It is not uncommon for concrete contractors to request the location of rebar, conduits, post-tension cables, electrical, or plumbing in order to aid in remediation risks. Additionally, savvy contractors have been using GPR technology to determine concrete slab thickness. This provides the concrete contractor with several benefits including: the proper assignment of concrete cutting and coring tools, and the ability to better quote the work required. Trained and certified GPR operators understand the benefits and limitations of the technology and how to determine the best course of action in challenging survey conditions. Yet, even the most knowledgeable operators will wonder if there’s a tip or trick that they can deploy in the field to collect better data. Here we explore the difference between suspended slab and slab-on-grade surveys, and infield processing techniques which operators can use to get the best out of their slab-on-grade data.
The detectability of the slab bottom depends on the underlying material and amount of steel within the slab. It is easier to see when a contrasting material such as water, air or metal is under the slab because they will have a stronger dielectric contrast. In Figure 1, the data is representative of an elevated concrete slab. Note the several hyperbolic reflections on the screen, this is indicative of a double rebar mat. Towards the bottom of the data image, there is a strong dielectric contrast at the concrete-to-air boundary and therefore products a clear indicator of the bottom of the slab.
In slab-on-grade situations, the bottom may be very weak or invisible if the slab rests on sand or another concrete structure (supporting beam, for instance) with similar dielectric properties. This can be challenging due to the low dielectric contrast for the concrete-to-sand boundary and intersecting hyperbolic tails from objects embedded in the slab. The former results in weak or non-existent reflections and the latter tends to mask the reflection from the bottom of the concrete interface. Figure 2 illustrates a reinforced concrete roadway with a strong reflective boundary towards the center of the roadway. This is an example of a void. The concrete-to-grade boundary is less reflective as the air-filled void.
2-D versus 3-D Data Collection: In most cases, GPR manufacturers recommend using 2D scanning for real-time locating and simple imaging services, and to use 3D data collection for complex survey sites. However, slab-on-grade surveys are an exception to this rule. It is recommended to conduct 2D scanning in these situations because the layer interface is a planer target which is more easily viewed from the side.
Depth Setting: Conducting a preliminary scan of the area will help the operator determine the appropriate GPR system settings. A common mishap is that the system is set to a depth that is less than the overall concrete depth. This inherently causes data loss at the deeper levels. Remember to always set the depth range 2-3 inches deeper than the expected slab thickness to ensure that the full slab thickness is captured.
Gain: Display Gain and System Gain can be used to brighten a weak back reflection. However, these settings should be used with caution. Display Gain and System Gain settings often influence the rest of the data. Some systems apply a correction factor to gain based on assumed material dielectric.
Migration: Migration eliminates hyperbolas by collapsing them into dots representing the actual targets. This can be helpful to make target identification more intuitive and makes the data easier to interpret. This is especially true for slab-on-grade because the tails of hyperbolic targets can sometimes intersect and hide the concrete-to-sand boundary reflection. By collapsing the hyperbolas into dots, the bottom of the slab can become more recognizable.
Cross Polarization: When detecting linear metal targets (pipes, rebar, etc.), antenna orientation relative to the target becomes important. Antenna dipoles (transmitter and receiver) are most sensitive to the metal targets that are parallel to them. In other words, if an operator is scanning across the slab with the GPR system in its normal orientation, it is sensitive to targets that are running perpendicular to the direction the operator is moving (parallel to the antenna dipoles).
Some systems can be modified to turn the antenna 90 degrees. This method is known as “cross-polarizing”. If the operator scans over a metal target that is again perpendicular to their direction of travel, the GPR system is not as sensitive to it. This “weakens” the amplitude of the metallic objects and may result in a stronger concrete bottom reflection.
In summary, GPR interpretation and survey efficiency is a skill that requires training, field experience, time, and practice. These tips are intended to help operators troubleshoot a very specific type of survey scenario. An operator may employ all of them, some of them, or only one of them in an attempt to conduct a successful survey. In some extreme cases none of the solutions may work, and only a trained operator will know what tools to use and when.
