30 Application of geophysical techniques
Interpretation of geophysical survey data should take account of prior knowledge of the site history and underlying geological structure derived from the desk study. For optimum interpretation of the data from a geophysical survey, adequate direct ground control should be available, such as boreholes or trial pits.
The geophysical data should be input to the ground model of the site to ensure a realistic correlation with the data. The overall model should be synthesized from all available geological information from the desk study, the intrusive investigation and field mapping to ensure that the model can be constrained and evaluated in practical terms. There should be close collaboration between the site geologists, engineers and geophysicists in the interpretation of the geophysical data.
The risks of the geophysical survey not returning the information required should be evaluated and discussed prior to the commissioning of the survey. In some cases, a feasibility study should be undertaken at the site to assess the suitability of the proposed geophysical techniques for the investigation of the geological problem.
The selection of geophysical methods in ground investigation should take account of five fundamental controlling factors:
- a) depth penetration;
- b) vertical and lateral resolution;
- c) signal-to-noise ratio;
- d) contrast in physical properties; and
- e) known limitations of the method.
NOTE 1 All five factors are intimately linked and are particularly relevant in the field of engineering geophysics, where the small-scale nature of the engineering site puts increased demands on the accuracy of the final interpretation. For example, with the current range of operational equipment, some geophysical methods, such as ground probing radar, have limited penetration into the ground, but can achieve excellent resolution of the near-surface geological structures when significant penetration is achieved.
NOTE 2 Excessive environmental noise can adversely affect the results of seismic and electromagnetic surveys, for instance, and if the signal-to-noise ratio is too low the required signal might not be observed within the ambient noise, although the use of signal enhancement techniques can improve the results for data analysis.
NOTE 3 Most geophysical methods used in ground investigation relate to the location of a contrast in the physical properties of the materials under investigation, such as the distinct change in seismic velocity at the boundary between two geological strata, or a difference between the magnetic properties of the material in a disused mineshaft and those of the surrounding ground.
NOTE 4 In the modelling of geological structures, the best results are obtained when the geological conditions are uniform and simple with large clear-cut contrasts in the relevant physical properties of the formations, for example, strata on either side of a fault.
Many geological boundaries are transitional, which can lead to a margin of uncertainty in the interpretation of the geophysical data when related to the engineering or geological boundary; for example, in a survey to determine the depth to competent rock, the results might be complicated by the presence of a weathered layer, or overlying boulders. The geophysical interpretation should indicate, and quantify where possible, the uncertainties that remain in the interpreted ground model.
In the location of geological hazards, such as natural cavities or buried mineshafts, geophysical surveying techniques should be laid out so that the area of anomalous ground conditions can be rapidly identified by comparison with data acquired over a known geological structure in the survey area.
On-site assessment of geophysical data should be encouraged to allow adjustment and optimization of the survey layout to improve the final product; this field assessment should be made by suitably qualified and experienced geophysicists. The increased capability for data review and numerical modelling should be balanced by an awareness that the quality of raw data has to be such as to warrant subsequent manipulations. The geophysical expert overseeing the acquisition phase should perform an on-going assessment of the quality of the data as it relates specifically to the purpose of the survey, and should liaise closely with the geotechnical adviser regarding the progress of the geophysical survey and the information the acquired data is expected to yield.
31 Specification and planning of a geophysical survey
The client or the geotechnical adviser should seek expert advice from an appropriately qualified geophysicist (see Clause 6) when considering the possible acquisition of geophysical data as part of a ground investigation. The geophysical expert may be an independent consultant engaged to act for the geotechnical adviser, or may be a senior technical expert in the specialist geophysical consultancy or contractor. The geotechnical adviser should be responsible for specifying the survey with significant relevant input from the geophysical expert.
NOTE If an inappropriate geophysical technique is specified and a rigid tender submission style called for, the client might not obtain the right information.
The recommendations given in 31.2 to 31.6 should be followed, where possible, when designing a geophysical investigation.
