The location and temporal extension of soil pollution are of paramount importance for land-use planning, agriculture, water resources, and soil de-pollution. According to the type of pollution, electrical resistivity or induced polarisation can be linked to the presence of pollutants.
The Syscal and Syscal Switch range will allow to locate, determine the spatial extension and monitor such pollutions. For large areas, the pollution extension can be determined more easily and rapidly using the Promis 10 slingram type EM profiler to realize resistivity maps.
Waste disposal objects need to remain under supervision during their whole lifetime. The first aspect of this supervision consists of checking the proper containment of wastes to avoid leachate leaks through the geomembrane and through the possible clayey layers. The second aspect deals with the monitoring of the homogeneous re-circulation of leachate in the waste disposal to ensure optimal biodegradation of waste.
The Syscal Switch range will allow to locate, determine the spatial extension and monitor the proper functioning of a waste disposal site.
In the following example taken from Guérin et al. (2004), the leachate of a bioreactor is pumped and re-injected through a borehole. Monitoring of the electrical resistivity allows imaging the recirculation of the leachate (expansion, velocity, diffusion). This type of measurement can be realized with a Syscal Pro Switch in monitoring mode to optimize the number and location of boreholes used for the reinjection of the leachate.
Archaeologists are increasingly looking at remote sensing methods as techniques to explore sites with minimum disruption to the surroundings. This work is delivering new means of mapping prehistoric and historic sites in three dimensions rather than traditional two-dimensional methods.
Magnetics is a primary remote sensing technique that offers both ease-of-use and cost-efficiency.
The main benefits lie in the ability to resolve details non-invasively, the wide range of artifacts and cultural objects that are detectable, and the low-cost of magnetics in comparison to other methods.
Magnetometry and gradiometry resolve many structures, including buildings, cooking sites, furnaces used for smelting, burial grounds, and other types of buried subsurface objects.
Data is typically acquired using a gradiometer – a two-sensor configuration that serves to reduce natural noise from sunspot activity (diurnal effects) as well as focusing the depth of investigation to the near-surface. Depths of penetration vary up to 10m depending on the type of target being investigated (i.e. highly ferrous as opposed to weakly ferrous).
A number of case histories are available from Symetrics and GEM. Note that these case histories have been digitized and appear with less resolution as compared with the originals. However, they still provide numerous examples of the application of magnetics to archaeological investigations.
Instruments and Data Processing Overview
Several different types of instruments are available for measuring a) total magnetic field (i.e. systems from Symetrics and GEM) and b) three components of the magnetic field (i.e. fluxgate systems). Total field systems offer a number of benefits over fluxgate systems, including high rates of acquisition and no requirement to calibrate systems during surveys for greater survey efficiency.
Processing of data is straight-forward, requiring a) downloading of magnetic and gradiometric data from the instrument to a personal computer and b) minor filtering for noise suppression related to geologic or other effects not of interest to the archaeologists. Simple software packages are available for these purposes from Symetrics and GEM.
Advanced users may also be interested in applying routines, such as Analytic Signal processing to convert dipolar total field anomalies to single peak anomalies that can be easier to visualize. Other advanced routines, such as modeling to determine the depth of magnetic sources, can also be applied.
Archaeologists work in some of the most diverse terrains possible. From the world’s largest historic site at Angkor Wat to the Indigenous burial grounds in North America to the ancient Roman fortifications that cover Europe, archaeologists are “breaking new scientific ground” every day. Our magnetometer has aligned its product offerings to meet these demanding requirements with instruments that are non-intrusive to the sites under study.
It's very high sensitivity optically pumped Potassium system is capable of resolving the most subtle contrasts in materials (such as those of clay bricks in soil).
The unique Overhauser system has a wide range of “detectability” for low contrast and high contrast (ferrous) structures while matching specifications of optically pumped Cesium instruments at a much lower cost.
