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There
are several of 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 |
A 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, and (ii) we fix the number of layers and try to
find values for the property and layer thicknesses. |
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. |
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 cylindar 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.
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