Shan Wei – Quark Scan

05 Apr 2023

An electromagnetic mapping example for underground utility locating

Underground Utility Locatingunderground utility locating, utility locating, utility markout

During an underground utility survey, we were searching for a pipe which was supposed to be made of reinforced concrete. The pipe was about 4 feet in diameter and the depth to its top was about 7 to 8 feet. It's a big pipe, but unfortunately it's too deep to see it using our ground penetrating radar (GPR). After considering all the options, we decided to try electromagnetic (EM) mapping due to target and site characteristics. The survey area was a landfill area. As the picture shown above, despite extreme variations of local geology, the target pipe still showed up in the EM mapping, although partially/intermittently. Its closeness to a shallower metallic water pipe made the data interpretation more difficult.…

14 Dec 2022

By Shan Wei

A Checklist for Underground Utility Locating

Underground Utility Locatingunderground utility locating

If you use private utility markout (underground utility designating/locating) services, this checklist is for you. By considering and discussing the items on the list with your service providers, you can get better results and minimize your risks. Although the checklist is inspired by health and safety checklists which are usually used at the beginning of each field day, it should be used during the planning/scheduling stage before utility locators arrive the site. It could be a costly mistake to think utility locators can locate all underground utilities under any circumstances; your coordination and cooperation are important and sometimes could be critical for underground utility damage prevention. This checklist is by no means complete or perfect, but hopefully it's a good…

05 Sep 2022

By Shan Wei

Phase II environmental site assessment – ordering a GPR survey?

Geophysical Survey, Ground Penetrating Radar, Underground Storage Tank LocatingGround Penetrating Radar, Oil Tank Sweep, Underground Storage Tank Locating

During a geophysical survey for a Phase II environmental site assessment, some underground storage tanks (USTs) were located. The size of the survey area was less than 1/3 of an acre and 7 steel USTs were detected. The ground penetrating radar (GPR) profiles showing these USTs are shown in the following picture. The first example in the picture (the one at the top left of the picture) is a very good example of using GPR locating USTs. GPR profiles similar to this one are often used by GPR manufactures and GPR service providers to show that GPR is an amazing tool for UST locating. Although this kind of nice profiles could be very impressive, they can create a wrong impression…

07 Jun 2022

By Shan Wei

Electrical conduits embedded in concrete have been marked through scanning — can we drill/core in cleared spaces near them?

Concrete Scanning, Construction, Ground Penetrating Radar, Underground Utility LocatingConcrete Scanning, Ground Penetrating Radar, utility locating

The answer to the question seems to be obvious. Since the electrical conduits have been marked through scanning, why can't we drill/core in those cleared areas? We will keep a few inches away from those marked conduits, isn't it good enough? Unfortunately, it might not be. Are you telling us that the conduits are not marked accurately? Well, ground penetrating radar (GPR) can be as accurate as within an inch in finding conduits and other objects embedded in concrete. However, GPR accuracy is not the main issue here. The main issue lies in the limitations of scanning technologies. Due to the nature of scanning methods including GPR and cable/pipe locators, generally there are no guarantees that all conduits can be…

30 Mar 2022

By Shan Wei

How are underground utility surveys affected by snowfalls?

Underground Utility LocatingGround Penetrating Radar, underground utility locating

Whether it's during or after snowfalls, or whether the snow has been plowed or not, usually snowfalls affect underground utility surveys in negative ways. If the snowfall is a safety issue and your site is closed due to the snowfall, then we cannot do underground utility surveys. Let's just leave it as that. Otherwise the survey might be able to be conducted, but often the results are less ideal. To decide whether you should go ahead to schedule an underground utility survey when a snowfall is a factor, there are several snow-related issues to be considered. You should consider them along with other factors such as survey purposes, budgets, project time lines, etc. Spray painting Spray painting doesn't work well…

10 Dec 2021

By Shan Wei

Ghost rebar in ground penetrating radar concrete scanning

Concrete Scanning, Ground Penetrating Radarconcrete structure scanning, ghost rebar, Ground Penetrating Radar

