Geoinformation, monitoring and assessment Environment

Posted December 1998

Groundwater Exploration by Satellite Remote Sensing in the Syrian Arab Republic, Part 1

by
Carlo Travaglia
Remote Sensing Officer
Environment and Natural Resources Service (SDRN)
FAO Research, Extension and Training Division
with the cooperation of
Osama Ammar
General Organization of Remote Sensing
Syrian Arab Republic


from "Groundwater exploration by satellite remote sensing in the Syrian Arab Republic", RSC Series 76, FAO 1998


Abstract

The study undertaken in the Syrian Arab Republic clearly indicates that the integration into a geographic information system of data extracted from earth observation satellites with those traditionally gathered, coupled with selected field investigations and the geological knowledge of the area under investigation, provides a powerful tool in groundwater search.

In the basaltic terrain of Southern Syria groundwater moves along faults and fractures, mainly the result of tectonic movements. The statistical analysis of the lineaments interpreted from Landsat TM data indicates two major directions of fracturing considered as tensional and thus "open" for the storing and transmission of groundwater.

Large freshwater springs occurring on the seabed at a short distance from the Syrian coastline were identified and located through analysis of thermal data. As they occur on the seaward extension of lineaments mapped inland, potential drilling sites were identified and positively tested by geo-resistivity.

The development of the "blocks" concept allowed for the identification of the most suitable lineaments and, mainly, of "waterways" and consequently of highly potential drilling sites in a large portion of the carbonate interior of the coastal landmass.

The methodologies developed by the project may be operationally applied in other similar environments in Syria, as well as in other countries where the geological factors affecting groundwater storage and transmission are comparable.


Introduction

Being aware of the positive experiences of the FAO Remote Sensing Centre in locating potential groundwater areas through satellite remote sensing investigations in semi arid countries (Burkina Faso, Yemen) and in karst terrain (Philippines), the General Organization of Remote Sensing (GORS) of the Syrian Arab Republic requested FAO technical assistance for strengthening its capacity in this field through a joint study in selected parts of the country, allowing for the necessary technology transfer by on-the-job training of its staff and preparation of detailed guidelines for the application of the methodology developed and tested in the field to other parts of the country. Project TCP/SYR/6611(T) "Strengthening Remote Sensing Capacity in Groundwater Exploration" was approved by FAO in February 1996 with the above objectives.

In view of distinct environments occurring in the country and to fully test new approaches and technologies, three different hydrogeological environments were selected, namely: 1. the basaltic terrain of Southern Syria, 2. the sedimentary terrain in coastal area, and 3. the interior alluvial plain (Aldao Basin) in the Syrian steppe.

Basaltic terrain of Southern Syria

Highly fertile soils, developed from basaltic lava flows and scoria, occur in Southern Syria. However, mainly in the eastern portion of this region, water is scanty and irrigation, therefore, limited. Even water supply for domestic purposes is insufficient in some areas. Groundwater development is decidedly needed.

Sedimentary terrain in coastal area

It is well known that large springs occur along the shoreline and that fresh water springs exist on the seabed, not far from the shore. Large amounts of fresh water are thus lost to the sea. The aim of the project was to locate the karst "waterways" and indicate the most suitable sites for drilling so that water can be tapped before it is lost.

Interior alluvial plain (Aldao Basin)

This basin is located in the central part of Syria. It is a slightly dissected flat terrain with a smooth surface. The basin is a floodplain with playas (sabkhas) and eolian deposits filled by Quaternary and Palaeogene sediments. It consists of marl, clay, conglomerates and sands. Locating fresh water potential boreholes in a similar environment is important for animal grazing and to assist in the development of the recently established olive trees and dates palm plantations.

During the course of the activities, it was decided to eliminate the interior alluvial plain, where some hydrogeological investigations were carried out by the Arab Centre for the Studies of Arid Zones and Drylands (ACSAD), and to expand the study to the carbonate interior of the coastal area, in view of the pressing need of water supply of the towns and villages occurring there (Fig. 1 below).

To achieve its objectives in the above environments, the project developed specific methodologies based on the integration of satellite remote sensing, GIS, traditional hydrological data and field investigations. Geophysical investigations and test drilling in selected sites were undertaken to confirm the methodological approach and the theoretical assumptions made during the study.

Furthermore, the project and its results in distinct hydrogeological environments should be regarded as an application of satellite remote sensing to groundwater exploration, for future use, not only in other areas of Syria, but in similar environments outside Syria as well.

