Geophysics. What, Why and When.
In terms of environmental geophysics, geophysics is: The subsurface site characterisation of the geology, geological structure, groundwater, contamination, and human artifacts beneath the Earth's surface, based on the lateral and vertical mapping of physical property variations that are remotely sensed using non-invasive technologies. Many of these technologies are traditionally used for exploration of economic materials such as groundwater, metals, and hydrocarbons, as well as in engineering applications such as built structures and infrastructure.
Geophysics represents a class of subsurface investigations that are non-invasive (i.e. that do not require excavation or direct access to the sub-surface). The exceptions are borehole geophysical methods that expand the use of holes already drilled to access the subsurface on a very localised basis.
What are the Types of Problems Addressed?
Generally, environmental and engineering problems covered by RSK Geophysics fall into the following categories:
- Urban (utility mapping, underground storage tank location)
- Infrastructure (highways, bridges, tunnels, etc)
- Civil Engineering Concrete Studies / Non-Destructive Testing (NDT) of Structures and Historic Buildings
- Groundwater (contaminant mapping) and Landfill Investigation
- Brownfield Site Characterisation
- Geologic Mapping
- Void Detection (solution features, mineshafts, etc.)
- Forensics (illegal burials, etc.)
- Unexploded Ordnance (UXO detection)
What are the Benefits of Geophysics?
- Non-destructive. It is ideal for use in populated areas, such as cities, where many of today's environmental and engineering issues arise. It also means an archeological site can be examined without destroying it in the process. The non-intrusive nature also means no disturbance to the ground which important in contaminated sites.
- Efficiency. It provides a means of evaluating large areas of the subsurface rapidly. Most techniques are undertaken at walking pace, with some items of equipment even towed by vehicles to increase the data collection rate.
- Comprehensiveness. Combinations of methods (i.e. multi-disciplinary methods) provide the means of applying different techniques to solve complex problems. The more physical properties that are evaluated, the less ambiguous the interpretation becomes. RSK regulary use two or more techniques to solve problems. See our example case study sheets.
- Cost-effective. Geophysics does not require excavation or direct access to subsurface (except in the case of borehole methods where access is typically by drilled holes). This means vast volumes of earth can be evaluated at far less cost than excavation or even grid-drilling methods. There are, or course, situations when digging holes is the most appropriate solution. We take the utmost care to provide the best advice, especially where that is when not to use geophysics.
- Proven. The majority of techniques have been in existence for several decaded and are mature, yet still relatively undiscovered and underutilised by decision-makers who face complex environmental and engineering problems.
What Geophysical Methods are Available?
The methods or techniques most commonly employed by us include:
- Ground penetrating radar (GPR)
- Seismic refraction (and / or near surface seismic reflection)
- Spontaneous potential (or "SP")
- Induced polarization (or "IP")
Click on each method to view an introduction and collection of example data.
How are Geophysical Methods Applied in Practice?
The implementation of geophysical methods is a structured process that consists of a number of key steps, including:
- Initial evaluation (i.e. what is the suspected problem/target, what initial information is known about the site, what additional information is required, and what are the desired outcomes)
- Determination of which geophysical method (or combination of methods) will yield the optimal results. Not all methods will be applicable as noted in some of the links above, therefore, it is critical to carefully assess which methods are most likely to provide data and information relevant to the problem of interest. Also, while some methods may provide information, they may not be cost-effective in a particular context.
- Identification of the scope (or size) of the required geophysical coverage.
- Assessment of the way in which the data and information are to be acquired, interpreted and presented so as to address the issue at hand.
After these basic questions have been answered and the project approved, the geophysical work will commence. Typically, the geophysical survey will be conducted along oriented lines (i.e. survey grids) over the desired area of interest.
Decision tree to decide when to use geophysics in a site investigation.
How is Data Processed and Interpreted?
A prerequisite is that the data are reliably and accurately interpreted and presented. Typically, data evolve through a "life-cycle" that consists of the following stages:
- Data acquisition. Acquisition of digital data records according to the specifications and capabilities provided for by instrumentation manufacturers.
- Processing. Synthesis of data and removal of cultural or other types of "noise" that might reduce the overall quality of results and ability to interpret results.
- Visualisation. Development of profile, plan and 3-dimensional maps and presentations for interpretation and presentation of results. These need to be immediately comprehensible by the non-geophysicist.
- Modeling. Application of mathematically or computationally derived models that replicate the sub-surface geologic conditions and provide synthetic data in close agreement with actual data.
- Interpretation. Integration of all data, visualisations and models into a final report based on the experience of the geophysical professional and conformable to geological and geophysical constraints. Typical results can be provided as interpreted cross sections and map drawings in CAD format.
Geophysics has many advantages, but each target type, and technique has is limitations, in particular from certain ground conditions and cultural interference. For example:
- EM and Magnetics suffers from interference from power lines, metallic surface objects such as fences, vehicles and buildings. The further away, the better.
- GPR penetration depth is significantly limited by clay rich ground conditions. Sandier soils antennuate signals less than clays. Made ground and reinforced concrete act to scatter the signal so that features below may be resolved less.
- Electrical resisitivity electrodes (and seismic geophones to a lesser degree) must be in direct contact with the soil therefore if hardstanding is present, holes must be drilled first. Arrays can also be quiet long to aquire sufficient depth and extent.
- Microgravity requires a 'quiet environment' in order to collect stable readings. Soft or steep ground, ground disturbed by nearby vibration sources such as heavy footfall and plant, or high winds all contribute to poor quality data
It is therefore important for site and grounditions to be understood prior to any site works. We are happy to visit the site first to undertake a reconnaissance walkover survey to help inform the survey design.
What are the Costs of Geophysics?
Cost is, of course, a key consideration. A typical geophysical survey will have a cost structure that comprises mobilisation, equipment costs, field engineer time costs, and costs for data processing and interpretation, and report writing and presentation. In general, geophysical surveys are almost always substantially less expensive than traditional non-technical means of investigation such as excavation or drilling.
Our policy of using only qualified geophysicists to undertake geophysical surveys, and ensuring that the same personnel acquire the data on site and process and interpret in the office, sets us apart from the competition.
Our senior staff are Chartered through the Geological Society of London, and all survey designs and interpretative drawings are signed off by Chartered professionals.
Please contact us for more information regarding anything you have read on this page.