Relative Dielectric Permittivity (RDP)
Relative dielectric permittivity (RDP), also called the dielectric constant, is a measure of the ability of a material to store a charge from an applied electromagnetic field and then transmit that energy. It is usually determined empirically from measurements in the field but can be directly measured in the laboratory. In general, the greater the RDP of a material, the slower radar energy will move through it. Relative dielectric permittivity is a measurement of how well radar energy will be transmitted to depth. It therefore not only measures velocity of propagating radar energy, but also its strength.
Relative dielectric permittivity is inversely related to radar travel velocity.
RDP is the ratio of a material's electrical permittivity to the electrical
permittivity in a vacuum (that is defined as one). Relative dielectric permittivities
of materials vary somewhat with their composition, but the greatest factor affecting
RDP is moisture content and its distribution. Bulk density, porosity, physical
structure and temperature will all affect the moisture content, but by themselves
are not the controlling factor, as they all affect moisture in some fashion.
Many erroneously view a measurement of RDP as a definitive measurement of how effective a material will be in transmitting energy to depth in the ground. In reality it is mostly a measurement of how effectively an electromagnetic wave can move, and therefore is more a measure of velocity rather than overall ability to transmit energy. For instance, the RDP of fresh water is very high (about 80), but radar energy can easily be transmitted through it without being attenuated (especially when frozen). A bed of peat, which is composed almost wholly of organic material and fresh water, also has a high RDP but will also allow radar transmission to great depths, but at much slower speeds than in saturated sand or other materials. The relative dielectric permittivity of air, which exhibits only negligible electromagnetic polarization, is approximately 1.0003, and is usually rounded to one. The RDP of many naturally occurring materials in the ground, in a totally dry state, varies little, usually between about 3 and 5. But if just a small amount of water is added to the material (which is almost always the case in natural conditions, even in the driest of deserts) the RDP will increase. In order to generate a significant reflection in a profile, the change in RDP between two bounding materials must occur over a short distance. When the RDP changes gradually with depth only small differences in reflectivity will occur every few centimeters in the ground and weak or no reflections at all will be generated.
It is always important to know the RDP (or velocity) of the material at each site being studied, as it will be used to convert radar travel times to depth.
Typical relative dielectric permittivities (RDPs) of common geological materials. Modified from Davis and Annan (1989) and Geophysical Survey Systems, Inc. (1987).
|Dry sandy coastal land||10|
|Average organic-rich surface soil||12|
|Marsh or forested land||12|
|Organic rich agricultural land||15|