Elements Strike and dip shown alongside cardinal directions on a horizontal plane. Observations about a structure's orientation can lead to inferences about certain parts of an area's history, such as movement, deformation, or tectonic activity. These can be done separately, or together using a tool such as a Brunton transit or a Silva compass.Īny planar feature can be described by strike and dip, including sedimentary bedding, fractures, faults, joints, cuestas, igneous dikes and sills, metamorphic foliation and fabric, etc. A clinometer measures the features dip by recording the inclination perpendicular to the strike. A compass is used to measure the feature's strike by holding the compass horizontally against the feature. Strike and dip are measured using a compass and a clinometer. Linear features are similarly measured with trend and plunge, where "trend" is analogous to dip direction and "plunge" is the dip angle. A feature's orientation can also be represented by dip and dip direction, using the azimuth of the dip rather than the strike value. They are used together to measure and document a structure's characteristics for study or for use on a geologic map. A feature's strike is the azimuth of an imagined horizontal line across the plane, and its dip is the angle of inclination (or depression angle) measured downward from horizontal. In geology, strike and dip is a measurement convention used to describe the plane orientation or attitude of a planar geologic feature. Example of strike and dip on tilted sedimentary beds Tilted layers of chalk, Cyprus ContactĬontact Enquiries for further information.For the angle made with the horizontal by the Earth's magnetic field lines, see Magnetic dip. Comparisons of water strike to rest water level can be used to identify perched water table (strike > level) or confined conditions (strike < level). This is the level at which water is first encountered. When drilling a borehole water strike may be recorded. These allow small localised water tables to develop over the lenses, which can overlay a regional water table in the surrounding rock mass. Perched water tables occur when there are lenses of impermeable material in a rock that is otherwise permeable. Semi-artesian is occasionally used to describe cases where a borehole penetrates a confined aquifer, but the water isn't under enough pressure to overflow at the surface, but artesian may also used in this case. In these areas a borehole that penetrates the confined aquifer will flow naturally. The term artesian is used to describe areas where the piezometric head in a confined aquifer is above the ground surface. Piezometric surface is often used synonymously for potentiometric surface. If it is drilled to measure water in a particular horizon and other horizons are cased off, it is called a piezometer. The level of water in an observation borehole will be a composite of all aquifers that are penetrated by the borehole. A borehole drilled specifically to measure water level is an observation borehole. If there are multiple aquifers present in a vertical sequence there may be several distinct water levels, and where there is a significant vertical flow component present in an aquifer the potentiometric surface may vary with depth. The groundwater level in a confined aquifer is more properly described as a potentiometric surface, and represents the theoretical level to which water would rise if the confining layer were not present. Confined aquifersĬonfined aquifers occur when a layer of impermeable rock or soil overlies an aquifer that is fully saturated, allowing the water within the aquifer to be pressurised. Often rivers will gain during summer and lose water in winter. However levels in an aquifer may be lower than levels in a river, in which case water may be lost from the river to the aquifer, or higher, in which case the river will gain water from the aquifer. In an unconfined aquifer the groundwater may be in hydraulic continuity with surface water, in which case the surface water can be considered to be an outcrop of groundwater. Strictly speaking the water table refers only to unconfined aquifers aquifers in which water is free to drain vertically from the ground surface to the aquifer. A pumped water level (PWL) refers to a water level measured while the borehole is pumped. Formal terminologyĪ more formal term is rest water level (RWL) which implies the water level in a borehole that has not been recently pumped, nor affected by nearby pumping. Above the water table lies the unsaturated zone. This is also referred to as the water table and represents the top of the saturated zone. Groundwater level is a term that is used in a relatively loose way, normally referring to the level, either below ground or above ordnance datum, at which soil or rock is saturated.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |