Geologists and Geophysists – What’s the Difference?

The differences between the practices of Geology and Geophysics are quickly diminishing, thanks to the emergence of reservoir modelling in petroleum geoscience and ore reserve modelling in the mining disciplines. In fact there is convergence with Petroleum and Mining Engineering as well. All three disciplines use the same software for different reasons, and all three contribute to the development of an economic model for a particular prospect that is the basis for a bankable feasibility study, the need for teamwork and a common vocabulary has become a top priority.

Basically, the difference amongst the three professions is one of orientation. Geologists are concerned with discovery and delimitation of an economic mineral reserve in three dimensions.  Geophysicists are concerned with establishing the form factor for a reserve, and contribute in determining whether or not there is continuity of the reserve between drill holes. They also contribute to reconnaissance level prospecting by conducting airborne gravity, magnetic and electromagnetic surveys of prospective regions. It is up to the geologists to drill the anomalies to find out if anything interesting is there.

Once delimited, the discovery becomes an economic opportunity. Engineers are concerned with the details of exploitation of the mineral reserve and placement of a dollar value on the well or mine end product. All three must consult continuously through the project to ensure value for the investor dollar.

The nature of their work requires geophysicists to be concerned with more dimensions than either geologists or engineers. Geologists and engineers usually think in terms of the three spatial dimensions and time. Geologists must also be concerned with all scales from nanometres for primitive cells of naturally occurring minerals through to the age, size and characteristics of the solar system and the galaxy and how interaction of forces within and surrounding the earth have affected the deposition of rock units on earth over the past 3 gigayears. Engineers are concerned with more human scale projects and perhaps a 25 to 35 year project life span.

Geophysicists deal with the same spatial scales as the other two professions, but must consider (with full mathematical rigour) a world with the following dimensions:

• The usual 3 spatial dimensions
• Time on scales from the origin of the universe (~8 gigayears before the common era) to nanoseconds (4^th dimension)
• Strength of materials and their mechanical properties (stresses, strains, moduli and their orientations) in both static and dynamic states of stress, which produces 81 possible physical states, all of which must be considered when planning and executing field surveys and interpretation of the resulting data  (5^th dimension)
• Hydrodynamics of constrained and unconstrained fluids in all three states (hides in the preceding 5 dimensions plus Newtonian physics, below)
• Chemistry of fluids (dissolved and suspended solids) and their materials properties (density, viscosity, pressure regime and temperature) and it influence on the mechanical properties of fluids, contained solids and bounding solids (hides in the same 6 dimensions as hydrodynamics)
• Newtonian/non-Newtonian interaction of matter in the earth and in free space (6^th dimension)
• Gravity fields in four dimensions (7^th dimension)
• Electrical fields in four dimensions (8^th dimension)
• Magnetic fields in four dimensions  (9^th dimension)
• Electromagnetic phenomena and their interaction with geological materials in four dimensions plus frequency and time domain (two more dimensions, numbers 10 and 11)
• Solar dynamics and the sun’s influence on the four fields (12^th dimension – includes subatomic particle physics)
• The interactions of all of the above.

Returning to modelling of mineral deposits for a moment, the mathematics behind the models force geologists to invade the turf they share with the geophysicists and in turn geophysicists have become more interested in the environments their instruments are describing. Calibration of survey data with real rocks on the ground and their characterization have become a joint imperative forcing the two main groups of geoscientists toward a common ground. Close collaboration combined with fewer practitioners who are doing ever more sophisticated work has been diminishing the intellectual distance between them. This trend will only increase over the next decade or so, as all the easy mineral targets have been found. 

In short, geophysicists are an unusual and rare (there are fewer than 1,500 in all of Canada) group of people who think in strongly non-linear, inductive ways and are often difficult for the rest of us to fully understand and appreciate. They always bring an integrative and synergistic attitude to team deliberations, since they must think at many more levels than their fellow professionals.

An emerging specialty is Engineering Geophysics. The ability to conduct a non-invasive, spatially continuous site survey allows for a quick and efficient way to identify foundation issues before serious project planning begins. Field surveys developed largely for the mining industry allows shallow investigation (top 10 metres) of engineering soils that will determine the structure of the subsurface; evaluate materials properties in situ; determine the location and behaviour of the phreatic surface and locate any chemical contamination of groundwater resources at the site, including the extent and morphology of the plume. Radioactive methods are also available, if the presence of radionuclide materials is suspected.

Modern geophysical survey methods can usually be used before site clearance begins, leaving no trace if the project owner decides not to proceed or to alter development plans in some way. Buried facilities locating technology is well developed and in most jurisdictions is required before any form of ground disturbance begins. The standard BFL technique is the use of a shallow penetration (3 to 4 metres) electromagnetic field system specifically designed to identify and mark utilities to reduce risk of contact during excavation operations.

It takes a special breed of people to wrap their heads around a 12 dimensional hyperspace. Geophysicists do it every day of their working lives.

 Tom Sneddon, M.Sc., P.Geol. is Manager of Geoscience Affairs at the Association of Professional Engineers, Geophysicists and Geoscientists of Alberta (APEGGA) in Calgary.