|Resistivity Meters & Imaging Systems|
|Magnetic Susceptibility Meters|
|Magnetic Resonance Systems|
Reconnaissance Study of the
Relationship Between Lineaments
and Fractures in the Southwest Portion of the Lake Chad Basin
Solomon A. Isiorho and Tom O. Nkereuwem
One of the problems facing drought-stricken parts of the world is the location of
potable sources of water. The delineation, verification, and study of the effect of
fractures on groundwater resources and movement are essential parts of the hydrologic data
set in drought-prone regions. In this reconnaissance study prominent lineaments were
mapped from Landsat images of the Lake Chad region. A Wenner array resistivity profile 14
kilometers in length was made perpendicular to a lineament identified on Landsat images.
The profile shows a segment with significantly lower resistivity values at the position of
the lineament. Low total dissolved solids (TDS) values were also recorded for shallow
groundwater samples near the lineament, suggesting it is a recharge zone. Prominent
Landsat lineaments in this area are thus probably fractures, as suggested by the
resistivity profile and TDS values, and should be investigated through more detailed
geophysical and hydrogeological surveys.
Locating a potable source of water in arid and semiarid regions
requires a knowledge of the local hydrology. The Sahel region of Africa has experienced
many droughts including those in the 1960s and 1980s. The shortage of potable water in
this region is even more serious with the southward advance of the Sahara Desert. Located
to the south of the Sahel region is Lake Chad. Slightly larger than Lake Erie, Lake Chad
on occasions has completely, or almost completely, dried as evident from prehistoric sand
dunes found on the lake bottom (Isiorho, 1989).
Lake Chad is located in a semiarid region with a low annual
rainfall (-30 cm) and a high evaporation rate (-2 in/ year; Roche, 1980; Carmouze, 1983).
These facts, coupled with the proximity of the Sahara desert, cause the lake to shrink
during periods of low annual rainfall in the active watershed south of the lake. Lake Chad
is the main source of water in the region, both at the surface and through recharge of the
underlying aquifer through seepage through the lake bed (Isiorho and Matisoff, 1990).
Structural features around Lake Chad have been mapped by
geophysical methods (Cratchley, 1960; Cratchley et al., 1984). Some of these structures,
although covered by the thick sediments (-549 in) of the Quaternary Chad Formation, may be
visible at the land surface (Durand, 1982; Isiorho et al., 199 1). Casual observation of
Landsat and shuttle images indicate the presence of linear features. Is there any
relationship between these linear features and known structures? Could these linear
features affect water movement between the lake and the surrounding aquifers or provide
zones of enhanced infiltration? This reconnaissance study is an attempt to answer these
questions for the southwest portion of the Lake Chad Basin.
The study area encompasses the southwestern portion of the Lake
Chad Basin (fig. 1) where annual rainfall is approximately 0.31 in (Eugster and Maglione,
1979; Jaekel, 1984). The average annual rainfall for the past seven years at New Marte
within the study area was 0.43 in . The annual high temperature is 30-45°C with relative
humidity averaging 30%. The southwestern portion of the Lake Chad Basin is a plain that
slopes gently towards Lake Chad. It is devoid of rock outcrops and is covered by
superficial deposits of sand and clay. All surface drainage is towards the lake.
The climate of the region is semiarid with two seasons: a long
dry season from October to April when a dry dusty wind blows off the Sahara desert with
daytime temperatures of 30-36°C (85-98 °F) and nighttime temperatures of 5-11°C
(40-50°F), and a short wet season, from about May to September with daily maximum
temperature of 34°C (90°F) and relative humidity of about 40-70 %. The vegetation in the
study area can be described as Savannah woodland which is divided into two zones: Sudan
Savannah to the south and Sahel Savannah towards the north.
The Lake Chad Basin is located in a tectonically active area with structural
features that extend northwest to the Air Plateau and southwest towards the Benue trough,
a failed arm of a triple junction (Burke, 1976; Ajayi and Ajakaiye, 1981). The locations
of present day rivers (Lagone, Chari and Kamadogu) and earlier streams are controlled by
structural features (Durand, 1982). The western shoreline of the present day lake, and the
direction of the perilacustrine ridge, correspond to known structural features (Durand,
1982). A number of positive Bouguer gravity anomalies correspond to neotectonic lineaments
near Lake Chad, and the structural history of this area is extremely complex (Cratchley et
al., 1984; Avbovbo et al., 1986). In general, the surface morphology is a direct
reflection of subsurface structures.