Satellite data/software used

In selecting the satellite data for the study, Landsat Thematic Mapper data were preferred, due to the availability of a large spectrum of bands, mainly in the near and medium infrared portion of the electromagnetic spectrum (em), the most suitable for lineaments and terrain analyses, as well as in the thermal portion of the em, which permitted the acquisition of further information. ERS-SAR data were also acquired over the coastal area for geomorphological/lineaments studies and comparison with Landsat TM information. Digital processing and GIS tasks were performed on a PC-based system using ERDAS 8.2 Imagine and ARC-Info 3.4 software.

Southern area

Area description

The area selected for study belongs mainly to the Syrian portion of the Yarmouk River catchment and is bounded in the North by the Al-Maneh Hills, a volcanic complex which constitutes the divide with the Damascus Basin; in the South by the Yarmouk River itself and by the boundary between Syria and Jordan; in the West by the Golan Heights (approx. 1 000 m asl); and in the East by the crest of the Jebel Al-Arab (1 760 m asl) and by the eastern edge of the large recent basaltic outcrop of Al-Laja, a distinct feature of the region. The pilot area has an approximate surface of 10 000 km2.

Geologically, the region is almost completely underlain by basaltic rocks of various kinds (lava flows, scoria, ..) belonging to several volcanic episodes occurred in Miocene to Quaternary, resting on Palaeocene marls.

Thickness of basalt varies widely, from over 1 300 m at Jebel Al-Arab to an approximate 200 m at the centre of the study area. Basalt thickness decreases southward to a few meters (10 - 20), as can be observed at the Yarmouk River canyon in the vicinity of Daraa, where the basalt-marl contact is evident and where many contact springs and water falls occur. Here the Paleocene sedimentary formation includes marl, limestone and bentonite interbeds. In limited areas near the border with Jordan, the basalt is even absent and the marl with limestone interbeds is outcropping as it is in the vicinity of Nasib village, SE of Daraa.

In general the study area is gently sloping southwestward and is drained by the Wadi Hrair, a right tributary of the Yarmouk River and by lesser rivers and creeks, usually trending around SW. Apart from the mountain ranges at its borders, constituting the main recharge areas for the aquifer, the region is characterized by large plateaus and plains, rolling terrain and heavily dissected basaltic outcrops, as a consequence of the different kind and age of basalts and their degree of weathering. The region takes its name from its largest plain, the Hauran Plain. However the plain, mainly in its eastern side, is frequently dotted by small basaltic cones, cinder cones and lava fields, usually aligned along the fractures from which the magma originated. Very good examples of colonnar basalt are frequent in the outcrops. Further eastward, the rough Al-Laja basalt field occurs.

Intensive de-stoning of the plains made available rich basaltic soil for intensive agriculture, with irrigation water pumped from creeks or wells. Areas of excessive rock outcrops are used as rangeland.

Average rainfall in the southern area is concentrated in the winter months, exceeding 450 mm in the east and 1 000 mm in the west, although in some dry years may be much less. A significant part of the precipitation infiltrates into the basalt cracks and joints caused by cooling and in the tectonic fractures which are numerous in the area. This secondary permeability allows for the storage and transmission of groundwater, locally in large quantities.

As indicated, in view of the gentle sloping of the region southwestward, of the occurrence of marls under the fractured basalt, large springs occur in the lowest portion of this system, in the vicinity of Daraa. One of these is the Mouzerib Spring, averaging in 1995 1 100 l/s discharge. The water of this spring is piped to supply almost all towns and villages of Daraa and Assuwayda provinces, as well as for irrigation of the southern portion of the Hauran Plain.

As a general scheme, groundwater flow is directed toward southeast from the Golan Heights, westward from the Jebel Al-Arab and southward from the Al-Maneh Hills, the first two being the main recharge areas. Once in the Hauran Plain, the flow is southwestward, according to the slope and fracturing of the region.

Wells have been drilled throughout the region and usually have limited production. However, in the central and southern portion of the Hauran Plain, owing to the limited thickness of the basalt layer and of its intense fracturing, wells have a much better production.

Methodology

The search for promising groundwater areas involved satellite remote sensing, GIS, traditional hydrological data and field investigations.

Consultation of a quite accurate but unfortunately not updated (1982 ?) listing of wells existing in the area indicated that production is usually very low (<1-5 l/s) but wells with above average production occur in a limited number. Thus particular attention was focused on those "strange" wells. From the above listing, information about their serial number, exact location/coordinates, depth of borehole, depth of water table and production was extracted.This operation was performed for all wells with production of 10 l/s and above. Location of each well and its attributes were reported as "well layer" on GIS. It was also observed that all wells of this class were moderately deep (70-120 m) and all drilled in basalt.

Similarly, a "spring layer" was created on GIS including the main springs occurring in the region with their attributes (name, location/coordinates, average discharge).

