Understandig Ground Formation

The Earth’s crust has often been compared to a sponge, in that it can soak up and hold water in pore spaces, fractures and cavities. This ability to store water depends very much upon geological conditions and on the host formation. For example, fresh, un-fractured, massive granite – a crystalline rock – has virtually no space available for water, whereas unconsolidated, or loose, river gravel and highly weathered cavernous limestone can store large quantities of groundwater and are capable of releasing it relatively freely. Sandstone and mudstone may be able to hold significant groundwater resources, but because of differences in grain size – and hence porosity – will release it at different rates. One may be a good aquifer, the other a poor one.

The rate at which water flows through a formation depends on the permeability of that formation, which is determined by the size of pores and voids and the degree to which they are interconnected. Permeability and porosity should not be confused, porosity being the ratio between the volume of pores/voids to the bulk volume of rock (usually expressed as a percentage).

The three principal characteristics of aquifers are transmissivity, storage coefficient, and storativity. Transmissivity is a means of expressing permeability, the rate at which water can flow through the aquifer fabric. Storage coefficient and storativity express the volume of water that can be released
from an aquifer.

Hydrogeology is the science of groundwater, and it is the job of a hydrogeologist to assess the groundwater resources in any given area. This is accomplished through the use of maps (topographic, geological), satellite images, aerial photos, field observations (geological mapping, vegetation surveys, etc.), desk studies (literature, field reports, etc.) and ground geophysics.

Ground geophysical surveys are now quite effective in locating water-bearing formations at depths down to around 100 metres. Methods include resistivity (vertical electrical profiling), natural-source selfpotential and electromagnetic methods (such as VLF), magnetic methods, and micro-gravity surveying.