Near surface geophysics and specialist surveying
We tend to take it for granted that the ground beneath our feet is solid. That foundations will support any structure we may wish to build. That existing buildings and infrastructure will remain at “ground level”. The truth is quite different for a variety of reasons that vary from region to region. Traditionally, site investigation has been achieved intrusively through trenching, trial pits or drilling, but how effective is random sampling at detecting isolated defects? The use of geophysics in site investigation is starting to grow, not necessarily as a replacement for intrusive investigation but as a tool for directing intrusive investigation to any locations of concern. This approach offers considerable benefits in terms of confidence that nothing significant has been missed and can also reduce costs by focusing costly intrusive methods only where they are likely to yield results of interest.
Geophysics has been around for a long time, so it might seem odd it appears to have been under-
Surveyors graduating with an MSc from Camborne School of Mines, taught by Neill, are now emerging with skills that include an understanding of site investigation techniques including practical experience in the use of geophysical methods. Being located in west Cornwall, these students benefit from a highly varied local geology that has been heavily mined for centuries. Each year shafts collapse where no shaft was known to exist, consuming roads, gardens and even houses. New constructions encounter shallow mine workings, trenches for cables and pipes encounter dykes of hard rock blocking their path and drainage schemes are complicated by a combination of natural faults in the rock and adits (ancient tunnels dug to drain mines).
So what is geophysics and how does it work? Well one of the strengths of geophysics (and one of its complications?) is that it is a term for a very wide range of methods and techniques that employ a variety of scientific principles. To keep things simple, the purpose of geophysical surveys is to identify something in the ground that differs from its surroundings. To identify something “different” requires contrast between that something and its background. The key word is contrast, so a surveyor will select the method or methods that will best exploit a difference between the physical properties of the “target” of the survey, and those of the background. For instance, an air-
Of course, the implication is that the design of a geophysical survey requires an understanding of the survey objectives. The surveyor should appreciate the nature of whatever they have been asked to look for, including size, shape and orientation if possible, and the depth of investigation required, as well the nature of the ground they will be working on in terms of composition, topography, vegetation and so forth.
A lot of people will have spotted surveyors pushing little carts along roads or pavements to locate utilities with ground penetrating radar, and for many, that is their sole understanding of geophysics. Some people will have expanded their knowledge of geophysics from the television by watching Time Team, which generally revolved around a technique called magnetic gradiometry. Each of these techniques has a role to play, but there are others that are likely to be more widely applicable when it comes to establishing the geotechnical integrity of a volume of ground.
Consider a common feature in rock, a near-
Modern equipment, and surveyors with the skills to acquire, process and interpret the large volumes of data they produce, have much to offer. For instance, the FlashRes 64 and FlashRes 96 Electrical Resistance Tomography (ERT) systems developed by ZZ Geo in Australia have proved highly effective in near-
Sadly, the application of geophysics is not always achieved as efficiently and effectively as it could be. Even a client that recognises the potential benefits of a geophysical survey is unlikely to have sufficiently detailed knowledge to precisely specify the nature of the survey they require. Different geophysical contractors are likely to offer different forms of surveys, often depending upon the equipment and expertise that they have available. When choosing between different methods, price is often not the best determinant of value for money. The best approach for a client to take is likely to be to specify the nature of the survey to be undertaken in terms of the potential “targets” that need to be identified should they be present. This should include realistic definitions of the “target” dimensions and composition, and a maximum depth of investigation.
With a survey properly specified and appropriate methods selected, one might expect that the survey itself should be plain sailing but success hinges on the skills of the surveyor at every step. Collecting high quality data requires good field technique but just as critically, even high quality data is pretty useless if it isn’t adequately georeferenced. This is where surveyors differ from pure geophysicists. Modern GNSS equipment can establish locations to 10 mm or so horizontally, and not much more vertically, but geophysics doesn’t always take place where there is good visibility of satellites and a mobile telephone signal for the all important real-
To many observers, the “magical” aspect of geophysics starts after the data has been collected and this perception is understandable given the relative lack of transparency in what happens next. Almost all geophysical methods produce data that must be processed in some form, and then the processed results must be interpreted. Much of the processing is likely to be performed using software, often quite specialised software, but the parameters of the processing will be controlled by the surveyor. The implication, of course, is that if the processing of the data is controlled by parameters set by the surveyor, the outputs of the data processing will vary depending upon the decisions made by a human being. This is invariably true, so the skill and experience of the person processing the data is critical to the ultimate success of the survey. Distinctions need to be made between data processing and image processing. Geophysical data is generally presented in the form of images and the way in which those images are presented will determine the level of detail that can be gleaned from them. A simple way to think of this is to consider a digital photograph. If there is a bright light source in the image, the remainder of the image will be dark under normal settings. By re-
If we assume that data has been adequately and accurately processed, it now needs to be interpreted and again, there is no substitute for skill and experience. There is also invariably more than once potential explanation for any data set, so expect a professional to express opinions in terms of probability. This brings us back to the issue of geophysical investigation versus intrusive investigation. Digging a hole of some kind generally produces conclusive evidence of what lies in the ground but only at the location of the hole. Geophysics can give us a strong indication but importantly, provides a location and depth for whatever it is that has attracted our interest amongst the data from across a site. When we really want to know where we stand, combining the two approaches will almost always yield benefits in cost per degree of confidence compared with the use of either method in isolation.
Some background Information on Geophysical Site Investigation
for the seriously interested
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