Lineament Analysis using Landsat MSS Data and Engineering Properties of some Rocks within the vicinity of Osara Dam, Itakpe-Okene area, North Central Nigeria.

*Kolawole, M.S.; Daniel, A. and Zakari, E. S.

Earth Sciences Department, Kogi State University, Anyigba. Nigeria.

*Corresponding Author

Introduction

A lineament is a mappable, simple or composite linear feature of a surface whose

parts are aligned in a rectilinear or slightly curvilinear relationship and

which differs distinctly from the pattern of adjacent features and presumably

represents a subsurface phenomenon (O’Leary et al., 1976). Geologic lineament

mapping is considered as a very important issue for problem solving in

engineering, especially in site selection for construction (dams, bridges, roads

etc) seismic and landslide risk assessment, mineral exploration, hot spring

detection and hydrogeological researches etc.

An effective approach for delineation of fracture zones is based on lineament

indices extracted from satellite imagery for a detailed structural analysis and

understanding of the tectonic evolution of the area. It also provides useful

information for geological mapping and understanding of groundwater flow and

occurrence in fractured rocks (Marghany and Hashim 2010). The few studies done

in Nigeria using modern remote sensing include that of Ananaba and

Ajakaiye(1987) that shows evidence of tectonic control of mineralization in

Nigeria from lineament density analysis and Goki et al (2005) that used digitally

processed Landsat 5 imageries to map mineralized pegmatites around Nasarawa

state. Other studies that focused on hydrogeological applications include those

of Bala (1987); Bala (1997); Edet et al.(1994); Odeyemi et al.(1999); Bala (2000b)and

Ayok(2009).

There have been reports of frequent lowering of the water level in the dam

reservoir, with the consequent disruptions of the electricity supply to many

parts of the country and neighboring countries. Ananaba (1991) attributed this

lowering of the water level to high evaporation, and or a possible leakage of

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the water through unmapped fractures in the vicinity of the dams. Lineament

analysis techniques using remotely sensed data help researchers identify

different structural regimes and mineralization zones (Kavak and Cetin 2007).

The construction of many infrastructural facilities by government and other

related agencies have sparked off unequal demand for rocks aggregates which are

products of engineering materials including marble, gniesses, and banded

ironstone that abound in the study area. Though, these rocks appear to be

homogenous on the surface, natural processes that the rocks have been subjected

to over the years have caused serious weathering and fracturing thereby

degrading their desirable engineering properties (Mclean and Gribble 1979). The

fractured and weathered state of these rocks cannot be ignored in dam

construction as a highly fractured rock, if saturated will not only reduce the

total quality of rock aggregate but their strength will also be degraded. (Akpan

et al 2010).

In this study, several image processing procedures such as Principal Component

Analysis, band rationing technique and directional edge enhancement technique

using convolution kernels filtering along with visual interpretations have been

used to delineate geologic lineaments in the vicinity of Osara Dam built to

supply water to Nigeria Iron Ore Mining Company (NIOMCO), Itakpe. Faults of

various types and sizes, alignments of tonal features, systematic fracture

patterns and dentritic drainage networks are some of the lineaments found in the

study area using ILWIS 3.2. Geological and geotechnical techniques were also

employed to identify and appraise the strength and engineering attributes of

some rocks at the vicinity of the dam.

Location, geomorphology and geology of the study area.

The Osara dam is located in Okehi Local Government area of Kogi State in the

north central part of Nigeria. It is bounded between Latitude 70 401 and 70 471

North of equator and longitude 60 151 and 60 261 East of the Prime Meridian. The

Dam site is situated about 6km north of the NIOMCO and it is accessible through

Lokoja-Okene highway and Abobo-Dam untarred road. The outcrops in this area were

accessed through these routes and cattle tracks, footpaths and river and stream

channels.

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The iron ore deposit mined by this company forms ridges that is approximately 1

km wide and 5km long and reaches a maximum elevation of about 500m above the

surrounding lowland which is 200 m above sea level (Olade 1978), the granitic

rocks and gneisses in the area also occur as inselberg or low lying . The

vegetation is typically a southern savanna with long grasses and shrubs. Annual

rainfall ranges from 1524 to 1916 mm in May to October, with surface temperature

of about 270C (Olade 1978).

