Earth Observation in the frame of EO-MINERS - Earth Observation methods
Hydrogeological and Hydrological
While hydrochemical investigations address the quality of ground- and surface waters, hydrogeology and hydrology are concerned with origin and quantity of waters. The majority of investigations are carried out in the field, but may be supported by some laboratory experiments.
Porosity tests - Porosity is a fundamental parameter governing the movement of water in geological materials. Seemingly trivial, its practical determination is hampered by a number of conceptual difficulties associated with certain types of materials. Its determination for silts and coarser materials is relatively straightforward and can be carried out by gravimetry, i.e. for a given sample volume it is difference between the weight of a wet sample and its dry weight (under standardised conditions). However, drying samples in an oven at 105°C over several hours will also release water sorbed to the surface of clay minerals and thus indicate a higher porosity than actually exists. In clays a certain fraction of the total porosity is made up of pores that are so small that due to the electrical double layer on the mineral surfaces water molecules are severely restricted in their movement. Therefore, one has to distinguish between a total and an effective porosity. An added difficulty is that the effected porosity may vary as a function of the hydrostatic pressure, the load on the sediment column and the ionic content of the porewaters. Poresize distributions can be assessed by measuring the resistance a probe exerts against the intrusion of a non-wetting fluid (traditionally mercury) under pressure.
Permeability tests - The permeability of soils and rocks to the movement of water is the crucial parameter in hydrogeology. The permeability of a geological material may be due to its porosity or due to the presence of water-conducting discrete features such as fractures. The principle consists of measuring the rate of flow for a given hydraulic head through a known cross-section. Such test can be carried out in the laboratory on (undisturbed!) soil samples or drill core specimens of rocks. For laboratory test the samples are emplaced in closely fitting (to avoid flow along the outer surface) cylinders. The top and the bottom of the cylinder is closed with permeable discs. The cylinder is connected to a water reservoir for which the hydraulic head is held constant. The quantity of outflowing water is measured per time unit. In field tests, a pipe is driven into the soil and the part above the surface filled with water to a predetermined level. The drop of water level then is recorded as a function of time. Since the porosity is influenced by the overall head applied, the load on the sediment column and the ionic composition of the waters concerned, these factors also influence the permeability.
As permeability test are performed on small samples their representativeness for geological bodies may be an issue. This is particularly the case for hard rocks with only discrete water conducting features. Pumping test provide a more integrative method of assessment.
Constant head (a) and falling head (b) permeability test.
Pumping tests - Are carried out to assess the properties of aquifers, such as hydraulic conductivity (The rate of flow through a unit cross sectional area of an aquifer at a unit hydraulic gradient), specific storage or storativity (a measure of the amount of water a confined aquifer will yield for a certain change in head), and transmissivity (the rate at which water is transmitted through a unit thickness of an aquifer under a unit hydraulic gradient. In principle, to carry out a pumping test water is pumped out of an existing well or a drill-hole lowered for that purpose and the draw-down in the well is observed as a function of the rate of pumping. In addition the developing depression cone of the water table (unconfined aquifers) or head (confined aquifers) can be observed in an array of observation wells or piezometric wells. The shape of the depression cone and the speed with which it develops allows estimating the hydraulic properties and their distribution in the horizontal direction.
Pumping test (© W.E. Falck/UVSQ).
Over the years variants of the simple pumping test have been developed that probe different aspects of the aquifer response and yield. Thus test can run dynamically with varying pumping rates or run until a steady state of the depression cone has been reached. Inverse test are also possible, so-called slug-tests, in which water is injected either as fixed amount and fall of the water level in a standing pipe is observed, or water is injected at a fixed rate.
The quantitative evaluation of pumping tests has spurred an extensive scientific and technical literature. Before the age of the digital computer, mainly analytical solutions were employed, the classical ones being the transient Theiß and the steady-state Thiem solution. Observed drawn-down can also be compared to pre-calculated so-called type curves. All these evaluations assume a homogeneous and radially symmetric (with respect to the pumping well) aquifer. Numerical 3D-aquifer models would allow a much more detailed evaluation, but the necessary data to support them are rarely available.