The possibilities of occurrence of ground water mainly depends upon two geological factors

1. The porosity of rocks:
   • The porosity of the rock, which is the major geological criteria for occurrence of ground water, is a quantitative measurement of the voids in the rocks. It is generally defined as the percentage of the voids present in a given volume of aggregates. Mathematically it can be expressed as:

$\text{Perosity}=\frac{\text{Total volume of voids in the soil aggregates i.e., the volume of water required to saturate the dry sample ($V_v$)}}{\text{Total volume of the aggregate (V)}}$

$n=\frac{V_v}{V}\times100$

• Porosity, in fact depends upon the shape, packing, and degree of sorting of the component grains in a given material.
• Uniform and well sorted grains give rise to higher porosity; whereas, heterogeneous grains with irregular arrangement decreases the porosity.
• The porosity of rocks and unconsolidated materials may vary considerably. It may be less than a percent or more than 50%. But generally, it does not exceed 40% except in very poorly compacted materials.

1. The permeability and transmissibility
   • As stated above, the ground water can get stored in the underground formations, only if, they are sufficiently porous. In other words, water is get stored in the pores or voids of the
   • The porosity of the rock, thus, defining the maximum amount of the water that can be stored in the rock. The permeability is therefore, defined as the ability of a rock or unconsolidated sediment, to transit or pass water through itself.
   • Transmissibility is another term which represents the same physical meaning, but only differing mathematically, as explained below
   • The capability of the entire soil of full width b and depth d (i.e. Area = b d) is represented by permeability; while that of the soil of unit width and full depth (i.e. b=1 and d=d, i.e. A=d) is known as transmissibility.
   • The permeability is measured in terms of coefficient of permeability, various methods are used to measure permeability.