The world’s leading manufacturer of ground penetrating radar (GPR) equipment, introduces the new 200 MHz (200 HS) antenna, the first of the next-generation high-performance GS Series, designed for applications that require deeper depth penetration. The new 200 HS antenna serves as the foundation for the GS Series, which is ideal for geophysical, geotechnical, or environmental applications that require high reliability under challenging survey conditions.
The newly designed 200 HS antenna is paired with the HS Module and wirelessly connects to a Panasonic Toughpad G1 or SIR 4000 control unit. The wireless HS Module incorporates system electronics, an internal GPS, and connectivity ports in an IP-65 rated housing. The 200 HS uses GSSI’s patented HyperStacking technology, which improves signal to noise performance and increases the antenna depth penetration, nearly double the conventional GPR antenna designs, under all soil conditions. The 200 HS is FCC, RSS-220, and CE certified.
The GS Series features a modular design that allows the user to select which controller best suits their needs; the rugged SIR 4000, combined with our new WiFi Module, or the Panasonic G1. Both controller options provide different advantages to the customer. The SIR 4000 builds on the same user interface and menu options that customers are familiar with to accommodate the 200 HS. The Panasonic G1 features a GIS map mode that will display GPR data collected on the left side of the screen and a location map on the right side of the screen.
Designed alongside the 200 HS are several survey accessories to enhance the ease of use in data collection. The antenna comes with a tow handle with various grip options. Optional accessories include a GPS mount and a four-piece wheel kit that will decrease the wear of the antenna on prepared surfaces such as grass and asphalt.
Source: Palawan News (www.palawan-news.com)
The Department of Environment and Natural Resources (DENR) is set to charge the management of a beach hostel in El Nido after the discovery of its “illegally installed” polymerizing vinyl chloride (PVC) pipeline within the easement zone over the weekend.
A statement sent to Palawan News Tuesday by the DENR MIMAROPA Region said the PVC pipeline was excavated from the beachfront of Outpost Beach Hostel in Barangay Corong-Corong through the use of the ground penetrating radar (GPR) by a survey team of the regional and central offices of the Mines and Geosciences Bureau (MGB).
“The pipeline measuring six inches in diameter and six meters in length was uncovered in front of Outpost Beach Hostel in Corong-Corong. It was also found discharging black and foul-smelling liquid directly into Bacuit Bay, one of the province’s ecotourism sites undergoing massive rehabilitation,” the statement said.
The DENR MIMAROPA said to confirm the source of the wastewater, the Environmental Management Bureau (EMB) used a green tracer solution into Outpost Beach Hostel’s last chamber to which the excavated pipe was connected.
The DENR regional and central offices survey team is seen in this photo using the ground penetrating radar (GPR) to detect the presence of the illegally installed sewerage pipe in front of the Outpost Beach Hostel in Corong-Corong, El Nido. (Photo courtesy of the Mines and Geosciences Bureau)
“After almost 20 minutes, the green solution drained into the said pipe, indicating that the said establishment was the one discharging wastewater from the tank. The EMB shall be conducting further investigation to determine if there are other sources of wastewater discharge aside from the hostel,” it pointed out.
Paul Sepulveda, one of the co-owners of Outpost Beach Hostel, reportedly admitted that they owned the pipe.
Nevertheless, the statement said DENR MIMAROPA regional executive director Henry Adornado ordered the immediate removal of the sewage line as it violates the provisions of Presidential Decree 1067 or the Water Code of the Philippines, which prohibits structures within the easement zones without permission from the government.
Meanwhile, the excavation site was filled with sand using the backhoe sent by the local government of El Nido. The end of the cut pipe was left open for sampling and analysis by the EMB.
“We have to remind everyone that we are preparing Bacuit Bay as Water Quality Management Area so we shall be conducting regular water sampling and analysis not only to Outpost Beach Hostel but also to other establishments to ensure they do not discharge untreated wastewater into Bacuit Bay,” EMB Regional Director Michael Drake Matias said.
