8 Detection (survey type B)

NOTE 1 The remote detection of underground utilities uses geophysical techniques. As the survey area is scanned, signals are received and analysed for anomalous responses. If the positions of these anomalies form linear strings they can be interpreted as features, such as utilities.

Some geophysical equipment only permits real-time data to be displayed without the facility for recording the data for post-processing and interpretation. This means that the operator has to interpret the signals whilst in the field and has to mark the results onto the ground surface being surveyed, i.e. no post-processing. Some geophysical equipment is capable of recording the data and some is capable of real-time mark up. The ability to post-process data can often help in understanding areas of complex utility networks so improving the confidence in the interpretation of the data. There is also the advantage of acquiring a record of what work was carried out in the field. This PAS specifies two techniques as a minimum to be used in detecting utilities:

  • a) EML, Electro Magnetic Locating: Detection of buried utilities via a hand-held receiver using electromagnetic and radio frequency signals that are present in metallic utilities as a result of current flow or re-transmitted low frequency radio signals (passive EML). Signals can also be induced from a transmitter at ground surface, by direct connection from a signal generator or from a sonde or tracing wire introduced into a pipe or duct (active EML). Most EML systems do not have the capability of recording what was detected on-site so it relies on the detected position and depth being marked on the ground surface as the survey progresses. This has the advantage of providing quick results but does not allow post-processing and retrospective interpretation of the data to be undertaken and has the disadvantage that no digital record is made.
  • b) GPR, Ground Penetrating Radar (sometimes referred to as ground probing radar): The use of radar waves from a surface transmitter that can penetrate through ground materials and are reflected back to the instrument by a change of ground material or other buried objects: In its simplest form, GPR has systems which have only real-time capability. As with EML, no digital record is captured. GPR systems where the data are recorded are usually surveyed in grids and then post-processed and interpreted off-site. This increases the confidence in the data. Accurate survey grids are established so that detected features found during post-processing and interpretation can be retrospectively located on the interpretative drawings. A digital record is generated with these systems.

Geophysical data by itself does not allow identification of the utility detected. The identification of the utility is achieved through a combination of on-site interpretation from both GPR and EML surveys together with on-site reconnaissance and correlation with utility records.

It is recognized that post-processing generally improves the interpretation of GPR data by resolving weak and intermittent signals or analysing multiple targets in order to gain a better understanding of areas of complex or more obscure utility networks. In addition there is the added advantage of acquiring a digital record of what work was carried out in the field. The practitioner should advise whether post-processing is necessary based on their understanding of the survey area and the requirements specified by the client. The client may choose whether or not to specify postprocessing. The use of post-processing is reflected in the method statement and also in the quality level determined and assigned to the results. For further information on on-site interpretation and post-processing see TSA's The essential guide to utility surveys – Detailed guidance notes for specifying a utility survey [NR1].

NOTE 2 The accuracy with which depth assessment canbe made depends on the technique being used and depth of utility (see Note 3). However, other factors, such as ground conditions, proximity of other utilities, material and method of construction, have an influence on the quality of depth data. Some techniques, such as ground conductivity, allow no depth assessment to be made; others might only provide indicative depth estimates.

NOTE 3 The accuracy with which horizontal and vertical position of the utility can be estimated depends on the depth of burial of the utility so that accuracies are expressed, in part, as a percentage of depth. The deeper the target utility the less accurately both the horizontal and vertical position can be assessed. This PAS applies to utilities buried no deeper than three metres.

NOTE 4 No detection technique can detect every type of underground utility in every location.

8.1 General

NOTE Table 2 specifies the minimum requirements for each method. Table 3 provides guidance on other techniques available.

8.1.1 Survey type В shall use geophysical techniques to detect and identify utilities within the survey area.

8.1.2 The quality level achieved shall be documented as QL-B1, QL-B2, QL-B3 or QL-B4 in accordance with Table 1.

8.1.3 If post-processing has been used to improve the confidence of the data, then each quality level shall be suffixed with the letter "P", i.e. QL-B1P, QL-B2P and QL-B3P.

8.1.4 Where post-processing is selected as part of the detection methodology, all data which can be post-processed, shall be post-processed.

NOTE Post-processing may be instructed within defined discrete sections of the survey area.

8.2 Methodology

8.2.1 Detection techniques General The detection techniques shall be deployed in accordance with Table 2. A minimum of GPR and EML techniques shall be used in detecting utilities.

NOTE Three or more techniques should be used, where it provides benefits in the detection capability, coverage, efficiency and/or accuracy. The GPR, EML and any other geophysical equipment shall be operated in accordance with the manufacturer's instruction procedures, calibration and any equipment process or limitations.

NOTE For information on the control, and application of GPR, see ETSI EG 202 730 V1.1.1 (2009-09) [5]. The following shall be recorded as a minimum as evidence of work carried out:

  • a) site name and location;
  • b) time and date of the site interpretation;
  • c) detection techniques used including the model and serial number of equipment;
  • d) weather conditions;
  • e) the names of the operator(s);
  • f) calibration method and calibration data obtained;
  • g) modes of detection for each geophysical survey instrument used;
  • h) photographs of the site (e.g. of on-site mark out, obstructions);
  • i) notes on site limitations (e.g. overgrown);
  • j) utility records available at time of the survey;
  • k) a polygon representing where any search sweep has been undertaken;
  • l) where post-processing is employed:
    • 1) the geo-reference to the start and end of each search transect;
    • 2)the data from all search transects;
  • m) where post-processing is not employed, the coordinate at any point where a utility has been detected and marked on the ground.

NOTE Where detection techniques allow recording of geophysical data, practitioners are encouraged to keep a record as evidence of detections and work undertaken. This allows the geophysical data to be reviewed by either client or practitioner at a later date. Any linear feature identified in the data whilst on-site shall be either followed to a node where the identity of the utility can be established or, where this is not practical, labelled as "unknown utility" in accordance with 11.4.

NOTE 1 This might involve tracing beyond the survey area.

NOTE 2 When detecting utilities over large areas (such as brown field sites), where the density of utilities is expected to be low, a perimeter search can be carried out in accordance with M2, Table 2 and the utilities traced across the site or to their termination. Where post-processing has been employed, the results of the data interpretation shall be presented on drawings.

NOTE The client might also request that on-site mark out is undertaken retrospectively. Where post-processing has not been employed, the data shall be marked out on-site in accordance with 8.2.2. Ground penetrating radar (GPR) GPR shall be deployed in accordance with Table 2. For a high density array (100 mm or closer antenna separation), the following requirements shall be met.

  • a) The collection regime for the main array shall be maintained throughout the survey area ensuring that any gap between swaths is no larger than the transect spacing specified in Table 2.
  • b) Survey speed shall be selected such that it allows scans to be collected where the centres do not exceed the antenna separation.
  • c) Positioning of the array shall be continuously monitored and recorded using either GNSS or total station and at an absolute accuracy of ≤ 100 mm.
  • d) Where a large (typically vehicle-towed) high density array cannot achieve full coverage over the whole survey area due to limited access, a handcart high density array or a single channel system shall be used for these infill areas. Electromagnetic locator (EML) EML shall be deployed in accordance with Table 2. Passive EML shall be deployed over the whole survey area. Where an active EML method can be used, it shall be used.

NOTE 1 Active EML involves the application of line tracers and/or sondes at the location of manholes and inspection chambers.

NOTE 2 Where it is not possible to lift a manhole cover because it is stuck or obstructed, or access is not permitted by the asset owner, this should be reported to the client at the earliest opportunity with a request for the cover to be made accessible or lifted, preferably whilst the practitioner(s) are still on-site. It is good practice not to attempt to lift damaged covers but to report the damage to the client for instructions.

WARNING. This PAS refers to physical entry into confined spaces, which is not to be attempted without suitably trained operatives and safety equipment. Attention is drawn to HSE's publication, Confined space – A brief guide to working safely (INDG258) [1]. Other technologies

NOTE Table 3 provides examples of other geophysical detection technologies.