And where economy is required, Symetrics also offers the world’s most feature-rich Proton Precession instrument – a tool with a classic value that complements any archaeologist’s toolkit.
The global shift to renewable energy is well underway, including the large-scale deployment of offshore wind farms. There are more than 3,500 turbines operating along European coasts, and a boom in the development of offshore wind energy appears imminent in areas such as Japan and the United States.
Many of the environmental effects of offshore wind are still unclear, due largely to the speed with which the industry has grown and the complexity of ocean ecosystems. An effective monitoring strategy can provide information and answers to questions such as:
· How do offshore wind farms affect ocean ecosystems?
· What are the long-term consequences of wind turbines on marine life?
· Do wind turbine platforms act as fish aggregation devices?
Our Portable Scientific Echosounders
Scientific echosounders (sonar) are an effective and versatile instrument for monitoring renewable energy projects. Our specialized sonar systems for the assessment of fisheries and aquatic habitat resources can be used to accurately measure the size, abundance, and location of marine organisms in the water column. These rugged, portable sonar systems are readily deployed from small research vessels or larger ships and used during mobile surveys under the harshest ocean conditions.
Hydroacoustic Baseline Studies
Baseline studies are a common and practical use for scientific echosounders, important in documenting the biological community prior to anthropogenic development such as renewable energy projects. Physical sampling with nets will provide species composition information which will be correlated with the hydroacoustic data. These results will be compared with data collected post-installation to evaluate the impact of the wind farm on the local fish populations.
Fixed Location Monitoring at Wind Turbine Platforms
On existing wind turbine platforms, sonar systems can be mounted in a fixed location for long-term studies to assess behavioral effects on marine life over time. Sonar data can be used to demonstrate fish aggregation effects and to observe diel and seasonal behavioral patterns in marine life. Transducers can be aimed vertically, horizontally, or in combination to maximize sampling where marine life is most likely present. The DT-X Extreme can be configured with multiple transducers and operate autonomously with data logging capacity for up to 30 days with no PC required, thus providing an efficient and effective tool for long-term acoustic studies.
Mobile Surveys for Habitat Assessment
Our echosounders can also be used around offshore wind farms to map and quantify important seagrass habitat areas and to delineate various substrate types such as coral, silt sediment, or rocky reefs. Data can be used to mitigate the potential loss of marine habitat and to identify and preserve vital, irreplaceable habitat areas. BioSonics offers specialized software for processing sonar data and creating full-color bathymetric maps that highlight important habitat features.
BioSonics Echosounders - Versatile, Accurate, and Rugged
Offshore wind farms are likely here to stay and the need for affordable, effective monitoring tools can only increase as more projects are given the “green light”. BioSonics scientific echosounders can be used in a range of applications for monitoring at proposed and existing wind farm installations. Every system is calibrated for scientifically defensible results and built to last in extreme conditions. Whether monitoring fish or marine life, quantifying seagrass, bathymetric mapping, or assessing aquatic habitat.
This image shows a concrete parking garage deck that has experienced damage. The impact echo test method can identify where the concrete is sound and where it must be repaired or replaced. We sell Impact Echo systems with Freedom Data PC (FDPC) and NDE 360 data platforms, as well as an economical Concrete Thickness Gauge (CTG) that is used with a Windows tablet. Contact us to learn how you can avoid situations like this.
Today we will discuss collecting and processing electromagnetic data with the portable UG12. The UG12 is one of the most reliable electromagnetic induction systems in the market that can work on both passive and active modes. On top of that, its price range makes it a favorite to many archaeologists, geophysicists, and geologists.
Horizontal measuring method:
The main application area for measurements is the horizontal method. Both antennas can be individually controlled. You can decide between automatic and manual measuring point recording. In general, the automatic measuring point recording for survey measurements is chosen when we deal with good terrains. The selected step speed and step distance must remain the same for the entire survey area.