The study area is within the southwestern Basement Complex of Nigerian which

forms a part of the Pan- African mobile belt that is situated between the east

of the West African Craton, north -west of the Congo cratons and south of the

Tuareg Shield as defined by Black et al (1979). The basement rocks comprise of

crystalline complex of gneisses, migmatites and quartzites (>2000 m.y.),

overlain by sequence of low-grade metamorphic rocks (~800m.y. old), and intruded

by a suite of granitic and charnockitic rocks (~600 m.y. old). In this area that

form part of the geology of Okene investigated by Olade (1978) consist of

strongly foliated gniess of granodiorite composition with alternating dark

(biotitic and/or honblendic) and light (quartzo-feldspathic) bands. Other

varieties of gniesses mapped in the area are of granite compositions that

contain either biotite or biotite-hornblende as the dominant mafic minerals

consisting of fine-grained leucocratic gniess that consist of about 70% of

quartz, 25% of microcline and 5% of biotite that forms a major topographic

feature and structural marker unit in the area. The iron –bearing rocks in the

area can in part be considered as ferruginous quartzites that comprise

alternating bands of quartz and iron oxides. Intrusive rocks include gabbro,

biotite-hornblede granites, pegmatites and numerous lamprophyre dykes.

Materials and methods

To detect lineaments from Landsat MSS data, two procedures were carried out.

Initially, visual interpretations were used to extract geologic lineament using

false color composites of bands 1, 4, 5 and 6 of the imagery. In addition to

this manual interpretation, the imagery of the area was evaluated using various

digital image processing techniques. The preprocessing and edge enhancement

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techniques used are contrast stretching, directional filtering, color composites

and color band ratio composites which according Kasak and Cetin (2007) are good

in enhancing geological features and to extract structural lineaments in the

study area.

For the engineering properties of the rocks, two representative samples of the

granitic and granite gniess were collected at ten different locations around the

dam site for laboratory tests. The tests conducted on the samples are specific

gravity measurement, compressive strength, and aggregate crushing value test, as

well as unit weight, void ratio and porosity test. The pycnometer method was

used for determination of specific gravity. The compressive strength of the

rocks was determined using an ELE instrument. The geotechnical property tests

were carried out at Rock Mechanics Laboratory, Federal University of Technology,

Akure Nigeria and result obtained compared favourably well with the ASTM D2938-

79 (1980) and ISRM (1979) standards and are summarized in Table 1 below.

RESULTS AND DISCUSSION

The Landsat MSS image of the area used was acquired on 21st April, 1992 and

processed at National Remote Sensing Centre, Jos Nigeria. The subscene covers

approximately 1000km2 including the dam site that occur at the southern portion

of the area.

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Fig 2: Landsat MSS 642 (RGB) false color composite image of the study area.

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(a)

(b)

(c)

(d)

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Fig 3: Individual bands of Landsat MSS (bands 1,2,4,6 marked a, b, c and d

respectively) of the study area

(a)

(b)

(c) (d)

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Fig 4: Ratio color composites (5/6, 5/4, 4/1, and RGB marked a, b, c and d

respectively) of the study area

(a)

(b)

8

01000020000300004000050000600007000080000

Leng

th of s

egmen

ts

(c)

Fig 5: RGB (NW, N-S, E-W) color composite image of directional edge detection

filtering on Band 5 (a) and derived lineament system from this (b) and bar

histogram showing the distribution of lineaments based on length of segments and

azimuthal direction (c).

Visual lineament interpretation: About 160 geologic lineaments were extracted

and interpreted visually using the RGB color composite ratio bands and RGB

directional filter band in the migmatite gneiss complex,

Lineament recognition criteria such as geomorphological trends, rectangular,

trellis and colinear drainage system patterns and distinct contrast differences

were used in this analysis. Distribution of these lineaments was clustered

particularly in the north and the NE regions of the study area, possibly due to

bias caused by solar illumination from the SE.

Figure 5 shows the azimuthal distribution of the data using a rose diagram. Most

of the lineament clusters in the NE part of the region are N 20°-30° E and N

60°-70° E trending lineaments. Whereas in the NW part of the region, lineaments

tend to cluster in N 50°-60° W direction. Another approach that has been used by

Qari (1991) is shown using an azimuthal histogram bar in Fig. 6.

