Groundwater Exploration

One key concern of humanitarian organizations supporting refugee and IDP camps is to provide drinking water in the required quantity and quality. Different data sources (e.g. Geological maps, hydrogeological maps, soil data, satellite data, SAR data, DEM) are integrated to provide hydrogeological reconnaissance maps.

This white paper provides an overview to the possibilities and constraints of remote sensing for groundwater exploration, and gives an overview to current data sources and techniques (link).

What for?

As a rule of thumb, 25 litres per person are required per day for drinking, cooking and personal hygiene. While the immediate needs of the camp population can be met by purification of surface waters and delivery of potable water by trucks, tapping on the local groundwater resources is usually the best option on the longer term. Groundwater is often fit for drinking without further treatment, because it has been filtered by the subsurface strata, and it is in the ideal case available right where it is needed.

Understanding the local hydrogeological situation is a prerequisite to successfully drill for groundwater. Our aim is to provide hydrogeological reconnaissance maps based on remote sensing, when detailed subsurface information from geological maps or drilling logs are missing or not available.

 


Groundwater Geology

 

The groundwater flow takes place in the voids of the subsurface. These can be the pores of a sedimentary body, or the fractures of a consolidated sedimentary or crystalline rock. The subsurface body itself is called an aquifer. Subsurface bodies that inhibit groundwater flow are called aquicludes. The most important feature of an aquifer is its permeability, which quantifies how easily water can pass through it. This, in turn, determines how much water can be abstracted in a given time.
The permeability of unconsolidated sediments depends on their grain size distribution. Coarse-grained sediments like gravels and sands have a high permeability, fine grained sediments like clays have a low permeability, and are basically watertight. The grain size depends on the depositional process that led to the formation of a sedimentary body. Rivers can produce a wide range of sediments, from coarse gravel beds to fine-grained clay deposits in oxbows. Deposits formed in lakes and by wind action are usually more fine-grained, and therefore have lower permeabilities.
In fractured aquifers, also called hard rock aquifers, the rock itself is basically impervious, and the water flow takes place in the network of fractures, called joints. These fractures come in various sizes from mm to km in length, and also their width can vary considerably. As these large structures can be the main conduits for groundwater, it is necessary to recognize their distribution and orientation during the exploration for groundwater.

 

 

 

 

Data and Methods

Data: Landsat and ASTER imagery, freely available VHR satellite data, radar satellite data, digital elevation models (SRTM, ASTER).

The main aim of hydrogeological reconnaissance is to display the location and orientation of aquifers and aquicludes in a given area, and to understand the flow of groundwater in them. The employed data sets and methods depend on the general type of the aquifer. Therefore, the first step is to differentiate if a porous or fractured aquifer is present. This can be done based on the topography, the presence or absence of hard rock outcrops, and regional geological maps.

Porous aquifers
In porous aquifers, the next step is to look for landforms that provide clues about the depositional environment. These can be alluvial fans, meanders, beach ridges, and the like. These landforms are observed in Landsat imagery or freely available VHR imagery, for example in Google Earth. The analysis consists mostly of visual photointerpretation and manual delineation of features. Radar data can be employed to distinguish grain sizes and to map soil moisture as indicator or groundwater near the surface.

      Fractured aquifers
In fractured aquifers, the aim is to determine the lithology and structure of the rocks. The lithology under favourable conditions be assessed based on converging evidence from texture (VHR imagery) and spectral response (Landsat and ASTER imagery), and regional geological setting. Digital elevation models help understanding the geological structure of an area. Inclined strata can be recognized more easily, and their dip direction and angle of dip can be estimated. Lineaments are the surface expressions of large-scale tectonic structures. Developing a methodology to extract these features from imagery or DEM data using semi-automatic methods in a way that can be operationalized is one of our scientific objectives. 

 

 Products

The main products are hydrogeological reconnaissance maps.

They should contain the following elements, depending on the geological situation:
-Topography: shaded relief, contour lines
-Lithology: rock types, classified as aquifer/aquitard/aquiclude; strike and dip of layers
-Folds: fold axes
-Lineaments: brittle or ductile, fracture or dyke
-Regional water flow
-Permanent and seasonal water bodies
-Recharge and discharge zones: springs, drainage systems
-Existing boreholes, water levels, depths
-In porous aquifers: groundwater isohypses (if known from borehole logs)
-Soil moisture (if known from radar data)
-Signs for anomalous vegetation cover, indicating groundwater close to the surface.
-Potential contaminants: Settlements, camps, industrial installations, etc.
-Roads and obstacles
-Potential drillsites