Besides regular effluent sampling, the DENR and the MGB have been conducting a GPR survey of the coastal areas of El Nido since March 18 to detect buried waste pipelines. They are calling business establishments to take Outpost Beach Hostel as an example to avoid interruption in their business operations.
“You cannot hide them (pipes) forever. We will eventually uncover them so we advise you to remove your illegal sewage lines and comply with the laws for your own good,” MGB MIMAROPA regional director Roland De Jesus was also quoted in the statement.
The DENR, EMB, and MGB in MIMAROPA vowed to impose the maximum penalty to any establishments found continuously breaking environmental rules and regulations; and employ unified action to ensure environmental protection remains as a top priority.
GSSI, the world’s leading manufacturer of ground penetrating radar (GPR) equipment, announces the release of a major software update for the StructureScan™ Mini XT – the newest generation of GSSI’s popular all-in-one concrete inspection GPR system. The update expands StructureScan™ Mini XT capabilities with an increased depth range, improved Focus Mode, and a new Auto Drill feature.
The update increases StructureScan™ Mini XT’s depth range by 20% to up to 24 inches for greater visibility in survey situations involving thick structural concrete and slab on grade. Additionally, algorithm improvements enhance the StructureScan™ Mini XT’s gain at greater depths.
The improved Focus Mode uses input from the StructureScan™ Mini XT’s 2.7 GHz high-resolution antenna to resolve closely spaced and bundled targets within concrete, offering precise visualization where traditional GPR hyperbolas would condense data into a singular dot. Users can sweep between raw GPR data and the easy-to-read focused view. The new Auto Drill feature searches for potential obstructions to a planned core location. The innovative software tool uses a specialized algorithm to identify possible obstacles to drilling operations by analyzing a user-selected position and size (1/2’ to 6”) on a 3D grid.
What makes GPR such a good tool to investigate archaeological sites?
Before geophysical methods arrived on the scene, archeologists had enjoyed centuries of success using excavation and shovel test grids to narrow down the most likely areas in which to dig. This time-tested site surveying method involves laying out a grid and excavating a unit, typically 50 centimeters by 50 centimeters. Archaeologists sift through the material to determine whether artifacts are present, and if so, from which layers. They then move on to the next unit, which may be 10 to 20 meters away.
However, this method has one major problem – if 20 percent of shovel tests contain artifacts using 10-meter spacing, there is a great likelihood that researchers can jump right over a discrete feature. It takes time and effort to excavate shovel tests on the scale necessary to accurately narrow down artifact locations. This process is labor and time intensive – and it results in a high potential to miss something.
Take the example of early sites from the 1600s, which may not contain a lot of cultural material – just a few bits of pottery, pipe stems, or nails. It is very likely that you may not recover cultural material from a shovel test even if you are right in the middle of the site.
In recent decades it has become clear that GPR and other geophysical technologies could really help with surveying sensitive archeological sites remotely and non-destructively. Targeting what to excavate saves time, money and protects fragile artifacts. GPR can also aid investigations comparing the site’s natural soils with archeological components.
GPR and other geophysical technologies are not generally used as “first phase” methods; rather, they are used when other information is needed to help refine the site, usually after an initial shovel test finds artifacts that point toward something interesting. They may also be used after plowing an agricultural field turns up artifacts, or if researchers have a detailed historical map that suggests a house or farm used to be at a site.
Such methods can pinpoint the best places to excavate and indicate which areas should be avoided. This is especially useful for large multi-acre areas, where GPR can be used to build a high-resolution map of what the site might have looked at when it was occupied. Surveying a few acres in high resolution could help locate all the roads in a farm complex, as well as all the paths, activity areas (blacksmith shop, yards), and even individual buildings.
On a smaller scale – say a researcher finds a house and a well – the GPR can be used to produce a more localized survey across discrete features to get a better idea of their size and depth, and to determine if the walls are intact and if the cellar hole is filled with rubble or clean material.
What are the geophysical methods archaeologists use in the field?