Vertical measuring method:
This method is used for smaller areas and when we require high resolution for small target structures. The measurement is done based on a grid keeping as straight as possible profiles. The profiles are either always unidirectional or meandering. With the meander-shaped measuring fixture, the running direction is changed by 180 degrees for every other profile.
Displaying Data in real-time:
Spectrum (frequency analysis):
All receivable locally Frequencies are shown in a dynamic display in percent as well as in a waterfall display as a different bright bar. The tool can be used locally to determine the frequencies that can be received there and to overwrite up to seven previously-stored frequencies with "change and save frequency".
Without storing the measured data, anomalies and suspected areas can be determined by "irregularly moving the measuring surface" with "plot".
The actual measurement recording is done with the subprogram "survey". During the measurement recording, the individual measuring points are displayed color-coded in real-time.
You can compare all 7 frequencies, a comparative advantage to other equipment in the market since you can save time in the processing.
The software allows interpretation of the data in 2D and 3D view, modeling, documenting, and printing. After processing the data these can be exported to programs that can process ASCII files, such as the Surfer from Golden Software.
The example below shows anomalies with high conductivity. The color assignment, the graphic view as well as the language, can be freely selected by the user. With a double click in the graph, you can get detailed information for each measured profile in the upper window single profile.
The main window shows the same anomalies. In the 2D view detailed information, such as X / Y coordinates, relative conductivity, and the approximate depth of the anomaly can be determined.
Residential and commercial inspectors, plumbing and utility contractors, and electricians are finding the use of ground penetrating radar (GPR) provides a host of safety, efficiency and revenue benefits. Rather than relying solely on the national 811 call-before-you-dig phone number to avoid unintentional digging into an underground utility line, general contractors, subcontractors, and inspectors are adding GPR units to their locating toolbox and integrating them with other locating tools to enhance accuracy on the job site.
GPR Technology Addresses Wider Needs
In the past 20 years, GPR utility locating equipment has been more readily adopted by surveyors and engineers. Already accustomed to deploying electronic equipment during the construction process, surveyors and engineers successfully used GPR to augment the 811 process.
In more recent years, engineers and project managers for subsurface utility engineering (SUE) contracts began routinely specifying that contractors do more to prevent unknown problems from buried underground utilities. GPR service providers established a niche serving electricians, plumbers, and contractors tasked with establishing utility locations.
Now, with many powerful, high quality and lower cost utility locating GPR equipment, inspectors, electricians, plumbing and utility contractors are asking why they are paying to use GPR equipment when they can purchase a unit and do it themselves.
Utility Locating Tools – 811 system, EM and GPR
Before contractors begin their projects, they utilize the 811 system to get the approximate location of all public utilities by marking them with spray paint or flags. Making that call technically satisfies a contractor’s legal responsibility.
Nonetheless, many believe that the 811 system is simply not adequate – the safety implications of hitting a gas line and the expense of idling their workforce has driven them to be more proactive in identifying underground utility lines. Most utility locators use an electromagnetic (EM) line locator to check for active utilities. Electric lines are harder to trace with GPR than EM, making EM much quicker and easier to use than GPR. While EM is faster, its positioning is not as accurate as that of GPR, which can provide horizonal and vertical position within a couple of inches. These two methods complement each other, since GPR works better for non-metallic objects and EM for metallic objects. If both tools indicate the presence of a pipe, it provides a higher level of confidence.
GPR Technology for Utility Location and Depth That Won’t Break Your Bank
The trend in the GPR utility world has gravitated towards the use of small, portable, and inexpensive units. Leading this standard is GSSI’s UtilityScan® system, released in 2017. UtilityScan was originally designed for municipalities, electrical contractors, and utility installers, but has since been adopted for use in environmental and archaeology applications due to its size and cost.