Figure 1. Map showing the study area in the Lake Chad Basin.
The Lake Chad Basin was formed by extensional tectonic forces during
the Cretaceous Period (Burke, 1976). Located within the basin is the Quaternary Chad
Formation, which is the youngest geologic formation that contains aquifers. The Chad
Formation is of major concern in this study because it is hydraulically connected with
Lake Chad. In the southwest part of the basin, the Chad Formation is composed of three
aquifers referred to as the Upper, Middle and Lower Aquifers. Although buried in the study
area, the Chad Formation is composed of argillaceous (fine grained) sequences with
arenaceous (coarse grained) horizons. A generalized section of the Chad Formation is shown
in fig. 2. The upper aquifer of the Chad Formation, made up of fine grained sediments
approximately 30 m thick, is hydraulically connected to Lake Chad (Carmouze, 1983; Isiorho
and Matisoff, 1990), and is separated from the underlying middle aquifer by approximately
100 m of clay-rich sediment.
The Chad Formation is overlain by aeolian sands, fluvial, deltaic and
lacustrine deposits approximately 1 to 6 m thick. Most of the fluvial deposits occur along
stream valleys which are made up of two units: the old alluvium and the young alluvium
(Hammand and Abdou, 1982). The old alluvium consists of deposits of old rivers, while the
young alluvium contains recent river beds and flood plains. Field observations indicate
the presence of silt and clay-sized sediments approximately 0.6 m thick in some places.
Along the New Marte-Kirenowa road, the silt-clay sediment overlying the Chad Formation may
be at least 1-1.5 m thick. Figure 3 shows the overlying sediments. The detailed geology of
the area is described by Raebum and Jones (1934), Barber and Jones (1960), Carter et al.,
(1963) and Avbovbo et al., (1986).
The objectives of this study are to verify that some lineaments mapped
from Landsat images southwest of Lake Chad are fractures, or fracture zones, and to
investigate the effect of these lineaments on groundwater quality within the area. This
paper describes how electrical resistivity may be used to accomplish these goals and to
determine the depth to groundwater immediately southwest of Lake Chad. This reconnaissance
survey is intended to provide a basis for planning future detailed surveys in this area,
for the purpose of characterizing groundwater resources.
Identification of lineaments has proven useful in hydrologic
studies especially where large areas are involved (Salomonson, 1983; Isiorho, 1988) and
many lineaments are believed to be directly related to tectonic activity or tectonic
features (Rowan and Lathran, 1980). "Lineaments are naturally occurring alignments of
soil, topography, stream channels, vegetation, or a combination of these features that are
visible on remotely sensed imagery and aerial photographs. The main assumption inherent in
performing any lineament analysis is that these alignments represent fracture zones or
other discontinuities" (Mabee et al., 1994). In this study Landsat and shuttle images
were analyzed and lineaments were delineated on acetate film. Several Landsat photographic
images and transparencies covering the same area acquired at different months during the
year (1987) and during different years (1972, 1975, 1976, 1985, 1986, 1987) were used in
delineating the lineaments. The images were studied using standard photogeologic
techniques under transmitted light and reflected light from a light table as described in
Isiorho (1985). Lineaments were drawn on acetate film placed over the images. Prevalent
lineament orientations were noted and compared to known geologic structural features in
the area. Only prominent lineaments (lineaments at least 20 km long that
were clearly visible on all images of the same area were
recorded. Some of the images were reexamined some months later to evaluate the
observers ability to reproduce lineaments at the same geographic location. New
lineament maps were made and were compared with the first set and the corresponding
lineaments were noted. These prominent lineaments were thus verified by their multiple
rendition in several images and at different times. One of these prominent lineaments was
then targeted for field verification.
Figure 2. Generalized stratigraphic column of the Chad
Figure 3. Photo showing the 1-1.5m clay/silt sized sediments overlying the Chad
Formation along the New Marte - Kirenowa road.