Archaeologists use several geophysical methods, including GPR, electrical resistivity imaging (ERI), magnetometry, and electromagnetic induction (EM or EMI).
GPR works by sending a tiny pulse of energy into a material via an antenna. An integrated computer records the strength and time required for the return of any reflected signals. Subsurface variations create reflections that are picked up by the system and stored on digital media. GPR is considered the most accurate, highest resolution geophysical technology. It works best in dry sandy soils with little salt content; the technique is not useful on the coasts where there is a high salt content, for example salt marsh. Dense clay-based soils are difficult to penetrate with GPR, it cannot see through metal and is also incapable of identifying bone.
ERI is used for mapping the depth of soils and rock. It involves placing stakes in the ground and measuring electrical resistance. Technicians must set up a row of about 24-48 sensors (metal stakes) along the ground typically in a straight line; information is only collected along that one line. This tool works well in clay soil, but takes longer and costs more to get the required data coverage than GPR. One can collect 80 or more profiles of similar length with GPR in the same time it takes to collect 2-4 profiles using ERI.
Magnetometers are passive sensors that measure the strength and sometimes the direction of a magnetic field. By detecting irregularities in the earth’s magnetic field, a magnetometer can indicate the location of items made of ferrous material. Archaeologists use them to measure human activity that increases magnetism. For example, old fire pits have higher magnetic readings, as do bricks, storage pits, and even old trenches. Magnetometers do a good job of finding ferrous objects, but do not provided accurate depth information like GPR.
Electromagnetic induction (EM or EMI) devices measure the change in mutual impedance between a pair of coils on or above the earth’s surface. Most EM instruments are comprised of two or more sets of coils, electrically connected and separated by a fixed distance. EM devices can simultaneously examine soil conditions and locate objects found beneath the surface of the earth spatially, but do not provide good depth information.
It is important to emphasize that these methods are often complementary, because each is better at measuring different things. For example, magnetometers are often paired with ER surveys. But here’s the vital point: Only GPR can provide true depth information that can be calibrated. Unlike other available geophysical methods, a GPR survey can indicate where an anomaly or archaeological feature is in high resolution spatially, enabling archaeologists to say how deep it is below the surface. That’s a huge advantage.
How to use GPR for surveying?
Before even starting to scan, it is absolutely critical to obtain as much information as possible about the site. GPR surveyors should seek out any historical maps and make sure they have access to the results of walkover surveys showing concentrations of archaeological features and artifact density. GPR surveyors should also have an idea of what researchers expect the GPR to show them so they can get a sense of what they should be looking for.
As part of this information gathering, researchers should pay close attention to what the landscape looks like. Is it at the side of a mountain where it may be difficult to access? Is it clear of vegetation or densely vegetated? GPR equipment needs to be pushed in a straight line and the antenna sits on the ground, so if a site is overly vegetated it must be cleared before conducting a GPR survey. Essentially, anything one would not want to go over with a lawn mower would also be difficult for GPR equipment.
Other factors surveyors should know is the time period being investigated, results from the initial archaeological investigation, and the density of archaeological features to be mapped. A pre-contact Native American site may contain mainly debris from making stone tools or food remains, so there may not be much to image. A historical 17th century farm complex might contain at least one building or a cellar hole, or perhaps a large underground feature that can help orient the site, like a barn, well, privy or farm lane.
What type of planning can GPR surveyors do to guarantee an efficient survey?
GPR surveys are a great tool to fill in gaps between shovel tests to ensure a complete picture of a site. Surveying budgets are always tight, so the key is to collect as much data as possible within the time and budget allocated.
Context is everything, so the right surveying parameters will always be based on the type of site and the findings of the initial archaeological investigation. Collection parameters will vary by the type of site and the density of features. Ideally, one should carefully consider line spacing parameters and direction of lines based on the specific site features.