UtilityScan is the smallest utility locating GPR system on the market, making it very simple to deploy. Weighing in at only 34 pounds, the UtilityScan is built for quick assembly, scanning, and break down. When folded down, the system can fit in the back of a small vehicle. The compact size makes it extremely portable and easy to maneuver around obstacles on busy streets and construction sites.
One key feature of UtilityScan is the robust wireless antenna tested for rugged job sites. UtilityScan incorporates GSSI’s patented HyperStacking technology, which has proven to increase depth penetration in challenging soils while also providing high near surface data resolution. UtilityScan is rugged, built to withstand any job site around the world. This system is IP 65 rated, making it the right tool to handle rain, snow and muddy conditions.
Another feature of UtilityScan is that it can be equipped with LineTrac®,which helps locate specific power sources situated underground, including AC power and induced RF energy present in conduits. LineTrac has coils that detect power radiated from electrical cables, combined with GPR radar into a single box. This feature lets users produce an overlay on the radar data that represents the presence of AC power and/or induced RF energy present in conduits. UtilityScan then integrates the EM and GPR readings and produces the image on the screen.
UtilityScan uses a wireless tablet-based system with a bigger screen, better viewing experience, and a simplified Android app-based user interface (UI). Perhaps the biggest shift into mainstream adoption has been the ease of use. Using a modern, user-friendly interface means operators need less technical experience to collect and interpret data, leading the way to faster onboarding than previously available.
GPR can provide contractors with more confidence than simply relying on the 811 system. GPR for utility locating is more accessible than ever with small, portable and inexpensive systems on the market.
Extreme cold weather conditions can significantly affect the quality of concrete, as well as its mechanical properties. In cold weather concreting, one should make sure that all the negative impacts of low ambient temperature are appropriately alleviated by taking the necessary precautions. In this article, we will review important steps that can ensure you will get the quality you are looking for. But first, let’s see what cold temperature is for concrete, and why it is critical.
Exposure to cold weather can have serious consequences on the Strength Gain, as well as the shot and long-term durability performance of concrete materials. In order to meet and exceed the minimum design requirements (sufficient strength and durability), it is important to protect concrete during the mix process, transportation, placement, and curing to avoid low strength and sub-standard durability properties. Codes and guidelines provide general recommendations for concreting in the cold weather. In this article, we will review the guidelines in Canada and the United States.
Standards for concrete based on weather
1- CSA A 23-1
In Canada, where temperatures tend to be quite low during the cold season. the following criteria is set by the CSA A23.1:
a- When the air temperature is ≤ 5 °C, and
b- When there is a probability that the temperature may fall below 5°C within 24 hours of placing the concrete.
2- ACI 306
American Concrete Institute definition of cold weather concreting, ACI 306, is:
a- A period when for more than three successive days the average daily air temperature drops below 40 ˚F (~ 4.5 °C) , and
b- Temperature stays below 50 ˚F (10 °C) for more than one-half of any 24 hour period.
Why Cold Weather Concreting is Challenging?
The hydration of cement is a chemical reaction. Extremely low temperatures as well as freezing can significantly slow down the reactions, thus, affecting the strength growth. In fact, freezing temperatures within the first 24 hours (or when concrete is still in plastic state), can reduce the strength by more than 50%. The minimum strength before exposing concrete to extreme cold is 500 psi (3.5 MPa). CSA A 23.1 specified a compressive strength of 7.0 MPa to be considered safe for exposure to freezing.
concrete rebars in snow
How to Protect Concrete In Cold Weather?
When concrete is properly produced, placed, and protected during cold weather, it will develop sufficient strength and durability to satisfy the intended service requirements (ACI Website). The following steps will help concrete suppliers and contactors in meeting project specifications:
1- Removing Ice and Snow
It is important to remove any ice or snow from the surface of formworks and rebar. This is specially important in the construction of slabs (with large exposed area).
2- Heating Water and/or Aggregates
It is important to order concrete with temperature between 10 °C – 25 °C. Concrete suppliers can achieve this either by heating water or aggregate; however, heating cement is not considered as effective.
