Geophysics is a valuable tool, but like any other tool it has advantages and limitations.
The main advantages of Geophysics are non-invasiveness, high productivity, 2D/3D coverage and applicability in challenging environments, as discussed in the previous post. However, the limitations of any non-intrusive technology are that the properties and conditions of the imaged targets cannot be directly observed without invasive techniques.
Here are the main limitations of geophysical imaging:
Site conditions and target parameters. Knowledge of expected site conditions and target parameters is required to evaluate the applicability of geophysical tools. The more information and data available, the more confident a decision on geophysical tool applicability and the “probability of success” prediction can be made. Also, there should be a contrast in physical properties between the target and the surrounding material. For example, it is easier to image the soil/bedrock boundary if the rock is competent limestone opposed to highly weathered shale, because of the stronger density contrast.
Resolution. Generally, the imaging resolution decreases with depth. Which means it is more difficult to “see” smaller and deeper targets. For example, rebar in an 8-inch concrete slab can be easily imaged with an appropriate GPR antenna, but a 2-inch diameter pipe at a depth of 10 feet is practically “invisible” to the same equipment.
Interpretation. Data interpretations are non-unique, and their reliability depends on external constraints (e.g., core samples, direct connection to utilities, as-built plans, additional geophysical data sets), understanding site conditions, and the geophysicist’s experience. While interpretation of some geophysical results is straightforward (e.g., a metal detector map from an underground storage tank (UST) survey), others require significant expertise.
Next week we will explore Selecting the Right Geophysical Tool for a Job.