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

​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 StructureScan Mini XT software update, version 1.5.0, allows users to manually adjust their gain – the artificial addition of signal in order to counteract the natural effects of attenuation. Previously, the Mini XT only provided an “Auto Gain” function, which adjusts the screen brightness to either brighten under-gained (weak) images or dampen over-gained (saturated) images. Auto Gain is typically recommended for performing real-time locating, allowing the user to initialize when the signal quality changes between profiles. Manual Gain allows users to change the amount of contrast applied to the top half (shallow) and the bottom half (deep) of the screen. With this update, users can also modify the contrast over the entire data image using the level option. ​

Ultrasonic Pulse Velocity Tester A1404 PULSAR has a software update with two new functions - Crack Depth Measurement (implemented in the instrument software) and SONREB for concrete strength evaluation. This gives additional functionality to our very successful concrete tester. ​

In cemeteries across the United States, there are “the forgotten” burials of unmarked and lost graves. Geophysical techniques, such as Ground Penetrating Radar (GPR), are needed to nondestructively locate these burials in cemeteries and in other locations. Here, we discuss the common causes for lost graves and present three keys to survey success.
Common Causes for Lost GravesThere are many causes for lost, unmarked graves, but two of the most important are cemetery age and population growth.
Historical cemeteries can go back hundreds of years. Over time, missing, fallen, or poorly placed headstones can complicate the assumed physical location of grave sites. The original documentation may be unavailable or rendered unreadable, further leading to confusion. For these and other reasons it is common for cemetery maintenance managers, or other stakeholders, to enlist GPR service providers to generate up-to-date burial maps or clear areas for new burials.
Modern population growth has led to increased infrastructure and city sprawl. As local and state regulations have evolved over time there are documented cases where contractors were given permission to build over known or forgotten burial grounds. In these situations, it is possible that civil and political pressure may lead to a GPR investigation to determine the existence of a cemetery, presence or absence of burials, whether the graves have been disturbed, and factors related to relocation recommendations. In other cases, cemeteries were relocated due to urban expansion but some of the graves could have been overlooked.
Due to the sensitivity of these sites, the GPR service provider’s challenge is to quickly explore the subsurface without disturbing the burials. Every cemetery is different, and local environmental and soil conditions can complicate the investigation. Below, we outline three steps to get started in mapping cemeteries.
How to Get Started: Three Keys to Success

1. Know your GPR Equipment – Experienced users with knowledge of archaeology, forensics, and burial practices are best suited for cemetery mapping surveys. However, for beginners there are many pathways for acquiring sufficient knowledge and experience. The first key to success is to determine the right type of equipment to use and to know one’s GPR equipment well. All the archaeological, forensic, or cemetery knowledge in the world is not a substitute for GPR gear and an understanding of how to use it effectively.
2.  Conduct Pre-Survey Research –
   • Practice: The best experience is hands-on practice. Consider reaching out to a local cemetery and ask permission to scan their grounds prior to conducting cemetery surveys as a service offering.
   • Research: Take time to research the age of the cemetery and how it may have been managed over time. For example, were any plots of land adjacent to the cemetery purchased or changed over time? Is there a clear property line boundary or is it possible that burials could be edging the property? Additionally, seek out research materials that describe GPR methods for cemeteries and become familiar with how burials appear in GPR datasets.
   • Directionality: Look at the existing marked graves in the cemetery to determine the direction and orientation of the graves. For example, if there is no surface expression, can the headstone or footstone provide directional orientation? This will aid in determining the direction of data collection and achieve a better-quality survey result. Ideally, the GPR should pass over burials perpendicular to their long axis (90 degrees).

​3.   Consider the Variables – Soil conditions, weather, and the surrounding environment are variables that can affect the quality of the GPR survey. Note these observances in your field notes and determine if other geophysical techniques can be used to compliment the GPR survey. Be especially cautious if the ground is saturated with water. If your shoes ‘squish’ when you walk over the site, consider returning once the water content has decreased. Look for large trees in the vicinity, signs of large rocks and animal burrows. These and other targets could generate ‘false positive’ indicators of burials.
With this application, it is just as important to understand the appropriate type of GPR equipment, as well as potential limitations, as it is to know about what you’re going to encounter onsite. As with all technical subjects, mastery of cemetery investigations with GPR requires practice and dedication. Armed with GSSI GPR and an understanding of burial characteristics, you can help locate and protect human burials.
We’re here to help – we’re educators and want to provide you with the tools to be successful. To learn more, here are some recommended readings:
Recommended Reading