3- Slab formwork temperature
Placing warm concrete over cold surface of formwork can result in integrity problems in concrete and low strength. It is recommended to warm up formworks prior to placing concrete.
Slab thickness < 1.0 m : 10 °C
Slab thickness > 1.0 m : 5 °C
4- Protect Concrete
CSA A23.1 specified that protection shall be provided by means of:
Heated enclosures / Coverings / Insulation
Note: The heat generated from hydration process should suffice in most cases, if appropriate insulating blankets of polyethylene sheets are used. Additional source of heat might be required based on area and temperature. Read More
5- Avoid Wet Curing
When temperature is expected to fall to freezing point, it is important to avoid wet curing.
6 – Control Temperature Gradient
The temperature gradient of concrete surface and ambient environment should not exceed those specified in standards, such as CSA A23.1
Common Problems during Cold Weather Concreting
Cold temperature (less than 5 C) can significantly affect the strength gain of concrete. It can also affect certain aspects of durability performance of concrete. The following section provides a brief review of some of these challenges and how engineers can verify concrete strength and quality:
1- Low Concrete Strength – Low Break
Strength is by far the most important parameter for concrete materials and structures. Structural engineers, and contractors want to make sure concrete has reached the minimum specified strength before moving with the construction process.
Temperature Monitoring and using Maturity method is a convenient solution to track concrete strength development. While maturity method has certain benefits, it often fails to precisely show strength in real construction sites. Certain challenges are:
Where you locate the temperature gauges is critical in evaluating temperature and strength. When sensors are places too shallow or too deep, the test results might not be representative of concrete strength gain.
You need to have certain benchmark curves for each and every mix that is used in the projects. Concrete that is used for footings, is different from the one that is used for columns and slabs. Therefore, you need different benchmarking that is project specific.
Strength measurements using maturity concept are good for determining the time to open the formwork, but you can not use the value for structural purpose.
Maturity is only effective for early age strength prediction. As strength gain curve flattens, the precision of the method will be limited, making it less practical for on-site evaluation of strength.
Combined NDT methods such as Rebound Hammer and Ultrasonic Pulse Velocity can be used to precisely evaluate the in-place concrete strength. The method can be used as a quality assurance process when all concrete samples are already used, and the strength value remains questionable.
2- Poor Quality – High Permeability
When development of concrete microstructure is halted as a result of cold weather, the durability properties might be affected. For example, concrete permeability might negatively get impacted by cold temperatures.
Engineers can use non-destructive testing methods such as Surface Electrical Resistivity to evaluate permeability of concrete.
3- Cold Joints
Managing Cold Joints are more critical during the cold weather conditions. Certain delays in construction process, or use of accelerators can impact the setting time of concrete and result in major integrity issues on or around the cold joints.
Different Nondestructive Testing methods can be used to assess concrete quality, and structural integrity around the cold joint area. Ultrasonic Pulse Velocity can be used to assess quality. Impact-Echo and a customized setup for Ultrasonic Pulse Velocity can be used to estimate crack depth.
Real-time evaluation of GPR profiles is quick and efficient, and a common method for projects with time constraints or where there are abundant obstacles in the survey area. The basic technique is to collect GPR profiles perpendicular to assumed burial orientation, and to identify potential burial-related targets and associated soil disturbances. Not all burials (especially older ones) will exhibit a hyperbolic target due to decay of wood coffins, but there should be evidence of the grave shaft as a result of excavation and filling of the grave. There are many environmental variables to deal with, therefore real-time cemetery surveying requires an in-depth knowledge of your GPR hardware, familiarity with local modern and historical burial practices, and a strong understanding of GPR theory and how cemetery targets appear on GPR profiles. In this post we will discuss the advantages and disadvantages of 2D data collection and offer advice on best practices. Real-time cemetery surveys can be challenging, especially for novice GPR operators, so we will start with a few key points on relevant GPR theory.