Ratioing effect to the determination of geologic lineament and structure: Figure

3 shows the individual bands, 1, 4,

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2 and 6, of the Landsat MSS data set of the region. Several analysis techniques

such as visual interpretation and compass gradient enhancement are routinely

used for lineament studies. In this study, ratio color composite subscene images

of the region were also used to enhance the lineament systems that were not

clearly enhanced by other methods. Ratio images generally enhance different

lithologies and features of a region. For example, various kinds of land-cover

types such as rocks, vegetation and water are represented in different colors in

ratio color composite images. Geological contacts can be enhanced using

different colors that can help researchers interpret lineament systems easily.

Figure 8 A shows a ratio color composite image using Landsat MSS 5/6 (red), 5/4

(green) and 4/1 (blue) ratios in RGB form.

Directional filtering: Edge enhancement filters are used in geological

applications to highlight faults and lineaments that occur in specific azimuths

(Kavak and Cetin (2007). The band 5 of the Landsat MSS dataset is one of the best

bands showing most of the features in the area and different filter types which

enhance particular spatial directions (Kavak and Cetin 2007). To enhance such

edges, filters are designed to map the contrast gradient orthogonal to the

preferred direction (Berhe and Rothery, 1986) that will detect and highlight

diagonal, horizontal and vertical edges in digital images (Richards 1986).

Directional filters are very useful for producing artificial effects suggesting

tectonically controlled linear features (Drury, 1986). Another characteristic of

these kernels is the distribution of nonzero weighting factors parallel to the

direction. Therefore, asymmetrical filters are ideal for lineament analysis

studies (Kavak and Cetin (2007).

Table 1: weighted kernel types of directional filters used in this study after (Kavak and Cetin (2007)

...............NW…........... ...................N-S………… ………..E-W…………

-1 -1 0 -1 -1 -1 -1 0 1-1 0 1 0 0 0 -1 0 1 0 1 1 1 1 1 1 -1 1

For this study, several directional filters, NW, N-S and E-W compass filters were

selected. Table 2 shows the size and weight of kernels used for the test area. The

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filters that have been used in this study had 3×3 kernel size. Some authors also

used these filters as first derivative kernels for airborne radiometric and

aeromagnetic data (Fernandez and Tahon, 1991; Lee et al., 1990). To provide a better

visual interpretation for every filtering type, contrast enhancement procedure was

also applied to three different edge detection filters. Later, filtered images taken

from NW, N-S and E-W were displayed in RGB color space mode Fig. 9. A total of 116

lineaments were observed using a color composite image of the test area. Similar to

the visual lineament interpretation results, the dominant directions were 20-30 (16

lineaments) and 60-70 (13 lineaments) azimuthal intervals. Similarly, 280-290

intervals (9 lineaments) represents densely populated interval in the NW direction.

According to the total length of the lineaments, 20-30 and 60-70 intervals

demonstrated the dominant directions values. Most of the longer lineaments are

concentrated in the NW direction, which is similar to the visual interpretation

results.

Engineering properties of rocks: the rocks in the study area have specific gravity

values that range from 2.739 to 2.756 (average of 2.746). The American Rock

Mechanics Association (ARMA) and International Society of Rock Mechanic (ISRM 1979)

recommends specific gravity of not below 2.65 for rock to be suitable for

construction. These results also conform to Bs 1377 specification for hard rock

materials.

Water absorption values range from 0.512% for granite to 0.671% for granite gniess

(average of 0.592%) which show that the rock fabrics are well cemented and non

porous. According to McWorter and Sunada (1977) the porosity of weathered granite

should be between 0.34-0.57 and the water absorption value should not be more than

0.7 per unit by weight. The fresh rocks on the surface have very high mechanical

strength properties with low water absorption capacity which is an indication of the

low porosity state of the rocks.