One prominent lineament was selected for a resistivity profile based on
its accessibility. The purpose was to verify that a fracture actually existed where
identified from the Landsat images. Such a fracture (or fracture zone) should exhibit an
anomalously low resistivity due to the presence of a shallow water table or elevated soil
moistures. Over four hundred resistivity measurements were made along a 14 km profile
along the Kirenowa-New Marte road which runs nearly perpendicular to the inferred
lineament. The Wenner array was adopted (instead of the Schlumberger array) because it
involved moving only one probe at a time after each reading. The electrode spacing used at
all stations was 30 m, as dictated by the available wire length and the available time for
profiling. Electrical soundings were also performed at two locations along the profile,
and ten soundings at other places to determine the depth to groundwater. The resistivity
soundings were then analyzed using a DC resistivity computer inversion program. The
presence of Earthwatch volunteers made it possible to collect a large number of
resistivity data points within a two-week period.
Figure 4. Landsat mosaic image of the southwest Chad Basin.
Figure 5. Map showing the prominent lineaments visible on all
the Landsat images, the resistivity profile (indicated as a broken line), and the TDS
values of some groundwater and lake water samples.
Seventy-five water samples were collected from the lake and open wells (dug wells with
diameters of 0.5 to 1.5 meter) for laboratory analysis. Many of these wells occur far away
from the Kirenowa - New Marte profile. The depth to water in some open wells were
determined by electric probe. Only water table samples with depths less than fifty meters
were used in this study. The water samples were collected either in the mornings or
evenings when the villagers collected (fetched) water. The water samples were collected in
250 ml plastic bottles, filtered, and acidified. The water samples were then analyzed for
major ions, pH, acidity, alkalinity, total dissolved solids, dissolved oxygen, carbonate
and sulfate using portable test kits and field instruments (e.g. Hach DR/2000
Figure 6. Photo showing flat terrain along the profile line.
Results and Discussion
Analysis of Landsat images indicate a NE-SW trend to the majority of lineaments.
Figure 4 shows a part of the Landsat mosaic image from May, 1985 over the southwest Lake
Chad Basin where some of the most prominent lineaments occur. Figure 5 shows the prominent
lineaments that were identified on all of the Landsat images. Approximately 50% of the 30
most prominent lineaments were observed in all images. Most of the smaller lineaments,
close to the lake, have NE-SW to E-W trends. These lineaments appear to be related to sand
dunes based on limited field observation. With the exception of a few with a N-S trend,
smaller lineaments are generally oriented in the same direction as the prominent
lineaments. The sinuous parallel lineaments, especially northwest of Baga, are old lake
shorelines. A prominent lineament starts northwest of Baga with a NNW-SSE trend, and
assumes an E-W trend -lineament south of Kirenowa; is the longest such feature (-240 km)
in the study area. This lineament is labeled Z in fig. 5.
The Kirenowa New Marte transect was chosen for the study because it was easily
accessible through a road that cuts across the lineament. The profile is approximately 14
kilometers long. Several parts of the profile were repeated at different times and days to
verify the stability of previous readings. The apparent resistivity readings in ohm-m were
plotted with distance without correcting for the topography (elevation) or depth to
groundwater levels. Correction was deemed unnecessary because only the general resistivity
trend was sought and the topography of the area is relatively flat with a relatively
uniform overburden thickness of approximately 1-1.5 in. This estimate is based on
Figure 7. Interpretation of a resistivity sounding
station about 2km south of Kirenowa. (The Figure depth to water in a well 5m from sounding
station = 24.1m)
Figure 8. Map showing the water table for the south-west Lake Chad Basin
(well=*; resistivity sounding +, resistivity sounding station shown on fig. 7+).
|soil profile exposures observed within the Kirenowa - New
Marte canal which also lies along the transect. The relative flatness of the study area is
exhibited in fig. 6, with two people at extreme ends of the line approximately 90 m apart.
The depth to groundwater increases southwards from approximately 15 m near Kirenowa to 30
m near New Marte. Figure 7 is a Wenner array sounding curve made approximately 2
kilometers south of Kirenowa. The depth to groundwater from the interpretation was 25.0 m,
close to the 24.01m measured from a nearby well approximately 5 m from the sounding
station. Available lithology well logs of the Chad formation within the study area are
similar to the generalized log shown in fig. 2. The spatial distribution of wells, as well
as resistivity sounding locations are shown in fig. 8. This figure also shows the
groundwater table map of southwest Lake Chad Basin. The general direction of groundwater
movement is southwestwards away from Lake Chad (Isiorho and Matisoff, 1990).