GPR surveys should always be collected on grids. Surveyors should place a larger grid over a feature so they can determine what is happening near the feature or is associated with it. A recommended practice is to bracket the area with space buffers to collect more information. This can be difficult, since projects are often restricted spatially by property boundaries. Development projects affected by the National Preservation Act’s Cultural Resource Management (CRM) requirements are generally restricted to the area of potential effect; surveyors do not usually have permission to survey beyond the project boundaries. For academic projects, one should keep surveying to collect as much data as possible in the allotted time. A GPR survey provides a digital archive of the recording process; even if the site is damaged or destroyed, the digital archive will remain.
The GPR surveyor conducts the survey and tells researchers where there are anomalies. In an ideal world, the GPR surveyor would later get feedback about the anomalies, with information on what was eventually found. This would enable surveyors to go back in and re-examine the data, providing a better sense of what particular data findings mean.
House sites and cemeteries are common geophysical survey locations. What are special considerations about using GPR at these site types?
Early American house sites are very feature rich, with numerous underground targets. Researchers are typically looking for former extensions of a house that have since been demolished, as well as kitchen wings, foundation walls, and even gardens and pathways. As noted earlier, it is important to conduct background research before surveying, including deed research and use of the Historic American Buildings Survey (HABS) and Historic American Engineering Record (HAER) collections.
Geophysical surveys can help with investigation of cemeteries – both formal ones with standing stones and informal cemeteries, with unmarked graves or single burials. GPR can be used to image internment, but, Hollywood portrayals to the contrary, it is incapable of identifying bones. Use of GPR is also hampered by the fact that there may be variable states of preservation across a particular landscape, depending on soil types and topographical features. One part of a cemetery from 1750 could be remarkably preserved, whereas a grave from a different part of the cemetery might be completely decayed.
Another factor is that older coffins were wood or brick, which are difficult to image with GPR. The technique targets hyperbolic reflectors (an upside down U); in the absence of those reflectors GPR surveyors rely on vertical disturbances in the soil profile that come from digging, which shows up fairly well in GPR data.
The accompanying graphic shows two-dimensional GPR data taken from a cemetery site. The data represents six burials, approximately 10 nanoseconds in depth.
Whats the added value in processing software?
Regular GPR profiles are interesting, useful, and powerful for people who can read them – but they are also limited. It is difficult to see the shape of a feature by just seeing a cross section. GPR data does not show up on the screen labeled – surveyors must interpret what the features are. This is where post-processing software can play a role.
Similar to software used to process camera images, post-processing allows researchers to downplay some features and highlight others. Noise is inherent in digital GPR data, and the post-processing software enables users to reduce or remove noise to accentuate what they are looking for. The accompanying figure shows a three-dimensional GPR data image that identifies nine anomalies that could represent burials. The data was processed with RADAN post-processing software.
One excellent software-based technique is called time-slicing, in which all the individual lines of data collected are stitched together using the assigned coordinates into a three-dimensional cube of the survey area. Horizontal slices (also called time slices) can isolate specific depths to show the soil layers and review lateral relationships and actual feature shapes. Time slices help researchers really see the shape of a feature, like a circular well or building foundation, or a long linear pipeline. The slices add an immense amount of interpretative data and are often the best way to illustrate findings to the general public.
To use the tool to its best advantage, GPR surveyors should collect the right information before beginning a GPR survey, carefully plan data collection parameters to get the highest resolution data at the highest percent coverage for each specific site, and use software based post-processing tools, especially time-slicing.
What is EM Locator Technology?
Electromagnetic (EM) Locator technology relies upon the detection of an alternating magnetic field. This magnetic field is created by the flow of current through a conductor, whether a wire, cable, pipe, or other conductive material. Often, additional ‘tracer wires’ are run in parallel to non-conductive pipes such as gas lines expressively for the purpose of facilitation such detection. AC power utilities, as long as current is flowing in the form of power usage, usually radiate a very detectable magnetic field as a side-effect of normal operation. It is also possible to intentionally induce a ‘tracer’ current (by inductive clamp or direct attachment) onto otherwise passive conductors in order to create a detectable field that can be followed.
