Section 5: Geophysical field investigations

27 General

The acquisition of geophysical data should be considered as part of any ground investigation and as an input to the ground model in combination with intrusive investigations. The interpretations of the geophysical data should be validated against all the available ground data. Geophysics should be used to:

• map sub-surface geology and groundwater;
• locate geological anomalies such as faults and voids or man-made anomalies such as shafts and buried structures;
• measure physical and/or engineering properties of the ground; and
• detect potential hazards, for example unexploded ordnances (UXOs) and utilities.

Early thought should be given to the applicability of geophysical methods to the various aspects of the ground investigation programme, for example, to assist in the siting of boreholes, trial pits, etc.

NOTE The choice of the correct geophysical method or combination of methods is essential to maximize the success of the survey and obtain the desired geological information in a cost-effective manner.

28 The use of geophysical surveys as part of a ground investigation

A geophysical survey should be planned to detect a particular feature, such as a geological structure, or the distribution of ground water. Geophysical surveying can be a useful "reconnaissance" tool to obtain an overview in areas where little is known about the subsurface or for interpolation between intrusive investigation points. The specific techniques to be deployed, and details of how they are to be used should be tailored to the physical properties that need to be measured, and the resolution and depth of investigation required. The survey results should be processed and interpreted using all available geophysical data, direct observations from boreholes and/or sampling, to input to the evolution of the interpretative ground model.

NOTE 1 The interpretation for some geophysical techniques might additionally involve a process of inverse modelling, enabling a consistency check of the assumptions made during the development of the interpretative model.

Each geophysical technique detects contrasts in a particular physical property and, therefore, a particular volume of ground should be investigated in different ways, using a combination of two or more geophysical or intrusive techniques, to build up a complete and accurate picture. Where warranted by the complexity of the site, the use of a combination of geophysical data sets can allow a more detailed and robust interpretation to be made. Subsequent data from borehole records or exploratory excavations should be used to add detail to this information and constrain the geophysical interpretations.

The geophysical survey should be implemented and interpreted by expert personnel and should be entrusted to an organization specializing in this work. The expert geophysical advisor (such as an engineering geophysicist with chartered status) should approve all survey designs and interpretative reports.

The geotechnical adviser should provide the geophysical expert with all information relevant to the requirements and planning of the geophysical survey.

NOTE 2 The principal role of the geophysical expert is to understand the requirements of the geotechnical adviser in terms of the information required from the geophysical investigation and the purpose for which the information is required. This understanding forms the basis of their advice regarding the possible use of geophysical techniques to reliably provide the required information to an accuracy suitable for the intended purpose.

29 Geophysical techniques

COMMENTARY ON CLAUSE 29

There are many different geophysical techniques, each based on different theoretical principles, such as seismic velocity or electrical resistivity, and consequently each produces different sets of information relating to the properties of subsurface materials.

Geophysical techniques can be used on land, at sea and in the air. In each case basic techniques are modified, but the same physical properties are involved irrespective of the environment.

Geophysical surveys can offer considerable savings in both time and money within investigations, and/or can significantly reduce the risk of encountering unforeseen ground conditions during construction; the benefits of a reconnaissance geophysical survey should be assessed at an early stage in a ground investigation to assist the planning of the subsequent intrusive investigation. This early survey can identify areas of the site where anomalous data are obtained, and which should be investigated by intrusive investigation. On sites where contamination is suspected, a geophysical survey may be carried out to form part of a preliminary risk assessment prior to drilling or sampling. Geophysical surveys may also be used to check the interpretation of the geological structure between the boreholes during the drilling programme. Later in the ground investigation, further geophysical surveys may be carried out within and between the boreholes and on the ground surface; these can determine the geological, hydrogeological and geotechnical properties of the ground mass.

The design of a geophysical investigation should take into account and incorporate, as appropriate, the four primary objectives of engineering geophysical surveys which are listed in Table 5. These are as follows:

• a) geological investigation;
• b) resource assessment;
• c) determination of engineering properties of the ground; and
• d) buried artefacts.

The applicability of the various methods for different geotechnical problems should be taken into account; a usefulness rating of the geophysical techniques is given in Table 6.

NOTE 1 Further details of the various geophysical methods to which reference is made in this section can be found in Telford et al., 1990 [35], Dobrin and Savit, 1988 [36], Kearey and Brooks, 1990 [37], Milsom, 1996 [38], Parasnis, 1986 [39] and Reynolds, 1997 [40].

NOTE 2 A detailed consideration of the use of geophysical techniques in civil engineering applications is given in Styles, 2012 [41].