Table 2 – Detection methods (normative)
Method1) (to be determined in consultation with the client) Survey grid/search resolution2) Quality levels achievable Typical application (informative)
EML3) GPR Other techniques 4)
General Post-processing
M1 Orthogonal search transect at ≤ 10 m intervals and when following a utility trace, search transects at ≤ 5 m intervals Use as applicable No ≤ 5 m survey grid B1, B2, ВЗ, B4 Used where the density of services is typical of an undeveloped area
M1P Yes B1P, B2P, B3P
M2 Orthogonal search transect at ≤ 5 m intervals and when following a utility trace, search transects at ≤ 2 m intervals Either:
a) ≤ 2 m orthogonal; or
b) high density array5)
No ≤ 2 m survey grid B1, B2, ВЗ, В4 Used where the density of services is typical of a suburban area or where the utility services cross a boundary of a survey area
M2P Yes B1P, B2P, B3P
M3 Orthogonal search transect at ≤ 2 m intervals and when following a utility trace, search transects at ≤ 1 m intervals Either:
a) ≤ 1 m orthogonal; or
b) high density array5)
No ≤ 1 m survey grid B1, B2, ВЗ, В4 Used where the density of services is typical of a busy urban area or for clearance surveys prior to operations such as borehole/drilling/ fencing/tree planting
M3P Yes B1P, B2P, B3P
M4 Orthogonal search transect at ≤ 2 m intervals and when following a utility trace, search transects at ≤ 0,5 m intervals Either:
a) ≤ 0,5 m orthogonal; or
b) high density array5)
No ≤ 0,5 m survey grid B1, B2, ВЗ, В4 Used where the density of services is typical of a congested city area
M4P Yes B1P, B2P, B3P
NOTE 1 In general the effort increases from M1 to M4 and the addition of post-processing. For areas with a greater density of utilities or areas considered high risk by the client, a detection method that has a higher level of effort should be selected.
NOTE 2 "P" indicates off-site post-processing has been included.
1) It is a requirement that a minimum of GPR and EML techniques are used (see
2) The tolerance for orthogonal transect centres and survey grids shall be ±0,1 m.
3) It is a requirement that passive EML is deployed over the whole survey area and that where an active EML method can be used, it is used (see
4) The transect centre depends on technique used.
5) A high density array comprises 100 mm or closer antenna separation.
Table 3 – Other technologies (informative)
Technique Notes
Acoustic transmission (sounding) Used to demonstrate connectivity of open drains only.
Drain tracing dye Used to demonstrate connectivity only for foul, surface water and combined drainage.
Earth resistance Used for detecting variations in earth resistance caused by shallow variations in soil, e.g. trench backfill.
Electromagnetic (EM) ground conductivity Used for detecting subsurface features. Often used to obtain information over large areas (≥ 0,5 hectare).
Gyro based pipe location logging Used for tracing the line of pipes where two access points allow the instrument to be deployed and recovered such as inverted siphons.
infrared (thermal) imaging Used for detecting thermal anomalies at the surface associated with underground features.
Magnetometry Used for detecting subsurface features, in particular ferrous based and fired clayware pipelines. Often used to obtain information over large areas (≥ 0,5 hectare). It is of limited use in urban and congested areas.
Metal detectors Used for detecting shallow ferrous objects.
RFID detection Used to relocate utilities that have been previously tagged with an RFID device. Only relevant where a check can be made against a record of its original placement.
Vibration acoustic Used to detect the horizontal position (not depth) of pipework where a vibration signal can be induced along the pipe.
NOTE 1 This table of other technologies is not exhaustive. A number of other geophysical, surveying and inspection technologies are available that might be useful in specific applications.
NOTE 2 Where these and other technologies are used, the same standards of record should be adopted as required for the more commonly deployed geophysical detection techniques such as GPR and EML
NOTE 3 The search resolution of these technologies should be agreed with the client prior to any fieldwork being undertaken.

8.2.2 Marking out detected utilities and survey grids whilst on-site

NOTE The marking out of detected utilities is a key element to the accuracy of a survey. Where wooden pegs are used, they shall be offset and placed to one side of the horizontal position of the utility. This offset shall be made clear by annotating the peg.

NOTE 1 Agreement of the client to the use of pegs should be sought.

NOTE 2 The use of wooden pegs may be used over rough ground and scrub or where a soft surface will not accept the marker. The duty of the utility shall be marked using an agreed letter code and/or colour.

NOTE 1 Where just one colour is used, it should have a stark contrast to the ground.

NOTE 2 Where a range of colours are used to represent different utility types then the colour code should be agreed beforehand with the client. Depth estimations shall be marked on the ground in metres followed by "d" to indicate depth (e.g. "0.9d" – this indicates that the utility has an estimated depth of 0,9 m). Where paint is used, it shall be biodegradable.

NOTE 1 Before using paint, permission should be sought. Consideration should be given to the durability of the bio-degradable paint to be used which might not be consistent with any permits issued.

NOTE 2 The width and length of individual marked lines should be kept to a minimum. Marking out should be sympathetic to the location of the survey with minimal use of a marker with preference to chalk and crayon in areas of high quality hard landscaping.

NOTE 3 Marking out of detected utilities can be with the use of spot marking aerosol paint. These need to be COSHH assessed and any empty/unusable canisters have to be disposed of as commercial waste in an environmental friendly way with the appropriate records kept. Steel pins, spikes or long pegs shall not be used to mark out detected utilities.

NOTE Steel pins, spikes or long pegs are not used as these could damage shallow utilities. Providing a prior assessment of utility depth has been made, the use of survey nails judiciously placed to mark out survey grids is acceptable. The marks representing the detection of the utility shall:

  • a) depict the alignment of the utility at the scale of capture;
  • b) include a mark at each change in direction, each junction and each point of termination.

NOTE Utilities running along curves necessitate a higher frequency of markings than utilities running in a straight line. The time between marking out and the recording of its location shall not exceed 48 hours.

NOTE The time between marking out and the recording of its location should be kept to a minimum. High pressure pipelines shall be marked up in accordance with the utility owner/operator's instructions.

8.2.3 Where the survey detects anomalies other than those caused by utilities, these shall be recorded as specified in 11.5.

NOTE The use of geophysics might detect anomalies, often referred to as radar anomalies, that are the result of buried features other than utilities. This is particularly true of GPR.

The types of features that may be detected include but are not limited to:

  • voids;
  • foundations;
  • thrust blocks;
  • tanks;
  • chambers;
  • basement extents;
  • ducts (not linked to any street furniture);
  • reinforced concrete.

PAS 128:2014 Specification for underground utility detection, verification and location