1. Introduction: Since the introduction of Remote Sensing studies in the field of hydrogeology, in a big way during June 1986, the word Lineament has become the synonym for copious water availability. This word in its true form just represents linearity of any feature captured during aerial photography or during satellite pass. Hence there exist an absolute need to understand the importance of lineaments in the field of hydrogeological studies: the term’s true meaning, way of its identification and understanding its role in transportation & storage of groundwater.
Aerial photographs often reveal important hydrologic information that cannot be seen clearly on the ground. Structural features, which are not identifiable on ground and some times even on aerial photographs, can be very easily identified on satellite images, due to its regional synoptic coverage. Faults, joint systems and abandoned river courses are some examples, which can be readily seen on satellite images. Aerial photography has been especially helpful in fracture-trace analysis. A fracture trace is defined as a natural linear feature less than 1.6 Km in length that can be identified on aerial photographs. In the past, the success of bore wells constructed in hard rock aquifers were usually considered as a matter of chance. Virtually dry holes have been drilled near open wells producing 10,000 to 15,000 LPH. Using aerial photographs in combination with satellite imagery, hydro-geologists now know that well yields in Basalt terrains depend on the presence and interconnectivity of joints and/or fractures and have developed a technique to reach such locations. Linear traces longer than 1.6 Kms are called lineaments. Crustal movements along the faults or shear zones in igneous and metamorphic rock terrains create rubble zones in which relatively large volumes of groundwater can be stored. The weathered width of fractures may range from 6 to 76 m or more (Fletcher G Driscoll, 1987). The trace is usually delineated by aligned set of sympathetic joints / fractures or other surface depressions. Thus, fracture traces represent zones of increased porosity and hydraulic conductivity in igneous and metamorphic terrains.
Scale of the imagery or aerial photograph is very important in the interpretation of lineaments. Any drainage or related linear feature looks as though it is rectilinear on small-scale images, due its generalisation. But, the same feature when studied on large-scale maps its true nature can be studied. Therefore, the clarity regarding the feature and the scale on which it is being interpreted should always be kept in mind.
2. Classification of Lineaments: During Remote sensing data acquisition, features on the earth’s surface are captured true to their existence. Any linear feature or curvilinear feature is generally termed as a lineament. The lineament can be broadly classified as:
i. Anthropogenic lineaments and ii. Natural lineaments.
i. Anthropogenic lineaments are also called as pseudo lineaments. They include features like Roads, Railway line, Air strips, Water supply pipelines, Dam lines, Canals, boundaries of larger agricultural plots, trees planted along the roads, trenches, vegetation clearances created for forest fire protection, terrace cultivation, salt pan activities, etc. Some times even the lines marked on the Remote Sensing data sheet itself are interpreted as lineaments. Except for interpretation of communication links or annotating the area, these features do not have much significance.
ii. Natural lineaments are important for all interpretations related to Geology, Hydrogeology, Agriculture, Pedology, etc. They include a. straight courses of drainages, Geological structures such as faults, fractures, bedding planes, intrusives, weaker zones, upliftments, formation boundaries, escarpments, soil boundaries, Mud flats, etc. Amongst the above listed out natural features identified as lineaments, drainage lineaments and structural lineaments are of much interest to hydro-geologists.
As indicated earlier the linear feature which are less than 1.6 km in length are termed as fracture traces and linear feature which is longer than 1.6 km are identified as lineaments. The lineaments which are less than 5 km in length are termed as minor lineaments, lineaments of 5 to 25 Km in length are termed as medium sized lineaments and lineaments having its length more than 25 kms are called as Mega lineaments. The above terminology used will also indicate the influence of lineaments on groundwater storage, its movements and the depth of the aquifers, which they influence. The mega lineaments are the ones, which mainly contributes to the recharge of deeper aquifers during the hydrological cycle. Areas having abundance of mega open lineaments receives groundwater recharge to deeper aquifers as a part of first hydrological cycle and the bore wells taping aquifers at a depth of + 100 M will start working shortly after first rainfall. Mostly this situation has been seen in part areas of Latur, Osmanabad and Beed districts of Maharashtra State.
Structural lineaments can be further classified broadly as:
i. Closed lineaments and ii. Open lineaments.
Closed lineaments include geological features like dykes, narrow elongated ridges etc. As the openings within the fractures or joints are filled with the intrusive material or the extension of morpho high stands with a solid core at the center, these lineaments are often termed as closed lineaments. These lineaments when interpreted as dykes either they can be further classified as positive lineaments or negative lineaments based on the relief factor. When the dyke rock is comparatively harder compared to the country rock they stand against the background area, like a narrow elongated ridgeline. Such type of structures can be identified on remote sensing database by the virtue of their shape and shadow on the opposite to sunlight side.
Similarly if the dyke rock is comparatively softer compared to the country rock or if it is highly jointed or friable then they will give rise to elongated furrows. In such conditions they can be called as negative lineaments. Such lineaments on aerial photographs can be interpreted by using 3-dimentional view and on satellite images generally they can’t be decisively interpreted as negative lineaments. Under such circumstances limited field checking must be undertaken before classifying them.
