GEOPHYSICAL TEST METHODS
Geophysical test is often used as part of the initial site exploration phase of a project and/or to provide supplementary information collected by widely-spaced observations (i.e., borings, test pits, outcrops etc.). Geophysical testing can be used for establishing stratification of subsurface materials, the profile of the top of bedrock, depth to groundwater, limits of types of soil deposits, rippability of hard soil and rock, and the presence of voids, buried pipes, and depths of existing foundations. Data from geophysical testing should always be correlated with information from direct methods of exploration.
There are a number of different geophysical in-situ tests that can be used for stratigraphic information and in the determination of engineering properties. The two most common methods are:
- Seismic Methods
- Electrical Methods
Also Read: Electrical Resistivity Test of Soil
Advantages of Geophysical Test
Geophysical testing offers some notable advantages and some disadvantages that should be considered before the technique is recommended for a specific application. The advantages are summarized as follows:
- Many geophysical tests are non-invasive and thus offer significant benefits in cases where conventional drilling, testing, and sampling are difficult (e.g., deposits of gravel, talus deposits) or where potentially contaminated soils may occur in the subsurface.
- In general, geophysical testing covers a relatively large area, thus providing the opportunity to characterize large areas with few tests. It is particularly well-suited to projects that have large longitudinal extent compared to lateral extent (such as for new highway construction).
- Geophysical measurement assesses the characteristics of soil and rock at very small strains, typically on the order of 0.001 percent thus providing information on truly elastic properties.
- For the purpose of obtaining information on the subsurface, geophysical methods are relatively inexpensive when considering cost relative to the relatively large areas over which information can be obtained.
Limitations of Geophysical Test
Some of the general disadvantages of geophysical methods include:
- Most methods work best for situations in which there is a large difference in stiffness between adjacent subsurface units.
- It is difficult to develop good stratigraphic profiling if the general stratigraphy consists of hard material over soft material
- Results are generally interpreted qualitatively and therefore useful results can only be obtained by an experienced engineer or geologist familiar with the particular testing method.
- Specialized equipment is required (compared to more conventional subsurface exploration tools).
Also Read: Electrical Resistivity Test of Soil