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3.6.2 Non-intrusive geological surveys

Contents The application of geophysical techniques Commonly applied geophysical techniques Selection of geophysical techniques Down-hole geophysics The application of geophysical techniques

Geophysical techniques provide an indirect means of characterising a site prior to any intrusive works. For contaminated land sites, geophysical methods that identify variations in the near surface structure or chemistry of the ground are required.

Many nuclear-licensed sites, e.g., defense and NORM industrial sites have a long history of development, and it is possible that records on the exact locations of disused disposal sites, underground storage tanks and demolished buildings have been mislaid. Operational sites have many sub-surface services (including electrical supplies, water supplies, gas mains, trade waste drains, radioactive waste drains, telephone lines and fibre optic cables), some of which may not be accurately located on site plans.

On nuclear-licensed sites, geophysical methods have two principal uses:

  • Identification of sub-surface services (and munitions only for defense sites), which may be a hazard for intrusive investigations.
  • Characterisation of the geological structure of the site and identification of sub-surface structures (such as buried tanks or foundations) or potential waste disposal pits.

A geophysical survey will not necessarily identify all features associated with the contaminated land or all services in an area. Safe excavation practices must be employed during the intrusive phases of the work (see Section 3.7.2 and Section 4 for information on procedures for undertaking excavations). Commonly applied geophysical techniques

The three methods that are of most use for the investigation of potentially contaminated land on nuclear-licensed sites are:

  • Electrical methods.
  • Magnetic methods.
  • Microgravity.
  • Ground penetrating radar (GPR).

These techniques provide characterisation of the near-surface environment, typically within 3 m of ground surface. Other techniques, such as seismic reflection/refraction and other gravitational surveys, provide information on the deeper structure at the site. These techniques are less likely to be used in contaminated land investigations and are not discussed further here.

Features that can be identified by the geophysical techniques discussed below include:

  • Buried objects (in particular concrete and metallic wastes).
  • Areas of disturbed ground (such as waste disposal pits).
  • Services (in particular metallic pipes or electrical supplies).
  • Buried foundations and sub-surface voids.

Also, but less reliably, variations in geology, plumes of contamination and groundwater saturation may be detected.

Recent innovations linking geophysical data acquisition with GPS data through sophisticated data processing software has significantly improved the visualisation and presentation of information. Transfer of the information to GIS formats with other layered data allows interpretation against mapped and digital layouts, particularly existing and historical building footprints and services.

Electrical methods are divided into two types: electromagnetic surveying and resistivity profiling.
Electromagnetic surveying uses electromagnetic induction to measure the subsurface electrical properties. Electromagnetic surveys generally produce an areal plot of apparent resistivity over the area surveyed and can be configured to look, with limited resolution, at different depths. These surveys can often identify buried objects (such as concrete foundations), disturbed ground and metallic services.
They are significantly affected by surface metallic structures and care is needed to avoid anomalous readings adjacent to features such as fences. Resistivity profiling is carried out by inserting an array of electrodes into the ground surface, passing electrical current through pairs of these electrodes and measuring electrical potential between other pairs. Interpretation of the results gives a depth profile or, using imaging methods, a cross-section of ground resistivity. Resistivity profiling is employed where resistivity data of good vertical and horizontal definition are required or where above-ground metallic objects reduce the effectiveness of electromagnetic methods. Resistivity profiling may detect buried metallic objects and changes in ground conductivity.

Magnetic methods are used to map variations in the earth’s local magnetic field caused by ferrous objects. Magnetic methods are primarily used to detect buried metallic objects such as cables, drums, pipes or waste materials. They can sometimes also be used to locate areas of fill material. Magnetic surveys can be used to estimate both the depth and mass of an object. The resolution of the method decreases with depth. Surface metallic objects may affect the results of magnetic surveys.

Microgravity techniques are based on measuring extremely small variations in the earth’s gravitational field which are caused by the presence of materials of different densities, or voids, in the subsurface. The presence of an anomalously high (or low) density buried object causes a localised high (or low) anomaly in the gravitational field. This technique is useful for establishing buried foundations, basements of tanks.

Ground penetrating radar (GPR) systems transmit pulses of electromagnetic energy at microwave frequencies into the ground and measure the amplitude and travel time of the returned signals. The systems are used to detect buried ferrous and non-ferrous objects including plastic pipes, void spaces, drums and concrete. The penetration depth of the electromagnetic radiation, and hence the maximum detection depth for buried objects, depends on the electrical properties of the soil. Selection of geophysical techniques

The geophysical survey design will depend both on the survey objectives and the site and ground conditions. In most cases, a specialist geophysical consultant should be employed to carry out the geophysical survey and to provide input into its design. As a guideline, a list of typical survey objectives and some appropriate geophysical techniques are listed in Table 3.41 below.

Objective Proposed technique

Locates services (Note: no technique will guarantee to detect all services. Safe digging practices must be used if services may be present). Electromagnetic profiling (both in-phase and out-of-phase components) on a 2 × 1 m grid across all accessible areas of the site to detect metallic services and cables.
Targeted GPR on a 2 × 1 m grid to detect the most significant plastic and ceramic services (such as gas services).
Cable avoidance tool (CAT) and signal generator, to be used at all proposed excavation positions to confirm absence of services.

Detection of buried pits. Electromagnetic profiling on a 2 × 1 m grid across all accessible areas of the site.

Locate underground structures (e.g., building foundations). Electromagnetic profiling on a 2 × 1 m grid across all accessible areas of the site.
Ground penetrating radar (GPR) targeted into the areas of interest.
Microgravity surveys targeted at the areas of interest.
Locate non-ferrous and ferrous metal items that could relate to buried munitions. Electromagnetic profiling on a 2 × 1 m grid across all accessible areas of the site.
Metal detector survey at sampling locations.

Table 3.41 Typical objectives of geophysical surveys and illustrative techniques to provide the required data

Guidance on use of geophysical techniques for groundwater pollution studies is given in Environment Agency [Burton-1994], [Digby], [EA-2006]. Down-hole geophysics

Geophysical logging of boreholes provides a range of measurement of various physical characteristics of the formations penetrated, physicochemical indicators of the groundwater flows and quality. A detailed description of all the techniques available can be obtained from standard geophysical texts and an industry summary is provided in [Burton-1994], [Digby], [EA-2006]. It is recommended that logs are run in all boreholes to maximise the data gathered.

Logging should be undertaken before borehole installations are fitted, and therefore sufficient time in the field characterisation program should be allowed. The data supplied by the logging is essential to good monitoring well design, to allow well screens to be accurately placed in flow horizons. For low flow sampling equipment to work effectively, placement of pumps and well screens should be dictated by accurate geological and geophysical information.

Logging of existing boreholes with closed-circuit television (CCTV) is a useful tool to ascertain borehole construction and condition, where installations are old and records poor. It is also a technique which can be used to verify installations on newly installed boreholes.