A NEW LOOK AT GEOPHYSICS

Article Contaminated Land Laboratories by

“We used geophysics once – It didn’t work!” – This was the somewhat challenging response which the author received from a young lady engineer at a water exhibition in the recent past. And, with the passing years of comment and observation, not an entirely unexpected reaction, for the use of geophysics in environmental and engineering projects has had a chequered, and not always positive, history.

Why is this? The search for, and exploitation of, oil reserves would not be where it is now if seismic methods, in particular, had not been available or developed to the current sophistication. Clearly engineers and environmental scientists are dealing with a shallow geological section – so shallow it can be dug up with a JCB – or at least penetrated by a set of relatively inexpensive shallow boreholes. But do we always want or need to act intrusively? Can we really characterise the inhomogeneous sub-surface with a few, one-dimensional, boreholes? YES, we say. We want to do it that way because that is how we were taught. We are suspicious of something which is remote sensing, something which smacks of Black Magic. We want to get into the soil/rock. Feel it. Touch it. Smell it. Well – can you do it over a hectare and be sure that you know all the local changes hidden there? How many boreholes at one-metre centres would this be?

Clearly geophysics will not always prove to be the cheapest solution. Where the ground conditions are known to be uniformly changing across the site and there is no anthropogenic interference, what better than a few boreholes to prove what was known (or at least suspected) to be the case. But what about the other sites where conditions are heterogeneous and man’s past activity has left a legacy?

Using geophysical methodologies, the variability of sub-surface properties can be measured on a grid, along a series of profiles, down or between boreholes. Two-dimensional profiles can be constructed and, given sufficient data, the three-dimensional sub-surface structure deduced. The science is volumetric and (apart from when used in boreholes) non-invasive. It provides spatial data. As a reconnaissance tool it can be used to identify the most technically effective borehole locations for the geoscientific programme. It rarely supplants the use of borehole measurements which are required to provide “ground truth” and calibration. It can considerably improve the understanding of the variability of subsurface conditions and, used wisely, can greatly improve the cost-effectiveness of a site investigation programme.

Early guidance on the use of geophysics by an experienced professional is essential if the programme of works is to be of material value to the investigation programme. The 1993 report by the Site Investigation Steering Group of the Institute of Civil Engineers, “Site Investigation in Construction”, distinguished between the Geotechnical Contractor and the Geotechnical Adviser. A similar distinction can be made between the Geophysical Contractor, chosen for his practical abilities and specific skills, and the Geophysical Adviser, providing an independent view to the client as a consultant. The Advisor understands not only the science, but also the capabilities of the contractor and the needs of the engineer and environmentalist.

For the requirements of the ground investigation to be achieved it is important that the needs of the main consultant and client are fully understood and the abilities of the current geophysical “state of the art” appreciated. With objectives adequately defined and site constraints understood, the most appropriate method can be chosen for the situation, the survey line layout optimised and the best station spacing identified to achieve a useful data set.

Interpretation of these data to arrive at a meaningful evaluation is dependent on a basic knowledge of the ground conditions at the survey site from which a model can be developed to fit the geophysical data measured. It has to be appreciated that these data relate to the lateral and vertical variation of the physical properties of the underlying soil or rock strata. The integration of “ground truth” and the derivation of a sub-surface model are dependent on close collaboration between the geophysics team and the engineering and environmental staff.

This link is vital and allows the development of a site model drawn from the corporate experience of the team – creating confidence in the method and the results obtained.

The geophysical approach should be to ask a series of questions up-front:

Objectives: What is the nature of the site and are there physical constraints anywhere? What are the perceived problems in this area and what accuracy is required? Can these be addressed by a particular methodology or is a combination needed?

Deliverables: What does the client’s consultant need/expect? How does he want it presented? Is the time scale realistic? What control may be available? Is the data to be integrated?

Methodology: How should the survey be undertaken? Can the client afford this best approach? What can be achieved by a more limited study? What techniques are really most useful? Is geophysics really appropriate?

Only by careful pre-appraisal of a site and the desired objectives can the “didn’t work” tag be avoided.

Whilst most geophysical methods were originally developed by the mineral and oil prospecting professions, most have now been refined to provide higher resolution solutions for shallow engineering and environmental studies. Not to be mistaken for a single “remedy”, geophysics has many facets: –

Seismic operates in Refraction, Reflection and Transmission modes. It has applications in depth to bedrock, reconnaissance of layered structures, foundation studies and rock fracture conditions and detailed geological structure. A wide range of resolution is available.

Gravity measurements can identify shallow voids, fractured or weathered zones in rockhead as well as define fault locations and basic geological structure.

The surface extent of natural radioactivity is related to the underlying geology. Thermography is also an areal method which can provide information on shallow sub-surface disturbed areas, drainage zones and springs. Electrical methods using direct or low frequency currents have a value in determining water bearing layers, measuring thickness of weathering and overburden, and identifying faulted or polluted zones.

Electromagnetic techniques with either a remote transmitter or controlled close transmitter can assist in structural and stratigraphic studies as well as locating manmade buried objects. The methods are also used in water prospection and archaeology.

Magnetic data are often used both in a reconnaissance mode and in detailed surveys for archaeological and pollution studies for detection of ferro-magnetic objects and related features.

Ground Penetrating Radar has gained popularity in civil engineering for identifying buried pipes, obstacles and cavities and non-destructive testing of road surfaces and concrete structures. For further information on these techniques, CIRIA, in conjunction with the Engineering Group of the Geological Society, has produced a useful reference book “Geophysics in Engineering Investigations” (CIRIA C562). The Environment Agency also has incorporated geophysical “Investigation Method Summary Sheets” within its two volume Technical Report “Technical Aspects of Site Investigation” (P5-065/TR).

The author is a specialist in the use of geophysics for near-surface projects and keeps in touch with several European centres that are further refining and developing technologies. Whilst many of these have benefited from the considerable advances in the use of geophysics for petroleum exploration, a great deal of research is now directed to particular applications in the engineering and environmental areas. Can you afford not to gain the benefit of this experience in expanding your knowledge of the application of geophysics to ground investigations?

John C R Arthur Top-Hole Site Studies Ltd