Whether for embedded utility detecting or non-destructive testing (NDT) of concrete slabs, ground penetrating radar (GPR) interpretation involving rebar is often unavoidable. Understanding how GPR waves travel in reinforced concrete, and consequently what corresponding GPR profiles look like, are very important for appropriate GPR data interpretation. This article is about a "ghost rebar" phenomenon often encountered during GPR scanning of concrete slabs. These ghost rebar are not real but can show up and look real in GPR profiles. Even in theory this phenomenon can be easily understood, ghost rebar can cause confusion for data interpretation in real situations, especially when the concrete is not homogeneous or there are complex rebar patterns. Let's take a look at a GPR profile as…

18 Nov 2021

By Shan Wei

Cross-polarized GPR scanning – a useful technique for locating utilities embedded in reinforced concrete

Concrete Scanning, Construction, Ground Penetrating Radar, Underground Utility LocatingConcrete Scanning, GPR, Ground Penetrating Radar, underground utility locating

Above is an image showing two concrete ground penetrating radar (GPR) scanning profiles along the same line with normal and cross-polarized antenna orientations, respectively. Although the electrical conduit is visible in both profiles, it's easier to spot it in the cross-polarized profile than in the normal orientation profile. In the cross-polarized profile, the reinforcement's signature pretty much disappear; the conduit's signature also dials down, but remain very noticeable. So their differences are greatly amplified in the cross-polarized profile compared to the normal orientation profile. This tells us that the two signatures most likely represent two different objects, and it's most likely caused by the difference in their diameters. Of course, there are other possibilities such as differences in orientations or…

04 Nov 2021

By Shan Wei

Be aware: underground utility marks could also be incomplete, not just inaccurate

Ground Penetrating Radar, Underground Utility Locatingunderground utility locating

The accuracy of underground utility marks on the ground surface (spray paint, pin flags or stakes) is often a concern for drillers, excavators and others breaking ground. However, an equally important but less considered issue is the completeness of marks. To understand it, we need to understand how underground utilities are detected. First let's talk about one of the primary tools for underground utility locating: ground penetrating radar (GPR). As we all know, while some underground utilities are apparent in GPR data, others aren't; and GPR data is subject to operators' interpretation. In the GPR profile below, an apparent anomaly indicating underground utilities is shown. The GPR operator could just mark them on the ground surface according to the GPR…

28 Oct 2021

By Shan Wei

An Oil Tank Sweep Example – Why Should We Use Ground Penetrating Radar

Ground Penetrating Radar, Underground Storage Tank LocatingGround Penetrating Radar, Oil Tank Sweep

This underground heating oil tank was detected under a concrete floor using a ground penetrating radar (GPR) unit (See figure above). If only a metal detector was used, at best it would be suspected as a potential candidate for an oil tank. Exploratory digging would be difficult due to the concrete floor. However, Since the oil tank footprint occupied almost the whole concrete floor and there were steel stairs/fences and brick walls nearby, the metal detector responses could be easily disregarded as interferences from aboveground objects and reinforcement in the concrete. As a result, the oil tank could be missed if only a metal detector was used. Fortunately, we always use a GPR with a metal detector for oil tank…

27 Oct 2021

By Shan Wei

Underground Utility Locating: Access! Access! Access!

Construction, Underground Utility Locatingunderground utility locating

During a survey for locating underground utilities, access to the survey area is apparently needed. However, due to the lack of understanding of how underground utility surveys are conducted, survey sites are often found not prepared well for access after surveyors arrive. As a result, survey results are often less ideal and sometimes surveys are not effective at all. Most people understand that a survey area needs to be “scanned”, so the access to the survey area, at least in a walkable condition, should be granted. However, this is only the minimal requirement for the access. The whole access issue is far more beyond that, especially when the ground penetrating radar (GPR) depth is limited. When the GPR performance is…