3. Survey Type B: Detection
Survey Type B needs careful consideration from both client and surveyor. Within Survey Type B, Methodology Types indicate the level of intensity of the survey. The Methodology Type can directly influence the level of accuracy or quality achievable in the deliverable data for each section of the surveyed utility.
PAS 128 calls for the client to indicate to the surveyor the level of accuracy or quality level that they require or would like to be achieved, if technically possible, in different areas of the project. The surveyor will then suggest the appropriate survey type(s) to potentially achieve this using a combination of appropriate geophysical survey methods. To aid this part of the commissioning process, we suggest that prior to a formal tender or quotation request being undertaken, dialogue takes place between the client and a specialist utility surveyor to discuss the different options and potential survey outcomes available.
When requesting a Survey Type B: Detection survey, the client should include the following text in the specification document:
Undertake a Survey Type B: Detection survey to a Methodology Type(s) appropriate to the project needs, as defined in Section 9 of PAS 128:2022, of the required site area (a survey boundary plan is provided in Section X/Appendix X*). The on-site survey shall use EML and GPR geophysical techniques as a minimum, plus other geophysical methods as appropriate.
The survey will also comprise markings to the ground to depict the position and depth of underground apparatus. These markings shall be in permanent spray paint/semi-permanent spray paint/wax chalk/flags/pegs*. Should any of the chosen marking types be impractical or not possible then the surveyor shall inform the client in writing and suggest an alternative marking system.
GPR post-processing and off-site data analysis shall/shall not* be required.
Visual inspection of all accessible manhole covers, inspection chambers and access points shall be undertaken as part of a Survey Type B survey. The data gathered from a visual inspection is categorised as Survey Type A and must adhere to Section 10.2.2 of PAS 128:2022. Data shall be presented in the form of photographs accompanied by chamber asset cards, reports or similar.
*Delete as appropriate
GPR Post-processing
GPR post-processing is a system where GPR transects are recorded on site and then post-processed via specialist software. This process endeavours to clean up the radar data, assist the surveyor to interrogate the data at varying depths and make informed choices via an office-based process. Non post-processed GPR data involves the site surveyor making decisions based on site readings from the GPR equipment.
The client should consider what level of confidence it requires from the data:
- On-site GPR data analysis is quicker, however fine detail may be missed.
- Post-processed GPR data is potentially more accurate and better quality, however it takes longer.
- Density of services, size of site and ground conditions can impact on the effectiveness of on-site readings.
- Consider the competence of the person completing realtime GPR readings on-site or post-processing and what geophysical knowledge they possess to allow accurate interpretation of data.
- Other certain ground conditions may make other geophysical survey methods more appropriate.
Methodology Types
The higher the Methodology Type, the more intense the on-site survey and potentially more accurate final deliverable data. There are six different Methodology Types: M1 being the simplest to M4P being the most complex. M1 and M2 Methodology Types can include GPR post processing, while M3P and M4P always include GPR post processing.
Broadly, the different types relate to:
- M1/M1P. Used where density of services is typical of an undeveloped area.
- M2/M2P. Used where the density of services is typical of a suburban area or where the utility services cross a boundary of a survey area.
- M3P. Used where density of services is typical of a busy urban area or for clearance surveys prior to operations like borehole, drilling, fencing or planting.
- M4P. Used where the density of services is typical of a congested city area.