Figure 9a. Resistivity profile along the New Marte-Kirenowa
road. The lowest resistivity readings correspond toe the lineament (fracture zone)
Figure 9 is the resistivity profile
along Kirenowa - New Marte road. The resistivity reading was not uniform from
point-to-point, but there was a general reduction in the resistivity readings until about
midway between New Marte and Kirenowa where the lowest resistivity readings were obtained
in the profile. Initially, change in the personnel was thought to be the reason for the
varied readings and as a result some of these areas were profiled again with new
personnel, and in a few cases, a different meter was used. Most of the low resistivity
values in the 9-12 km segment were below 10 ohm-m (for example, 6.0, 6.0, 2.0, 0. 1, 0. 1,
and 4 ohm-m were some of the low resistivity readings recorded) with 0.09 ohm-m being the
lowest apparent resistivity reading. Some areas of low apparent resistivity readings were
visited the same day or at a later time using the same resistivity meter or another meter.
No significant differences in apparent resistivity were recorded.
The segment (approximately 3 km wide between 9 and 12 km) with lowest
apparent resistivity corresponds to the inferred prominent lineament from the Landsat
images. Figure 9 b is a "blow up" of the area corresponding to the lineament
from fig. 9 a. The low resistivity readings are probably due to the presence of shallow
moisture in the subsurface materials. The moisture hypothesis is further corroborated by
the presence of a high density of trees/shrubs relative to areas outside the fracture
zone. In several of the Landsat images, the lineament has a dark tone due to the presence
of vegetation. The fracture zone is estimated to be approximately three kilometers wide.
During the summer of 1991, seventy-five water samples were collected
from the southwestern portion of the basin with most samples from the lake and near-shore
region. Total dissolved solids (TDS) in the groundwater generally increased with distance
from the lake. The TDS value attains a minimum where the inferred fracture is located
(fig. 5), but increases again, south of the apparent fracture zone. A 2-sample t-Test
(df=11, t-stat=-5.56, alpha=0.05, p=0.0002) rejects the null hypothesis of similarity
between the TDS values (from the fracture zone compared to samples taken outside the
fracture zone). This seems to corroborate the inference that the lineament is a fracture,
and the low TDS values observed in the region can be attributed to the fracture acting as
a local recharge zone. Groundwater recharge by direct rainwater through the fracture zone
would have lower TDS values because of the short distance traveled by groundwater relative
to groundwater in the surrounding area. However, limited chemical data precludes the
establishment of a definite relationship between the lineament and water chemistry of the
study area. More water samples are needed to fully discover the impact or influence of the
lineaments on the water chemistry of the region.
The relationship between lineaments/fractures and water movement can be
verified by performing pumping tests around, and near, the lineaments. Detailed
resistivity surveys or other geophysical surveys (such as EM) should also be carried out
within a fifty kilometer wide region around the lake, together with the installation of
monitoring wells for both water sample collection and water level measurements. The
relationship of the lineaments to both groundwater production and the fate of the recharge
water from the lake should also be examined.
Figure 9b. Resistivity profile segment within the lineament.
The study has identified a prominent lineament in the southwest
portion of the Lake Chad Basin that appears to be a bedrock fracture zone. This prominent
lineament was investigated with resistivity profiling and a 3 kilometers wide low apparent
resistivity zone was found to coincide with the position of the lineament. Although
groundwater TDS tends to increase with distance from the lake, the groundwater TDS is at a
minimum along this low resistivity zone. Soil moistures also appear high in this zone.
This suggests that groundwater within the fracture zone may be recharged, at least in
part, from precipitation.
Financial support for this project was received from the
Earthwatch Research Corps, the National Geographic Society, and Indiana University -
Purdue University Fort Wayne. The Chad Basin Development Authority, the Geology Department
of the University of Maiduguri, Karen Wehn, and the Earthwatch "Lake Chad
Project" volunteers were instrumental in the completion of the present project.
Thanks are due to D. Chowdhury and C. Drummond for reviewing the manuscript. The anonymous
reviewer and editors comments helped in the clarification of points in this paper.
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