LineTrac technology is a multi-sensor fashion technology that combines the power and advantage of GPR with the specific target-identifying capabilities of EM location. Linetrac operates in both “Power” (50 or 60 HZ Utility Power) and in arbitrary “Frequency Mode” up to 50KHz, using a customer-supplied transmitter.
We provide two models of EM locator technology. The LineTrac system for our UtilityScan series and the LineTrac XT for our StructureScan Mini XT system.
Ultra-wideband (UWB) signals have been used for decades in geophysical applications. When UWB signals are used in geophysical instruments, the sensor is moved to detect and map underground objects. When used in GSSI’s LifeLocator®, the sensor is stationary to detect motion and breathing characteristics.
UWB signals are generally defined as electromagnetic transmitters whose bandwidth is at least 25% of the nominal center frequency. UWB operates in the time domain; almost all other transmitters operate in the frequency domain.
Geophysical instruments are moved across a surface and transmit very short bursts of electromagnetic energy into the ground. Reflections from buried objects are received at another antenna. This technology can detect targets such as plastic pipes and voids underground, and inside walls and floors. Any change in dielectric property of materials will cause a reflection. Reflections from targets will arrive at different times depending on their distance from the antenna (and will also vary depending on the type of material through which the signal passes).
For LifeLocator®, the transmitted and reflected signals are primarily passing through debris and air. The sensor is stationary and detects moving objects. As with geophysical analysis, the approximate distance of the object causing the reflected signal can be determined by the particular time delay in the signal return. The monitored area may approximate a cone with a beam width of 120 degrees and a range as large as 30 feet. There may be concrete walls or other structures and obstacles in the radiation path.
Total average power is transmitted is ~1% of a cell phone
Working in the time domain, distance to the motion can be measured
The reason for using a UWB signal instead of a single frequency transmitter is improved motion resolution, distance measurement and obstacle penetration. Lower frequencies within the transmitted pulse carry further, especially when looking through walls and floors; however, resolution is coarser with lower frequencies. Since the resolution is a function the wavelength of the transmitted signal, higher frequencies will provide finer resolution. In the simplest terms, the wide spectrum of the transmitted signal accommodates most motion and obstacle types. In essence, a maximum number of frequencies are transmitted (both low and high frequencies) with the notion that some frequency will be reflected and sent back to the receiver.
GPR works by sending a tiny pulse of energy into a material and recording the strength and the time required for the return of any reflected signal. A series of pulses over a single area make up what is called a scan. Reflections are produced whenever the energy pulse enters into a material with different electrical conduction properties or dielectric permittivity from the material it left. The strength, or amplitude, of the reflection is determined by the contrast in the dielectric constants and conductivities of the two materials. This means that a pulse which moves from dry sand (dielectric of 5) to wet sand (dielectric of 30) will produce a very strong reflection, while moving from dry sand (5) to limestone (7) will produce a relatively weak reflection.
While some of the GPR energy pulse is reflected back to the antenna, energy also keeps traveling through the material until it either dissipates (attenuates) or the GPR control unit has closed its time window. The rate of signal attenuation varies widely and is dependent on the properties of the material through which the pulse is passing.
Materials with a high dielectric will slow the radar wave and it will not be able to penetrate as far. Materials with high conductivity will attenuate the signal rapidly. Water saturation dramatically raises the dielectric of a material, so a survey area should be carefully inspected for signs of water penetration.
Metals are considered to be a complete reflector and do not allow any amount of signal to pass through. Materials beneath a metal sheet, fine metal mesh, or pan decking will not be visible.
Radar energy is not emitted from the antenna in a straight line. It is emitted in a cone shape (picture on left). The two-way travel time for energy at the leading edge of the cone is longer than for energy directly beneath the antenna. This is because that leading edge of the cone represents the hypotenuse of a right triangle.
Since it takes longer for that energy to be received, it is recorded farther down in the profile. As the antenna is moved over a target, the distance between the two decreases until the antenna is over the target and increases as the antenna is moved away. It is for this reason that a single target will appear in the data as a hyperbola, or inverted “U.” The target is actually at the peak amplitude of the positive wavelet.