SIM, Multiple Technologies Asset
SIM requires multiple technologies to be used in an investigation. This multiple technology strategy provides for redundancy in findings, thus increasing the precision of the investigation. The primary two technologies for SIM are the traditional pipe locator/electromagnetic receiver and ground penetrating radar, (GPR).
As with any investigation, more data points will bear out the best results. Results that are as close as possible to accurate and precise will yield the best outcome. In the case of locating underground utilities in a non-destructive way, accuracy and precision can be understood as the following:
Accuracy can be defined as the closeness that can be achieved to a known value. This is difficult when working with the potential of unknown values. Accuracy can be confirmed by exposing the located target and therefore comparing results to what is true. This is not typically done due to the general acceptance of the technologies used as being relatively accurate for practical excavation purposes.
Precision can be defined as detecting the same target in the same location multiple times. Precision is increased when more than one investigative technology is applied. This high level of precision in a SIM investigation is possible due to the use of GPR and an electromagnetic receiver. However, if one of the technologies suffers inconclusive results based on site interference or an unfavorable soil type, one technology may need to be accepted as the only reliable results.
This discussion of accuracy and precision is different for scanning concrete for critical targets like embedded PVC conduits. The primary technology used is GPR. Accuracy and precision are a function of calibrating the equipment to the known wave speeds in concrete. Thus, a layout of targets on the surface of the slab will be accurate conservatively within ¼” to the right or left of the center mark of the target identified. Depth can be as accurate as +- ¼”. Precision is achieved through several scans taken of the same target in different directions.
Additional to GPR being used to image steel reinforcement in the slab, SIM also calls for a passive sweep of the pipe locator/EM receiver to confirm the area has no energized conduits. GPR is capable of imaging a plastic pipe in concrete but cannot determine if the conduit is energized.
Understanding the two primary technologies of SIM is a requirement of the field technician. Operating principles and the basic technology theory are key to maximizing scan data on site.
Technology 1, The traditional pipe and cable locator is used to detect electromagnetic fields associated with electric current flow on a buried pipe or cable. The equipment is comprised of a receiver and transmitter. The receiver will register magnetic fields and frequencies passively or that have been induced or conducted.
This equipment has several ways of being applied to the SIM investigation:
Technology 2, Ground Penetrating Radar (GPR) uses an electromagnetic pulse directed into a subsurface material by an antenna. This pulse propagates through the material at a particular speed determined by the atomic structure of the material.
When the pulse encounters a different material or subsurface target there is a speed change thus a reflection back to the antenna is created as the pulse continues to move away from its source with less energy. The reflective energy is then received by the antenna, digitized and imaged on the system controller screen.
The image displays the reflection or subsurface target in almost real time. Depth of target, amplitude/brightness of reflection and relative size of target are detailed in the image.
Data collection: Data is collected by moving the antenna and control unit over the area of suspected subsurface targets. As the antenna is passed over the ground or concrete, the image builds on the controller screen. When a target is realized, it is marked by the field technician with paint or flags.
Technology 1 Expectations: The traditional electromagnetic (EM) locator will only yield good results based on the technician’s ability to apply it to the site. The four modes of operation detailed below will locate, identify, and trace various underground utilities. The application of SIM on site is to exploit more than one of the four methods, thus assuring a high level of accuracy and precision.
Direct Connection: A direct connection to the underground utility is normally the first option due to its high level of accuracy. This method sends an induced frequency through a conductive utility or tracer wire, thereby creating an electromagnetic field that can be detected by the receiver. This method can only be used if there is access to the conductive surface of the utility in question.
Induction: Induction can be used when there is no access to a conductive surface of the utility. An induction clamp can be placed around a utility or the transmitter can be used to emit a field at a high enough to induce a frequency on to conductors in the area. The induction clamp works best when the conductor is grounded at each end. Inducing directly with the transmitter can work very well for a utility for which there is no access but the generated field will also induce other conductors in the area so it can be difficult to isolate and individual utility.
Passive Operation: Passive Operation is normally the final option but should always be performed. A passive sweep can be used to confirm markings from other methods but will also indicate unknown utilities that are emitting an electromagnetic field. This method is limited to detecting conductors around which there is already an electromagnetic field being created. It will detect either 60 Hz frequencies created by electrical current, cathodic protection frequencies, or various radio wave frequencies being carried by conductors within the utility. The type of utility can only be determined by tracing the utility to a surface feature or a logical endpoint.
Sonde/Rodder Method: Temporary Insertion of a sonde and/or traceable rodder can be used to trace nonmetallic utilities such as sewer and storm drain lines. A sonde is a battery-powered transmitter that can be pushed through a pipe in order to pinpoint specific locations along the pipe with a high-powered signal. A traceable rodder allows the technician to send a conductor through a nonconductive utility, induce a frequency through the rodder, and thereby trace the nonmetallic utility for the entire length of the rodder. This method requires access to the inside of the pipe and care must be used to ensure that the pipe is a sewer or drain line as opposed to an empty conduit which could potentially lead back to a live electrical circuit.
EM Depth Expectations: Utilities can often be located at depths of up to 20’ or more depending on a variety of factors. Accuracy Expectations: the equipment has the ability to pinpoint a utility with a very high level of accuracy but this accuracy depends on the ability of the technician to use the equipment properly and to recognize when there will be problems with accuracy such as distortion in an electromagnetic field.
Technology 2 Expectations, Ground Penetrating Radar, (GPR):
GPR has many advantages; it can detect both metallic and non-metallic objects in the ground and in concrete. It also can image previously disturbed soils, thus detecting previous excavations if the soil types are different enough to cause a wave speed change.
GPR depth penetration in soils for utility locating can range from approximately 2 feet to 12 feet. Performance is reliant on soil conditions and the frequency of the antenna. Higher frequency antennas will achieve less depth penetration but with higher resolution and lower frequency antennas will achieve more depth penetration but with lower resolution. The same rule applies to concrete. Typical depth penetration in concrete is 18 inches and sometimes more.
Heavy clay soils attenuate the GPR energy rapidly and create a barrier for the radar signal. Penetration depth can be less than a few feet whereas penetration in sandy soils can be much greater. GPR’s ability to image an object depends on contrasting wave speeds between the targets and the surrounding soils so objects such as clay drainage piping or PVC can be much more difficult to detect than a metallic object.
Depth Expectations in soils will vary, 2-12 feet. Depth Expectations in concrete can be as much as 18 inches or more depending on antenna frequency.
Accuracy in soils, + - 6 inches either side of markings and as much as 15% of the total depth.
Accuracy in concrete + -¼ inch to either side of markings and + - ½ inch in measured depth.
This performance can vary from site to site due to soil conditions, correct settings and access to scan area.