31.2 Geophysical trial survey
If there is any doubt about the feasibility of the geophysical investigation, a test or trial survey should be conducted to determine the appropriate method(s) and field techniques. Certain methods might be useful under one set of subsurface conditions but offer little or no information in others. The results of the trial geophysical survey should be used to refine the specification of the main survey or, in some cases, lead to a decision not to proceed with any further geophysical work.
31.3 Main geophysical survey
The main geophysical survey should include the following.
- a) The geophysical grids, traverse lines and sections should be set out and accurately located in space and elevation to a local or national datum.
- b) Data reduction and analysis should be undertaken in-step with the acquisition programme. Basic quality control should be undertaken during acquisition, and repeat measurements made immediately by the geophysicist as required. The geophysicist should analyse the data collected at the end of each day (or week, if more appropriate), make all check calculations possible, plot the data and evaluate it for consistency with other available data (geophysical, geological, borehole and pits) already to hand. This enables errors and inconsistencies to be identified and reacquired, if necessary, to obtain better data. The survey programme can also be modified to take advantage of any newly obtained information.
- c) The raw field records should be kept for any subsequent re-evaluation or re-interpretation and should be made available to the client as required but these are not normally included in an engineering geophysical report. The records should be annotated and stored so that another geophysicist could receive them and proceed to an independent interpretation if necessary.
- d) The initial interpreted ground model derived from information earlier in the field programme should be reviewed and updated as necessary because the geophysical results are sensitive to features such as geological strike and topography.
NOTE For example, in a seismic refraction survey, the data in one direction might appear to indicate a simple two-layer situation, whereas the data for a traverse crossing perpendicularly could be best represented by a three-layer case, suggesting the possibility of a thin, "hidden-layer" of variable thickness.
The geophysical data set at a site should be treated as a whole and not piecemeal, as the premature or incomplete use of data acquired early in the survey programme can be misleading.
31.4 Integration with other aspects of the investigation
The programme for the investigation should be designed to include boreholes positioned to calibrate the interpreted ground model derived from the geophysical information and to obtain detailed information on specific problem areas indicated by geophysical anomalies. As additional subsurface information is obtained, the initial geophysical interpretation should be refined, and plans for the remainder of the investigation may be revised to optimally fill in the gaps in the required subsurface information. Downhole geophysical borehole logging should be used, where possible and appropriate, to further refine the geophysical interpretation and the resulting ground model.
If the borehole and the geophysical survey programmes are carried out at sites where small-scale geological features might be present, failure of the geophysical surveys to detect any such feature (e.g. a cavity or narrow zone of shearing in the rock mass) should not be taken to mean that such features do not exist.
31.5 Submission of report
The final report should incorporate all the geophysical results, presented in the agreed format and scales, together with any correlation with drilling results, etc. The report should typically contain the following:
- a) the remit and intended objective of the commissioned work;
- b) a statement of the information available at the start of the survey (e.g. from the desk study, field reconnaissance, ground investigations to date and previous geophysical surveys);
- c) details of the equipment and the acquisition parameters used;
- d) descriptions of any difficulties with equipment or environmental conditions or access that are relevant to the accuracy or coverage of the data acquired;
- e) the processing functions applied to the raw data before interpretation;
- f) a description and graphical representation of the interpretations made, illustrated by plots of all of the acquired data, or example sections of data as agreed, such that it is clear to the geotechnical adviser the basis upon which the interpretations have been made;
- g) the accuracy and resolution of the information derived from the survey, referencing borehole or other ground investigation data available as appropriate, referring in particular to the limitations of the information acquired; and
- h) recommendations for any further ground investigation (geophysical, or targeted intrusive).
31.6 Feedback of subsequently acquired data
Any refinement of the ground model should include additional information derived from subsequent ground investigation work to provide constraints on the interpreted ground model previously derived from the geophysical survey. The geophysicist should update their interpretation with appropriate changes of the report and diagrams.