GPR Theory for CemeteriesThe performance of your GPR system depends on multiple environmental factors and local soil conditions. These include water content, soil conductivity, soil texture, and other factors. There are also hardware considerations, and chief among these is the GPR antenna frequency. Of equal importance is the burial container (or lack thereof) and whether it is constructed from wood, concrete, brick, or a modern synthetic material.
Water content is the single most important factor for GPR. Some water is required for GPR to penetrate the subsurface, but too much water can severely limit the depth of investigation. Water slows down the GPR velocity, increases dielectric, and can mix with soil chemistry to increase conductivity levels. In high-dielectric conditions hyperbolic targets are narrow and more difficult to see, reducing the chance of observing a burial target. Certain soil textures, like clay and silt, hold more water and may have inherently higher conductivity. High soil conductivity is like a lightning rod for GPR energy, whereby the GPR signal is dissipated into the ground and does not return to the antenna when reflected by a target or layer. This vastly reduces depth penetration, and in extreme cases may limit data collection to one or two feet below the surface.
Another soil-related factor is the presence of gravel, larger rocks, or boulders. Gravel can create “clutter” in the GPR profile, while cobbles and boulders create confusing hyperbolic targets that can look like coffins. You might also encounter animal burrows, and these too will generate hyperbolas. The key to differentiating actual burials from “false positives” is to look beyond the targets and to evaluate the entire GPR profile. Rocks, roots, and animal burrows may appear as hyperbolic targets, but they usually will not have a soil disturbance above them. Burial targets must have an associated soil disturbance, and this might appear as broken soil layers or an anomalous area above the target. Roots and animal burials may extend across the project area for tens of feet, but a human burial will not. Marking potential targets with paint or pin flags will help you visualize the length of targets and assist in ruling out some of them.
Your choice of GPR antenna is of critical importance. Higher frequency antennas, like 900 MHz and 2700 MHz, may exhibit impressive resolution but they cannot penetrate down to typical burial depths. Alternatively, lower frequencies greatly improve the depth of investigation but they sacrifice resolution in the process. As burials can be somewhat deeply buried, and they may not present large targets in profiles, we recommend a 400 MHz or 350MHz HyperStacking® antenna for most cemetery surveys. These antennas provide the ideal interplay between depth and resolution and will provide adequate depth penetration without generating unwanted soil clutter in your data.
Tip: Become familiar with local soil conditions, including soil composition and depth to bedrock/ledge, and ask to be present when someone is digging a new grave. If you cannot see a representative soil profile, check out the USDA NCRS soil map and other online soil science resources.
The various materials used for burial containers can degrade over time, especially if constructed from wood. Under the right circumstances wooden coffins may persist for a long time, though it most cases they will collapse and erode away fairly quickly. In these cases, you should understand that you will not be looking for the target; you will be looking for the hole, or soil disturbance, that the target is/was in. Brick and concrete vaults last much longer and should be easier to locate. However, these containers are larger than wooden coffins and the top of the container may only be 1-2ft below the surface. If you are only looking at the 4-6 foot depths, you might miss them! Lastly, you might encounter cemeteries where burials were previously exhumed. In this scenario the remains and container are removed, and all that is left is a filled-in excavation that is often larger than the original grave shaft.
Survey Area Considerations
Most of us cannot choose our project areas, which is why GPR operators rarely visit golf courses during work hours. As such, you might arrive at a project area and discover numerous issues that will slow down or complicate the survey. You should visit the project area prior to the survey and assess the ground conditions. If you are unable to visit the site in person, use Google Street View or Google Maps to check it out. Formal cemeteries are usually well-maintained, but older sections or those with burials of paupers or indentured persons may be overgrown with vegetation. A good rule of thumb is “if your lawnmower can’t go over it, the GPR can’t either”. Ask your client to clear brush, vines, and other problems; this will save you a lot of time and greatly improve the data quality.
