Many acqifers are found in porous rock like sandstone.
Aquifers are formations or groups of formations that contain enough water-saturated material to yield significant quantities of water to springs and wells. By using geophysical techniques, such as ground-penetrating radar or seismic refraction, potential aquifer users can determine characteristics of aquifers, including the size, type of aquifer and vulnerability to contamination, allowing them to select aquifers that will yield sufficient supplies of clean, usable water.
Two of the most commonly used geophysical techniques in groundwater studies are ground-penetrating radar and seismic refraction. In ground-penetrating radar, radar pulses are transmitted from antennae into the ground. When they hit rock or sediment boundaries, they are reflected back to the antennae. Differences in the returning signal provide information on the nature of the buried sediments and rocks. In seismic refraction, compression or shear waves are generated at a known distance from a recording array. The velocity of these waves is altered by the type of sediment or rock they travel through. Measuring the time from the waves’ generation until they reach the array provides insight into the rocks and sediment between the wave source and the array.
Sand & Gravel Aquifers
Aquifers can be found in a variety of unconsolidated and semi-consolidated sediments, as well as in sandstone and carbonate rocks. Water is distributed throughout the pores found between the individual grains in these sediments and rocks. These aquifers can range from a few meters to hundreds of meters in thickness. Sandstone aquifers in the western United States can be more than 6,000 meters thick. Aquifers can also range in size from a few square acres to thousands of square miles. The Nubian Sandstone Aquifer System, one of the largest in the world, underlies all or parts of Egypt, Chad, Libya and Sudan. Geophysical techniques can determine the areal extent and depth of aquifers, and the porosity of the rock or sediment.
Non-permeable rock (including metamorphic or igneous rocks) has only microscopic spaces between the grains. However, water can move though faults and fissures in the rocks. Some cities in the northeastern United States rely on these fracture aquifers for their drinking water. Because the fracture patterns can be complex, they can be difficult to locate. Geophysical techniques can determine the location and distribution of these fractures and provide accurate maps of these aquifers.
It is possible for chemicals or salt water to enter an aquifer and contaminate the water, making it unsuitable for use by humans. Geophysical methods can detect those characteristics that make an aquifer vulnerable. Because water can quickly and easily permeate aquifers in unconsolidated sediment, there is little time for contaminants in the water to be filtered out or decay naturally. The closer the unconsolidated aquifer is to the surface, the greater the risk. In coastal areas, paleo-channels, buried rivers and streams that are filled with sand and gravel can allow ocean saltwater to enter aquifers, making them unsuitable for drinking water or crop irrigation.