Compressive strength results for the rocks vary from 78.46 to 79.55 mpa (average of

79.005mpa). American Society for Test and Materials (ASTM D2938-79, 1980) recommends

that for granite to be suitable as a construction material as well as a foundation,

it must have compressive strength of 75-131 mpa. This strength of the rock is

influenced by the mineralogy of the material, rock structure and weathered degree of

the rock that increases with the rate of loading.

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Aggregate crushing values of the rocks vary from 28.5 for the granite to 28.1% for

the gniess; this implies that the rocks when crushed will for good aggregate that

will be useful for construction works.

Weathering and fracturing of the rocks seems to be topography controlled as hard

rocks at ridges are fresher than those along stream channels and swamps (Akpan et al

2010).

Table 2: A summary of the mean engineering properties of the granite and granite gniess mapped in the study area

Rock type Measured Parameter Mean results

Biotite and Honblende Granite

Specific gravity 2.739Aggregate crushing value (% (fines))

28.5

Compressive strength (mpa) 79.55Water absorption and Porosity(%)

0.514

Banded Gniess

Specific gravity 2.756Aggregate crushing value (% (fines))

28.1

Compressive strength (mpa) 78.46Water absorption and Porosity 0.671

CONCLUSION

Visual interpretation and digital image processing methods had very high

correlation in this lineament analysis based study. Filtering directions were

selected according to the features of the thermotectonic events that dominate in

the area. In this study, NW, N-S and E-W directional filtering processes were

analyzed and interpreted in terms of the thermotectonic evolution of the region.

Furthermore, rationing method helped remove topographic effects and enhance

several features in the area. Generally, result shows that clearly extensive NE-

SW trending lineament systems that most of the lineaments were clustered in the

northeastern part of the region which corresponds to one of the imprints of Pan-

African orogeny. Result shows that developed in the region. There are also twoprominent sets of fracture with both roughly oriented to NNE- SSW as mapped by

Ajakaiye et al (1986) and highlighted by Ananaba (1991) as St Paul’s Fracture Zone

(FZA) megalineament of Nigeria. the dominant foliation defined by alternating

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dark and light bands and prominent gneissossity trends roughly N-S with

variation between NE-SW and NW-SE. E-W foliations are the oldest and mostly

preserved as relicts.

Principal fractures directions are in the N-S, NNE-SSW, NNW-SSW and NW-SE and to

a lesser extent E-W, with the N-S fractures marked by considerable shearing and

brecciations. The NE-SW and NW-SE conjugate sets are mostly strike slip faults

with the north easterly characterized by dextral sense of movement. The NE-SW

and NW-SE conjugate sets are mostly strike slip faults with the north easterly

characterized by dextral sense of movement. Some of the important fault systems

are the Ifewara , Zungeru ,Anka and kalangai with wide zones of mylonites,

cataclastic and silicified rocks. They are interpreted to have resulted from

transcurrent movements. These lineaments are as long as 25km or less which could

be part of subsidiary faults suggested by Garba (2002).

Field and remote sensing studies indicate that most of the lineaments are

extensional fractures that correspond to either dikes emplacement or normal

faults which were subsequently reactivated into strike-slip shear fractures. The

NW–SE and NNE–SSW lineaments represent dilatational fractures (Garba 2002). The

NNE–SSW trending lineaments are the oldest. The N–S and WNW–ESE lineaments form

conjugate shear fractures and are younger than the NNE–SSW lineaments (Danbatta

and Ajibade 1995). The N-S trending lineaments are prominent around

metasedimentary rocks while NE-SW and NW-SE sets of lineaments are mostly common

around gneisses and migmatite rocks (Kogbe 1981). Blomquist and Wladis (2002)

reported that lineaments represent zones of weakness in the bedrock and are

assumed to also represent a topographic depression that streams usually flow

along which could be tectonically controlled. It has been proven in other area

that the area of higher points of fracture or lineament intersections usually

become a good target for economic mineral exploration (Sabin, 1987).

The fresh rocks on the surface have very high mechanical strength properties

with very low water absorption capacity. This low water absorption capacity is

an indication of low porosity state of the rocks. The results show that the

rocks that outcrop in the area possess good engineering properties and

characteristic and are therefore suitable for the use in the dam construction

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and other civil engineering works. The test results also compare favorably well

with standard values of ASTM D2938-79 (1980) and ISRM (1979).

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