Request maps of the cemetery and other relevant information. You might ask if there were ever any stones in the vicinity, or if historical records suggest the presence of burials. If you are surveying along a cemetery wall, ask when the wall was built. The wall could partially or completely cover burials that predate its construction, or there could be burials outside the formal cemetery boundaries.
Trees are ubiquitous in cemeteries, and though they add to the aesthetic their roots generate many “false positive” targets. Be cautious in areas with large and abundant trees and expect to find filled-in holes where trees fell in the past. Treefalls create soil disturbances that may look like grave shafts or mass burials.
Tip: Be wary of cemetery surveys after a large or prolonged rainstorm. If your boots “squish’ when you walk, or you leave deep muddy footprints, consider postponing the survey until the area dries out.
Cemetery targets are highly variable, so your system settings must be optimized to account for many different scenarios. This effort begins in the field, where you’ll want to configure an adequate depth range, perform a range gain/manual gain on “normal” background levels, and set a relatively accurate dielectric for depth calibration. Your goal should be to survey deep enough to image burials, optimize your gain settings to highlight lower amplitude areas and stratigraphic disturbances, and calibrate your depth scale for accurate assessment of target depth.
Burials are rarely “six feet under”, and depth to interment may vary wildly across the same cemetery. Coffin burials are usually four to six feet deep, but the top of a modern concrete burial vault or historical brick vault will likely be only one to two feet below the surface. Do not set the time range/depth to six feet; always overshoot the depth until the bottom 25% of the profile is attenuated. If your dielectric is inaccurate your depth scale will be as well, and this could lead to shallower-than-expected penetration. Deeper penetration will also reveal any potential stacked graves (more than one burial in a grave shaft) or will account for any historical or industrial fill emplaced over the cemetery. It is also important to look for marker beds or other relatively shallow stratigraphic indicators that will have to have been cut through to place an interment.
In addition, understanding dielectric changes and amplitude responses are critical for the GPR interpretation. Metal coffins and wooden coffins will all look different on the screen on your GPR system. The larger the dielectric change, the stronger the reflection and the brighter the target. The smaller the dielectric change, the weaker the reflection and the dimmer the target. The metal coffin will show up as a bright white to black to white target, while an an air-filled coffin will appear as a weak black to white to black target.
Tip: In 2D profiles, closely spaced graves can look like layers rather than individual targets. Furthermore, closely spaced burials, even with coffins, may not exhibit discrete grave shafts for each burial since all of the individual shafts could effectively coalesce into one large “trench” with no interior separations.
While 2D locating is fast and markable in the field, potential errors interpreting that data in the field could be minimized by collecting in 3D as well. Three-dimensional locating will maximize the data capabilities and achieve more certainty with post-processing the data. GSSI academy can help with RADAN and interpret data.
Credits// Author: GSSI
Ultrasonic Crosshole Test or Corsshole Sonic Logging (CSL) is a widely used nondestructive test for assessing Pile Integrity and quality control of drilled shafts (piles). CSL test is a variation of Ultrasonic Pulse Velocity. In theory, the pulse velocity in concrete is a function of the modulus of elasticity, density, and Poisson’s ratio. The uniformity of concrete shaft can be assessed by measuring the pulse velocity.
In crosshole sonic logging, a number of access tubes are installed inside the reinforcement cage prior to placing concrete. For the purpose of testing, the tubes are filled with water to provide acoustic coupling to the ultrasonic transducers.
The very basic format of the test involves at least two parallel tubes installed. Two transducers (a transmitter and a receiver) are lowered down to the bottom of the shaft, and are pulled up. The transit time of an ultrasonic pulse through the concrete between the tubes is measured by a data logger. As UPV transducers are pulled up, the UPV is measured and recorded versus the elevation. This provides engineers with a vertical profile of signal transit time. A modern CSL system uses an automated depth encoder to precisely record the position of the probes inside the access tubes.