Closed lineaments: The dykes identified as closed lineaments in Basalt formations of Maharashtra state generally have 3 sets of joints at the surface. These can also be called as biscuit type of joints as the individual fragments comes out from the formation just as biscuit from its pack. These joints get closed towards depth and generally after a depth of 20 M the dyke rock becomes hard, compact and massive. They act as open lineaments at the surface with respect to their hydrogeological characters, but at depth they act as real closed lineaments. By taking the benefit of this understanding many percolation tanks have been constructed in northern Maharashtra across the dykes. Open dug wells constructed in the command area of such percolation tanks by tapping these submerged dykes, will yield copious water till the storage in the percolation tank exist.
Open lineaments are mostly the structural lineaments wherein the fractures, joints and shear zones exist. Hydrogeologically, such lineaments represent weaker horizons on the earth’s surface conducive for groundwater storage and its movement compared to the background area. They are generally interpreted as having formed as strain due to the stress during structural disturbances of the area. Such lineament locations representing weaker zones on the earth’s surface are mostly occupied by the drainages. In other words drainage net works classified as rectilinear, parallel, trellis, radial, etc. having formed due to structural disturbances in the area, can be classified as open lineaments. Open lineaments might have also been developed due to structural faults, along flow contacts, flow terminal locations, etc.
Open lineaments can also be classified as concordant and discordant with respect to their attitude with respect to the main drainage of the watershed.
Groundwater conditions on / close to / away from open lineaments are quiet different. Analysis done by researchers of IIT, Powai; in Palghar and Wada tahsils of Thane district, have proved that the groundwater potential on the lineament is comparatively high compared to background areas. Further buffer areas on both the sides of lineaments can be marked having similar but quantitatively less effects.
Both closed and open lineaments can be vertical or inclined. Inclined lineaments can be identified on remote sensing data products due to their linear signatures and also due to difference in geological formations encountered on either side of the lineament. For example along an open lineament occupied by drainage coarse if the alluvial deposits are seen on one bank and hard rock exposure on the other side of the lineament, then it can be interpreted as an inclined lineament.
Lineaments can be further geomorphologically classified as concordant and discordant based on their relation with respect to drainage network.
Concordant lineaments are those, which are parallel or almost parallel to the main drainage of the basin/sub basin. Discordant lineaments are those with their strike direction across the main drainage of the basin/sub basin. Few major lineaments in the coastal districts or in parts of northern Maharashtra have been observed to acts as concordant act at few locations and in as discordant at few other locations. Concordant lineaments generally act as major groundwater transporting zone parallel to main drainage. Under such condition groundwater potential on the lineament depends upon whether the segment of the lineament under study is located in initial stage, middle stage or tail stage of the watershed (viz. Depends on the drainage order). Also the existing obstructions across the lineament due to morpho-high, dykes, etc. determine the ground water storage. Similarly discordant open lineaments act as potential storage zones only when it is located close to the drainages or located at the gravitation lows, otherwise on a morpho-high presence of lineaments accelerates the groundwater movement towards gravitational lows, creating a dry aquifer situation which is a disadvantageous hydrogeological condition for groundwater storage.
3. Studies for locating lineaments in the field: One of the biggest problems faced by a beginner practising hydrogeology is to get the confirmation about the interpretation of the lineament and to locate/ transfer them in field. During several introductory lectures, I have come across students complaining about the genuineness of the interpretation. This can be over come by following basic rules of field Geology. Following steps for the beginners is enlisted:
i. Interpret the geological and structural features by using standard FSC of a suitable scale.
ii. Transfer the interpreted details to a standard geo-coded annotated map of known scale (preferably on to a topo-sheet base map of 1:50000 or I: 25000 scale).
iii. By using Clino-compass and standard map reading techniques transfer the Remote Sensing interpretations to the ground.
What is expected on ground if both interpretation and field transferring is correct:
a. Favorable conditions for groundwater storage,
b. High density of good yielding wells / bore wells,
c. Drastic changes in the yield of the wells/bore wells for those located on the lineaments and away from the lineaments.
d. Closely spaced vertical fractures in the well sections in the strike direction of interpreted lineaments and absence of the same in background areas.
e. Change in quality of the groundwater from the wells located on and away from lineaments.
f. Geophysical surveys if conducted by using Wenner configuration then, the apparent resistivity will fall drastically on reaching lineament location and further continuation of the survey will indicate that apparent resistivity values will go back to the background area values.
g. Here as a word of caution it is necessary to mention that, before recommending the sites for additional / new groundwater withdrawal structures, care should be taken to analyze the present day groundwater development in the lineament location and balance potentials available in the area for further development. Otherwise the entire exercise of remote sensing interpretation for targeting of groundwater will become futile.
h. Structural lineaments provide additional favorablity to the groundwater storage and its movement, to the area of its existence. This situation can be used advantageously for inducing additional recharge to the groundwater storages. In the areas of over exploitation artificial recharge structures can be favorably located on lineaments.
4. Conclusions: The term Lineament represent various structural, geological, hydrogeological situation on ground and to use the term as a synonym to the existence of copious groundwater is not correct. Beginners in the field of practicing hydrogeology should be very careful in interpretation and assessing the role of groundwater storage and movement to the structural lineaments.
PS: This is a part of lecture notes prepared for introducing the subject of lineaments, targeting newly recruited hydro geologists. Hydro-geologists Interested in having the full text along with illustrative satellite images can send their request through e-mail. E-mail address: drbschandrasekhar@gmail.com
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Monday, September 10, 2007
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