Surface drainage ("drainage layer") was also input into the GIS with information extracted from topographic maps.

The above three GIS layers, with information extracted from traditional data, were then correlated with information originating from satellite data. To avoid potential errors, all data sets were prepared at the same scale of 1:100 000.

Remote sensing data were essentially used for the location of the fractures occurring in the area. As the greatest portion of the area is flat to gentle rolling with a few clear morphological expressions of fracturing, fracture zones may be observed indirectly through increased soil moisture along them. The period of acquisition of Landsat TM data was therefore selected based on the above, namely in spring and late summer, as in other seasons the soil is saturated by the moisture, thus appearing almost uniform in tone.

Table 1: Southern area, satellite data used
Landsat TM Path/row Acquisition date
174/37 18 May 1996
21 September 1995
173/37 9 September 1995

Lineaments analysis was performed both visually on 1:100 000 prints of FCC 453 (RGB) and directly on the computer screen on a series of data such as the above FCC and single near-mid infrared bands (bd 4,5). Enhancements (edge, ..) and several convolution filters (3x3, 5x5, horizontal, vertical, directional) were applied to band 5 of the May and September data. Results of the lineaments analyses were compiled into only one overlay, after critical review of all data sets. This "lineaments layer" was then transferred to the GIS.

The large amount of water discharged by springs near/at the Yarmouk River and the good production of wells in selected parts of this basaltic basin indicated a good transmission of groundwater from the recharge zone to the plain and this is obviously possible only along open, tensional fractures. Soil resting on fracture zones with good amounts of water percolating into them should be moist and probably cooler than the surroundings.Based on this assumption, an attempt was made to use the thermal channel of Landsat TM (bd 6) to evidentiate this. Notwithstanding the scarce spatial resolution of this channel (120 m, resampled at 28.5 m for correlation with the other bands) and the hour of acquisition of the data (around 9.30 a.m.local time) which was not the most suitable1 , it was possible to map linear thermal anomalies, corresponding to areas cooler than the surroundings, after simple digital enhancements of the band 6 of the September data. A critical review of the results through checking with topographical maps, allowed for the removal of a buried pipeline and of several creeks and irrigation canals with flowing water. The remaining thermal lineaments often coincided with lineaments mapped previously. However the presence of a thermal anomaly indicated the occurrence of water into them; this new information was then transferred to GIS as "thermal layer".

The collection of data concerning the lineaments observed in the Landsat TM scenes, both imagery and digital enhancements, was analyzed statistically. For this purpose, the strike and length of each lineament was measured. Strikes were divided into classes of 5* and the total length of the lineaments belonging to each class computed. Subsequently, the length of each class was divided by the total length of the lineaments. This procedure, which takes into account the lineaments trends as well as the mapped length, was applied to all lineaments identified in the study area.

By means of these operations, major groups of lineaments, assumed to be traces of near vertical faults and fractures, were observed to cluster in preferred orientations.

Two major directions of fracturing are evident: N65-70E and N45E, the first with a much higher frequency than the other. In the basaltic terrain of Southern Syria the volume available for groundwater storage is restricted to the interconnected system of fractures, crushed zones, joints and fissures in the rocks (fracture porosity). Such interstices are mainly the results of tectonic movements. In this kind of rock, the amount of groundwater available is entirely dependent on the storage and rate of infiltration in the faults and fractures. This, in turn, depends on whether the fracture is open or tight. It can be said quite simply that a tight fracture contains no water while an open one may produce a considerable yield of groundwater. In most cases this can be related to tension or shear phenomena in the ruptural deformation of the rocks.

Tensional faults, that is those parallel to the direction of the tectonic stress or orthogonal to the direction of crustal extension, may be believed open and somewhat wider than shear faults, which are orthogonal or inclined with respect to the direction of tectonic stress and consequently tend to be tighter. Thus, it should be much easier to recognize tensional faults in a satellite scene than shear faults and this should be reflected in the lineaments frequency histogram. This should be particularly true for the area in question as there are, as already indicated, very few morphological expressions of fracturing.

It may be concluded that lineaments trending according to the directions indicated above may represent tensional fractures or faults and thus may be open,an essential condition for the storing and transmitting of groundwater.

The intersection of lineaments having the above indicated trends is definitely a good prospective site to investigate. However,regional fractures trending N65-70E were preferred by us as the fracture zone was wide and thus the aquifer recharge, from infiltration of rainfall, from creeks and from other water bearing fractures, considerable.

