Geophysics can “see” without the need to physically see or touch the subsurface. In general, it is the application of measurements and physical methods to the study of the physical properties of the Earth.

Knowledge of the subsurface is extremely important for a variety of applications e.g., risk assessment, construction design, environmental impact etc.

Geophysical surveying is a faster and more cost-effective approach than conventional geotechnical methods alone, e.g., drilling, CPT, test pits etc. Of course, geotechnical methods cannot be substituted, if anything, they are crucial for “ground truthing” due to the “non-uniqueness” of geophysical data. However, geophysics has the ability to increase the spatial coverage, bridging information between boreholes, target borehole locations and adds valuable information to increase confidence in designing and risk assessment.

Geophysical surveys can be carried out in locations and terrains where geotechnical methods cannot be deployed. Geophysical equipment can often be deployed beneath bridges and power lines, in heavily forested areas, at contaminated sites, in urban areas, on steeply dipping slopes, in marshy terrain, on pavement or rock, and in other areas that might not be easily accessible for a drilling or CPT rigs. In addition, geophysical surveys are generally considered less dangerous than drilling since there are fewer risks associated with utility encounters and operations. Lastly, geophysical surveys can enable engineers to reduce the number of required boreholes.

The primary difference, from an end result point of view, is that refraction will provide P-waves velocity distribution of the different layers, which will qualify the condition of the subsurface, and reflection will provide a clearer image of the stratigraphy and structures but without direct correlation to strength.

There are many geophysical methodologies and many variations within each methodology, as well as many different types of equipment for each variation.

Such large variations were developed to cater for the many different applications and client needs. Each project is unique. This uniqueness can be determined by the location, terrain, geology and, most importantly, the project needs. All methodologies have strengths and weaknesses. It is important to select the right geophysical approach to avoid waste of time and money.

All clients have different expectations and geophysical knowledge. That is why is important to open an honest communication with an experienced geophysicist to discuss achievable results, maximising value for money and attain the best outcome for the project.

Due to the numerous geophysical methodologies and applications the costing can vary greatly.

Mobilisation and demobilisation of field personnel and equipment:
This, of course, varies depending on the location, the quantity and type of equipment, and number of personnel.

Equipment:
Geophysical equipment can be very expensive. A good SBP system can cost between $100K to $200K, and a SRT system between $50K to $80K. Therefore, the daily rental during fieldwork will reflect the costing of the equipment.

Qualified and experienced field personnel/geophysicist:
The number of personnel involved in the fieldwork will be determined by the methodology used, e.g., small ERT projects in urban areas will require 1 person, however, most fieldwork will require 2 or 3 personnel per crew. A large project could involve a geophysical crew up to 4 people, an explosive crew (normally 3 people), site manager, logistic coordinator, Health & Safety officer, and possible multiple crews.

Processing software:
As for the equipment, processing software can be pricey, e.g., a good processing package software for SBP is about $15K (1 licence!). Each methodology requires a different processing software and for some, more than one.

Experienced geophysicist for processing, analysis, interpretation, and reporting:
As for all the above, this stage is dependent on what methodology(s) is used and the amount of data acquired.

This stage is where the experience and expertise of the geophysicist is paramount. An experienced geophysicist not only will provide the best outcome from the data but will be able to achieve the best outcome with efficiency without waste of time and money.