disturbances are contrasted in-depth comparatively 
on the surface.
The high intensities of the horizontal-gradient over 
anomalous areas are interpreted as reflecting the fact 
that the pockets of disturbed material tend to become 
more consolidated at depth. We find that the analytic 
prolongation procedure of the horizontal-gradient 
helps to better constrain the location of anomalous 
areas on the surface. The overall effect is that of 
scanning the anomalous bodies.
We have described an analytical procedure to identify 
the anomalies of a specific problem in the phosphate 
mining industry. The results proved satisfying. Data 
processing procedures as horizontal-gradient were 
found to be consistently useful and the corresponding 
prolonged maps may be used as auxiliary tools for 
decision making under field conditions. 
It was found that the maps of horizontal-gradient 
at different depths were particular useful to the 
surveyors for improving and constraining their 
estimates of phosphate reserves in the deposit. 

Figure 8. Horizontal-gradient apparent resistivity intensity map in Ω.m-1 of figure 6 
(contrast density level between the disturbances and the normal phosphate-bearing rock corresponds  
to contrast of nuances of gray color. 



130

Saad Bakkali

Figure 9. Downward prolongation of horizontal-gradient apparent resistivity intensity maps .



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Analysis of Phosphate Deposit “Disturbances” using the Horizontal-Gradient 
Responses of Resistivity Data (Oulad Abdoun, Morocco)

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