Remote Sensing :

In this, Remote Sensing and geophysical techniques play a significant role especially in hard rock areas. Remote sensing data gives the idea of flood to be generated from the respective catchments and indicates the measures to be taken to prevent soil erosion and sediment loads into the rain water harvesting structures. It also indicates the possibility of desilting the existing tanks, If they are located in highly permeable zones and also to demarcate the area of benefits, through satellite pictures. In hard rock areas, it also helps to demarcate lineaments and their extent and also the permeable horizons.

The Remote Sensing unit of the Anna University in Chennai have identified the favourable ground water recharging zones in all the 384 Revenue Blocks of Tamil nadu and developed block maps, utilizing the satellite pictures.

Further Geophysical techniques are also highly suitable and effective means of determining the characteristic formational features for suitable site selection for artificial recharge structures. The same is briefly explained below :

1. Electrical Resistivity Method :

This method is highly effective in differentiating sand predominant zones from clayey sediments. Under suitable condition this method also differentiates between saturated and unsaturated zones. Resistivity methods are equally suitable in hard rock terrain to identify and differentiate fractured and compact rock system. The location with maximum thichness of weathered / fractured rock system is better suited for effective ground water recharge. The example of interpreted resistivity sounding curves, showing the presence of saturated / unsaturated granular zone is shown below :

Half current electrode seperation resistivity sounding (VES) to differentiate unsaturated - saturated zone in loose sediments

The following Vertical Electrical Sounding (VES) curve ascertains depth to compact rock system and this information is essential before construction of sub surface dykes. The location is chosen specifically at a place where the depth to compact rock is minimum so as to reduce the cost of construction of artificial recharge structures.

2. Electro Magnetic Technique :

Electro magenetic methods make use of the response of the ground to the propagation of electro - magnetic fields, which are composed of an alternating electric field and magnetizing force. The EMR technique projects the lateral variation in EM resistivity of the formations. In unconsolidated sediments, a permeable formation can be distinguished from a low permeable formation based on EM resistivity variation with it's high value indicating good permeability.

3. Seismic Refraction Technique :

This method consist of measuring the time of travel of compressional waves generated by an energy source, which is usually hammer blow or a weight drop system, or a small explosive charge fired in a hole. The most important use of this technique is to determine the thickness of both saturated and unsaturated sediments overlying the consolidated bed rock. A downward increase in velocity of different layers make the refraction possible. A schematic presentation of the location of sub surface dyke in hard rock terrain where the compact rock depth is minimum.