### Read : Vertical electrical sounding

##### Archives Posted on :- 2020-05-17 21:47:57
• A two-layer situation is encountered often in electrical prospecting. two-layer cases were interpreted with the aid of characteristic curves.
•  These theoretical curves, calculated for a particular four-electrode array, take into account the change in depth penetration when current lines cross the boundary to a layer with different
resistivity.
• The electrical boundary conditions require continuity of the component of current density J normal to the interface and of the component of electric field E tangential
to the interface.
• At a boundary the current lines behave like optical or seismic rays, and are guided by similar laws of reflection and refraction. For example, if u
is the angle between a current line and the normal to the
interface, the electrical “law of refraction” is • Characteristic curves can also be computed for the interpretation of structures with multiple horizontal layers, The apparent resistivity curve for a three-layer structure
generally has one of four typical shapes, determined by the vertical sequence of resistivities in the layers.
• • The type K curve rises to a maximum then decreases, indicating that the intermediate layer has a higher resistivity
than the top and bottom layers.
• The type H curve shows the opposite effect; it falls to a minimum then increases again due to an intermediate layer that is a better conductor
than the top and bottom layers.
• The type A curve may show some changes in gradient but the apparent resistivity generally increases continuously with increasing electrode
separation, indicating that the true resistivities increase with depth from layer to layer.
• The type Q curve exhibits the opposite effect; it decreases continuously along with a progressive decrease of resistivity with depth.
• • Once the observed resistivity profile has been identified as of K, H, A, or Q type, the next step is equivalent to the one-dimensional inversion of the field data.
• The method assumes the equations for the theoretical response of a multi-layered ground. Each layer is characterized by its thickness and
resistivity, each of which must be determined.
• A first estimate of these parameters is made for each layer and the predicted curve of apparent resistivity versus electrode
spacing is computed.
• The discrepancies between the observed and theoretical curves are then determined point by point.