The physical properties of geological materials are what link geophysical data gathered at the surface to interpretation of subsurface geology. These properties are determined by the microscopic and macroscopic characteristics of the materials and their environments.
Measurements of physical characteristics are taken in controlled laboratory settings, or on site in the field under in-situ conditions. Controlled measurements in laboratory may be complicated by lack of homogeneity in samples, or by changes in physical properties caused by isolating the sample from their environment. On site measurements by their nature incorporate more of the in-situ environmental conditions that would impact a geophysical field survey data, but it is then more difficult to isolate environmental factors that may alter the measurements at one site versus another. As a result, the measured physical properties of a geological material may vary greatly between particular samples, and the environmental conditions of the sample while it is being tested.
Some materials demonstrate heterogeneity, anisotropy, or the physical characteristics inherently dependent upon environmental factors such as saturation. Thus, even in ideal conditions, particular environmental conditions may substantially alter the measured property. Thus, the physical proper- ties of geological materials are more accurately a range of values most commonly associated with a specific material under specific conditions.
In addition to the physical properties themselves demonstrating variability, our ability to measure those properties is also limited. Measurement limitations in the lab or in the field are anything that reduce the ability to quantify a sample, including resolution, sensitivity, precision, bias, repeatability, reproducibility, and uncertainty. Bias in measurements may be quantified by inter-laboratory or round robin testing programs of materials with a known value, but cannot be quantified in materials without accepted reference values. ASTM International1 establishes and maintains a series of standards for consistent practices to take measurements of physical properties in order to increase the repeatability of the tests, and to reduce the impact of variation from human factors in the process.