Geophysics foundations:  Model types produced by geophysical work

There are several generic types of physical property distributions commonly considered when interpreting geophysical data. Models are invariably simpler than the real Earth, and the degree of simplification depends upon the geometry used for data acquisition (single points, lines, random locations, etc), as well as on the methods employed for interpretation. The five common generic types of geophysical models are illustrated in the following table.

 Model type Description Typical geoscience tasks Uniform halfspace A "uniform halfspace" means the earth beneath the surface has the same physical property value for as far as the measurement system can detect. A "uniform wholespace" refers to a volume with the same physical property above, below and all around, as might be encountered by an instrument in a borehole. Mapping shallow apparent conductivity for site characterization, contaminated ground, or other shallow investigation projects Buried objects When buried objects are the focus, the earth is usually considered to be uniform all around the object. The object itself may be represented with a more or less complicated set of parameters. Finding or characterizing buried utilities, tanks, UXOs, or other objects 1D (layered) models Properties vary only with depth In 1D models, the physical property is assumed to vary only in one direction (usually depth). The earth is commonly divided into layers (cells), each of which has a constant value of physical property. There are two variants of the 1D model: (i) layer thicknesses can be fixed and we try to find only the values of the property, or (ii) we fix the number of layers and try to find values for the property and layer thicknesses. Surveys that are designed to yield 1D results are often called soundings. Results are often displayed in a way that resembles a drill core (mouseover on the image). Layered Earth problems, such as hydrology, overburden thickness, clay layer detection, etc. 2D models Structures are unchanging perpendicular to survey lines In 2D models, the physical property is assumeed to vary in two directions, usually depth and the direction parallel to a survey line. Surveys that yield 2D results are interpreted as a plane under the line. The assumption is that the structures extend without change either side of the survey line (mouseover on the image). Cross sections through regions of geologic, engineering or scientific interest. Three dimensional (3D) models may be estimated if several lines are surveyed 3D models There are arbitrarily complicated structures In 3D models, the physical property varies in all three directions. Detailed geologic structure characterization such as defining ore bodies or other geologic features

For 2D and 3D models, structures within the earth may be considered as simple geometric shapes, or as continuously varying distributions of a physical property. Simple shapes (spheres, blocks, cylinders, etc.) are easy to describe - they are models with few parameters. For example a cylinder is fully described by a fixed radius, depth to top, length and density. Continuously varying physical property distributions are models with many parameters - the Earth's structure must be described as a function of physical property versus position. In the next section, we expand upon the distinctions between discrete simple geometric shapes and continuously varying models.