1. Bevan, B. W. The search for graves. 1991. Geophysics, 56(9), pp. 1310-1319
   • Can be downloaded from Bruce Bevan’s Academia.edu page (requires login or sign up)
2. Conyers, Lawrence B. Ground-penetrating radar for archaeology. Altamira Press, 2013.
3. Conyers, Lawrence B. Interpreting ground-penetrating radar for archaeology. Routledge, 2016.
4. Conyers, Lawrence B. “Ground-penetrating radar techniques to discover and map historic graves.” Historical Archaeology3 (2006): pp. 64-73.
   • Downloadable from Conyers’ website: http://www.gpr-archaeology.com/wp-content/uploads/2018/01/graves.pdf
5. Leach, Peter. RADAN 7 for archaeology, forensics, and cemeteries. Geophysical Survey Systems, Inc. 2019, pp. 70 Downloadable from GSSI’s website: https://www.geophysical.com/wp-content/uploads/2019/04/MN43203A-RADAN-for-Archaeology-Cemeteries-and-Forensics.pdf

Recent Articles

1. Guzzo, Paul, “Are there graves under Tropicana Field parking lots? Archaeologists want to find out,” Tampa Bay Times, July 2, 2020, https://www.tampabay.com/news/st-petersburg/2020/07/02/are-there-graves-under-tropicana-field-parking-lots-archaeologists-want-to-find-out/
2. Fortin, Jacey and Diaz, Johnny, “A Long-Lost Black Cemetery in Tampa May Have Been Found,” New York Times, November 27, 2019, https://www.nytimes.com/2019/11/27/us/tampa-black-cemetery-school.html
3. Schreiner, Mark, “USF Researchers Plan Return to Dozier,” WUSF Public Media, October 2, 2019, https://wusfnews.wusf.usf.edu/post/usf-researchers-plan-return-dozier

 RADAN® is GSSI’s post-processing software for ground penetrating radar (GPR). Short for Radar Data Analyzer, RADAN was first developed by GSSI in 1984 and released in 1987 to post-process GPR data. This software allows users to select the processing functions that best suits their needs. RADAN is Windows™ based, which provides a familiar and easy-to-use environment for all levels of user experience. In 1994, RADAN changed from a disk operating system (DOS) to Windows. ​

Geologic and environmental investigations are integral in determining the geology of any work site. Ground Penetrating Radar (GPR) used as Non-Destructive Testing for subsurface exploration remains one of the safest, quickest, and highest resolution survey options available. Researchers and professionals have been using GPR for geophysical investigation for nearly a century and the applications are seemingly endless. From depth to bedrock, ground water exploration, ice and snow investigation, geomorphology, bathymetry, stratigraphy and sedimentation, structural investigation (along with geohazards), and prospecting, we offer a wide range of antenna frequencies with never before seen depth penetration and data quality. ​

Ground penetrating radar (GPR) offers an accurate, non-destructive solution to mapping the subsurface of the earth. With GSSI GPR antennas, it is simple to locate features of interest and subsurface layers in real time, up to 100 feet or more. 
We offer many different analog and digital antennas, giving you the freedom to choose the right combination of depth penetration and resolution. High frequency antennas provide higher resolution, but typically offer limited penetration. Lower frequency antennas collect deeper data, but they do not image small targets or closely-spaced soil boundaries. Whatever your survey requires, we’ve got you covered. ​

In the restoration industry, the level of understanding of moisture in building materials is generally very high. However, there is a tendency to treat concrete with suspicion and it is not uncommon for waivers to be used to reduce liability for the drying contractor. This is understandable considering there are many unknown variables in concrete, partly due to it being the only building material fabricated on site. Adding to the confusion about moisture in concrete is a mistaken assumption that measuring the vapor in concrete equates to a measurement of the total moisture content in the concrete. This assumption has contributed to many mistakes being made in the flooring and restoration industries. A better understanding of the meaning of measuring both the moisture and the vapor will go a long way to removing much of the confusion and allow for better decision making.