Capabilities of Ultrasonic Crosshole Test
Crosshole Sonic Test is a great test for identifying Anomalies in concrete shafts such as soil inclusion, poor quality concrete (low density, low modulus), and major voids. In general, the transit time of ultrasonic pulse between every two access tubes is measured using a high precision data acquisition system. The resolution of the scan along elevation can be controlled by the rate of the withdrawal of the transducers in the tube (normally performed form the bottom to top). The resolution of the scan at each elevation depends on a number of parameters, such as the selected frequency (pulse wavelength), the number and horizontal spacing of access tubes. A modern CSL is equipped with transducers that can operate between 25 to 50 kHz, allowing detection of defects as small as 2.5” to 4” (in each horizon). It is recommended to keep the spacing of the probes to about 12’ (3.6 m).
Analysis of CSL Results
Two main method are often used by geotechnical engineers around the globe: 1) The Waterfall method, and 2) First Arrival Time (FAT). In addition, 2D/2D tomography maps can be generated to illustrate the location and extent of damage at each elevation.
Waterfall Method for Analysis of CSL Test Results
In the Waterfall method, the results are presented as a profile based on complete ultrasonic pulse time histories. As a general practice, the positive peaks are presented by a dash line, whose width matches that of the original peak, and each negative peak is illustrated as a gap, creating a dashed line. These measurements are presented as a series of dash-lines along the shaft elevation, which allows for a more detailed review of the wave train, even when the first arrival cannot be detected. Figure shows an example of the waterfall method.
First Arrival Time for Analysis of CSL Test Results
In this procedure, the arrival time of the first peak in the ultrasonic pulse wave train is (First Arrival Time, FAT), and the overall amplitude of the early part of pulse is measured. A practical challenge in the FAT method is distinguishing between peaks in the wave train, and the noise in the construction site. This is often performed through filtering out the noise by assigning a threshold value.
Crosshole Tomography or tomographic analysis is an alteration of the Crosshole sonic logging that will provide image of an anomaly withing the shaft. Such image shows the shape and position of the affected zone. If regular CSL (horizontal setup) shows there might be a defect in the shaft, additional tests can be performed with transducers offset vertically to provide angled pulse paths. This will enable geotechnical engineers in locating the area and size of the defect.
It is important to note that the Crosshole Tomography comes with all limitations related to effectively measuring the travel path of the pulse (which will not be straight line in most cases). The positioning of the access tubes might change along the pile length. If the CSL test results does not show any anomalies in the shaft, performing tomography test does not provide any extra information.
Key Advantages of Crosshole Sonic Logging
The interpretation of the test results in CSL test is relatively easy (compared to other tests, such as the low strain pile integrity test).
In theory, there is no limitations with regards to shaft length or diameter. The test results are not affected by skin friction, variation in soil stiffness, or damping characteristics.
The test can be further enhanced by implementing a diagonal positioning of the probes (in which the elevation of transmitting transducer has an offset with the receiving transducer). This would enable engineers in creating 2D and 3D maps of defects inside shaft.
Disadvantages and Practical Limitations
The main disadvantage of the test relates to the fact that most access tubes are installed inside the steel reinforcement cage. This would limit the amount of information that can be obtained from concrete area that lies beyond the the steel cage (which happens to be the most problematic area in most cases).
CSL test does not provide information about small horizontal defects.
Another practical consideration is the installation of tubes. As the shaft diameter increases, the minimum number of access tubes is also increased. This will increase the number of paths that need CSL measurement (labor intensive and time consuming).
# what is crosshole sonic logging? # non destructive testing
The GS Series is a high-performance digital, wireless GPR system purpose built for geophysical investigation. Identify and mark characteristics easily with patented HyperStacking Technology and an integrated GPS.
We are glad to announce the antenna is now available for rentals or sale.
Learn more here