The information thus stored in the GIS consisted of three layers (wells, springs and drainage) originating from traditional data and two (lineaments and thermal) generated through analysis of satellite data, all layers being at the same scale of 1:100 000. The overlaying of the five layers, either in selected associations or all together, permitted the identification of the most promising groundwater potential areas. Infact, it was immediately evident that all springs were located on lineaments and that the largest majority of wells producing 10 l/s and above was either on lineaments or very close to them, that is in the fracture zone (Fig. 2).

Furthermore, the overlaying of the drainage and lineament layers indicated areas of recharge from creeks and wadis to the aquifer.

Fig. 2: Overlaying of wells, springs and lineaments layers

Results

The basic knowledge of the geology of the area associated with the information provided by the GIS overlaying of the mentioned layers permitted the identification of many sites suitable for further field investigation, but only eleven were retained, based on practical considerations.

In fact, the NW part of the basin has a good supply of surface water channelled from dams on the Golan Heights and the area in the vicinity of Daraa is either served by the Mouzerib Spring or from wells. Attention was thus focused on areas in real need of groundwater development, mainly in the Assuwayda province. Field visits were thus undertaken to identify by GPS the exact ground location of potential sites, as identified through the RS/GIS analysis, to collect further field data and indicate to the operators the exact place for geophysical investigations.

In four locations (Mohajeh village, Slakhid site, Nasib and Bosra Al-Hanja villages) selected in the office on the basis of our methodology, the team found wells already operating, In Mohajed there are two wells producing 10 l/s each and in Slakhid a group of five wells producing over 50 l/s in total. In Nasib there is a well producing 20 l/s and in Bosra Al-Hanja there are "good" wells in the vicinity. The wells were recently drilled by the municipalities concerned on the basis of geophysical investigations and were not reported in the well listing in our possession, unfortunately not updated.

However, the occurrence of wells producing above average and drilled on sites considered promising according to our study, was a clear indirect confirmation that the methodology developed by the project was correct. Consequently geo-electric surveys orthogonal to the fracture trace were undertaken in the remaining six suitable places of the Assuwayda province, as one potential site was not tested due to difficult terrain (Fig. 3, Tab. 2).

Table 2 : Location of potential drilling sites in the southern area
Serial No. Location Latitude Longitude
North East
1. Walga 32 44 35 7 36 32 11 5
2. Overlaped land 32 43 47 36 29 26
3. Mleha Al-Sharkieh 32 45 21 36 23 75
4. Najran 32 50 29 8 36 27 05
5. Ghasm 32 34 12 36 23 20
6. Irra 32 37 42 36 32 86

Fig. 3: Location of proposed drilling sites

Geo-resistivity tests were carried out by vertical electric sounding according to the Schlumberger method (four electrodes laid out using the Schlumberger configuration) and produced positive results, clearly indicating the occurrence of groundwater in the fracture investigated. The diagram of the test effected at the Najram site is reported in Fig. 4.

Fig. 4: Geo-resistivity test, Najran site

Comments

The successful well drilled in Najran, with its production above average (10 l/s), clearly proves that the methodology is correct. Based on this, the Municipality of Assuwayada has decided to drill in the other sites indicated by the project.

It should be said that the GIS layers prepared during the study indicate many other favourable places for groundwater extraction, which might be exploited according to needs.

However, it is important to note that under the present circumstances only through time-consuming indirect extrapolations it is possible to extimate the water balance of the basin, as the number of private wells, both for domestic and irrigation purposes, and their production is practically unknown, and scant, dated information exists about spring discharges and river flows. It is evident that all those data should be collected without delay to avoid over exploitation of the aquifer, as the practice of drilling wells, often in a wildcat way, is spreading quickly among the inhabitants of the region. Furthermore, it is evident that the exploitation of the aquifer upland decreases water availability in the lower areas. This is already happening in the basin, as indicated by the discharge of the Mouzerib Spring, averaging 1 420 l/s before 1985 and 1 100 l/s ten years later.

The potential pollution of the aquifer is a problem which should be duly considered by the relevant authorities. The aquifer is actually recharged through joints, fractures and faults in the basalt, with consequent little filtration. Furthermore, creeks and wadis intersecting these fractures or laying on them play the largest role in this recharge action. There is, thus, a clear risk of groundwater contamination from sewage near the largest cities and from herbicides, pesticides and fertilizers in the areas of intensive agriculture.

Farther east of the study area a large region of basaltic rocks occurs, dotted by Quaternary cinder cones aligned according to precise trends and associated with vast expanses of recent lava fields. The area is almost uninhabited due to lack of water. Locating sites for groundwater extraction would have a valuable impact on the living conditions of the inhabitants and on development. It is thus recommended to expand the present study to this area. The methodological approach, however, will have to be reconsidered to take into account the fact that some specific lineament trends are of no use for groundwater search as they constituted the gateway for the basalt extrusion.

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