The Importance of Temperature and Equilibrium Equilibrium Relative Humidity (ERH) measurement, as per the ASTM F2170 standard, is a way of measuring the vapor in an airspace in equilibrium with the moisture in concrete as a percentage of the maximum vapor which the airspace can hold. For the results to be meaningful, it is important that the air is in equilibrium with the moisture in the concrete, which is why it is referred to as ERH not RH. However, this does not mean ERH measurement is a measurement of the moisture content. Relative Humidity is used rather than Absolute Humidity because the temperature of the air has a dramatic effect on the amount of vapor the air is capable of holding, making the absolute humidity measurements such as Grains Per Pound meaningless for this purpose.

To explain this simply, the Equilibrium Relative Humidity and the Humidity Saturation Point will both be affected in the same way by the changing temperature. As such, the ERH remains meaningful at different temperatures whereas the absolute humidity levels change. This highlights the need for the humidity to be in equilibrium with the moisture in the concrete, again explaining why it is referred to as ERH and why the temperature must remain stable during the measurement phase. Variables when ERH testing ERH measurement per ASTM F2170 gives useful information about the condition of the concrete but due to variables including the unknown amount of un-hydrated cement present and the unknown level of air permeability of the concrete, it cannot be considered an accurate measurement of the moisture content. To understand this more clearly, the vapor saturation level needs to be considered. It is often assumed that vapor will always saturate at 100% RH but this isn’t necessarily the case.

Take a salt calibration check for example: there is humidity saturation identified by the presence of liquid water but the RH is at 75% due to the presence of salts. Combining Non-Destructive Impedance and ERH Testing Adds Value Measuring the impedance of the concrete is also commonly used to identify the moisture content of the concrete. ASTM F2659 was written to standardize the methodology for carrying out this test. This is a practical site test, which is popular due to its ease and speed of use. However, there are variables which make the moisture content readings less meaningful, especially if we don’t know the vapor level within the concrete. For example, different ambient conditions will alter the Equilibrium Moisture Content (EMC) and different aggregates can absorb and adsorb moisture differently, also affecting the EMC. Therefore, a combination of measuring both ERH and Moisture Content is advisable. The combined approach can be compared to looking down a rifle and lining up two iron sights, instead of one. Ideally, an ERH of 85% will line up to a moisture content of 4% and 90% ERH will line up to 4.5% EMC but when they do not line up there is much information which can be gained about the concrete which would otherwise be unknown if relying on one of these test methods alone.

Many things can be learned about the concrete by combining different test methods, including identifying concrete which is highly alkaline which can have a low ERH to MC level, or identifying concrete with a high Vapor to MC level, which indicates a good quality concrete. Concrete needs a minimum RH of 85% in order for hydration to continue to take place, this hydration will continue to reduce the MC while, ideally, the RH remains at 85% or above until all the hydration is complete. Consider the Ambient Environment and Age of the Concrete Further testing of the ambient conditions is important to improve the meaningfulness of these results, as there are conditions from the environment which can also affect the relationship between the MC and ERH. The age of the concrete also needs to be taken into consideration as most moisture or RH specifications are made for new concrete where construction water is still present. Older concrete, where the moisture should be in equilibrium with the average ambient conditions, is also easier to understand with a combination of testing. Achieving Faster, More Efficient and Reliable Testing With meaningful and simple training, a technician is able to make much more informed decisions based on the data from combined tests, thus removing many of the unknowns as to the condition of the concrete.

Although more testing is carried out, the combined testing should actually be more efficient and much faster. Quick tests such as impedance are used to create a moisture map and more difficult testing such as ERH testing should be used as much as necessary to confirm the vapor condition of the slab, rather than as a method of moisture mapping. This is over time-consuming and doesn’t add to the value of the data. There is little doubt that there is a lot of room for improvement when it comes to testing concrete for moisture that can lead to potential flooring issues. The current methodology has many an expert scratching their head trying to understand the rationale for it. The view of this author is that the combination of ERH and Impedance measurement along with ambient measurements is enough to answer most questions about the condition of the concrete. However it is not the only way that this can be done. There are different methods developed in different parts of the world including tests which combine both humidity and moisture testing and there are several ways to achieve more meaningful information. The first step in improving one’s knowledge is to move away from the idea that one test competes with another, - a doctor would not for example refuse to use an X-Ray because an MRI is better. He or she would use the most practical means available to achieve the information needed to treat a patient, and that’s just pragmatic and good practice.

By Andrew Rynhart