Geophysical soil surveys are most often conducted before the commencement of various cubature (colloquially construction) and linear (e.g. road) investments, although they are also used in industry. They enable the detection of potential threats – both to the environment and the planned structure. In the article below, you will learn what they consist of and what research methods are most frequently used.
What is a geophysical soil survey?
Let’s start with the basics. The primary goal of a geophysical survey is to analyze ground conditions (e.g. hydrogeological) and detect any problems or anomalies that may affect a planned structure – a residential building, a road, etc. These include tectonic faults, voids, tunnels, and physical objects.
Geophysical survey is necessary especially in difficult geological conditions. A good example of this is mining areas. If a road, housing estate or house is to be built on such land, the contractor is obliged to check whether the current conditions allow for such an investment and how it will affect the stability of the ground.
The most commonly used geophysical research methods
There are at least a few ways to analyze the geophysical situation. They are selected depending on the specifics of the terrain, as well as the type of investment itself.
GPR surveys – what do they involve?
GPR method, consists in the analysis of the phenomenon of wave reflection from the surface separating the layers of a geological medium with different electrical properties. In the georadar survey, electromagnetic waves from short to ultra-short are used. The survey is performed using single-channel devices (antennas), where the signal reflected from the boundaries is recorded by one receiver (receiving antenna) placed at a constant distance from the transmitter (transmitting antenna).
In this way, it is possible to detect the aforementioned voids and ground subsidence. It also allows for the identification of waterlogging and the flow of pollutants in the soil-rock medium, the control of the stability of geoengineering objects or the identification of the presence of objects that may hinder the investment (pipes, collectors, etc.). Importantly, GPR measurements work well when determining geotechnical and geological boundaries (e.g. fracture zones).
Geophysical measurements of the ground – seismic method
They involve generating artificial seismic waves in a continuous medium, including rocks and soils, which are then analyzed for ground conditions as well as:
- occurrence and level of groundwater,
- the presence of objects, voids and karst forms,
- intensity of subsidence,
- determining geological boundaries,
- assessment of the workability of rocks for construction purposes,
- monitoring of hydrotechnical facilities
- analysis of rock medium properties (cracking level, fault, elasticity parameters).
The accuracy of this type of research is largely dependent on the quality of the equipment used, which is why from the Investor's perspective, cooperation with proven, specialized entities is of great importance.
Electromagnetic method in soil testing
Electromagnetic tests, also called conductometric tests, consist in the induction of a primary electromagnetic field and the analysis of the secondary field., which generates a geological medium. This allows you to check any changes in the ground's resistance, and therefore to detect underground obstacles, leaks, voids, and even the level of contamination.
The electromagnetic method not only facilitates investment planning, but also reduces the likelihood of potential threats occurring during construction. The investor can learn about the properties of the ground and detect potential obstacles.
Electro-resistance research methods are based on the analysis of the parameters of the electric field, which is artificially generated in the soil-rock environment. The flow of electric current is caused by the difference in voltage applied at the ends of the conductor (electrode). Usually, a set of 4 electrodes is used in the study (two of them are current electrodes used to transmit current, and the next two are potential electrodes used to receive current).
This method is used, among others, to determine the lithology of the soil-rock medium, especially when the test result can be related to other geological data. Electrical resistance testing is also useful:
- in the location of glacitectonics zones (e.g. disruptions in the continuity of the substrate caused by the movement of the glacier/ice sheet),
- in determining the lithological composition of embankments and earth structures,
- recognition of zones of inhomogeneity,
- in hydrogeology, e.g. searching for aquifers or separating the levels of salt and fresh water in the coastal zone of oceans and seas,
- in mapping shallow deposits,
- in mapping the continuity of impermeable layers in the locations of planned landfills,
- detection of voids and loosening zones in the soil.
Other geophysical research methods
Among the frequently used methods of geophysical analysis, we can also mention: microgravimetric method.
It examines the Earth's gravity, or more precisely, its changes. In this way, it is possible to find voids in the ground and loosenings. This is therefore crucial for the success of the planned investment.
Geophysical soil research are a key element of design expertise – both before construction and road investments. Their execution is essential for their success, which is why it is worth trusting specialists in this matter, who have the appropriate equipment and knowledge about the most effective measurement methods.
