CSL Tomography Explained: From 2D Imaging to 3D Pile Defect Visualization

When assessing the integrity of deep foundations, Crosshole Sonic Logging (CSL) provides engineers with valuable information about the internal condition of concrete piles and drilled shafts. While conventional CSL testing can identify changes in ultrasonic signal behavior, CSL tomography takes the analysis further by helping visualize where potential anomalies may be located within a pile.

Modern systems such as Piletest's CHUM combine high-resolution CSL data acquisition with 2D and 3D tomography capabilities, allowing engineers to move from individual ultrasonic measurements toward a more comprehensive representation of pile conditions.

What Is CSL Tomography?

CSL tomography is an advanced method for interpreting Crosshole Sonic Logging data to create two- or three-dimensional representations of a pile's internal condition.

A conventional CSL test uses ultrasonic probes positioned in water-filled access tubes installed within a concrete pile. An ultrasonic pulse travels between a transmitter and receiver, while the system records measurements such as First Arrival Time (FAT) and Relative Energy (RE).

Changes in these measurements can indicate areas of potentially inferior concrete or other anomalies.

Tomography uses measurements collected from multiple tube combinations and depths to provide a visual representation of the information. This can help engineers understand not only whether an anomaly may exist, but also its approximate position and spatial relationship within the pile.

How Are CSL Measurements Collected?

The process begins before testing takes place. Access tubes are installed within the reinforcement cage and become embedded in the concrete during pile construction.

After the concrete has cured sufficiently and the tubes have been checked, ultrasonic transmitter and receiver probes are lowered into the tubes. The tubes are filled with water, which provides the transmission path required for the ultrasonic signal.

Two important measurements are:

• First Arrival Time (FAT): the time taken for the ultrasonic signal to reach the receiver.

• Relative Energy (RE): an indication of the energy or attenuation characteristics of the received signal.

For a deeper explanation of the testing process, see Ultrasonic Pile Testing.

What Information Does 1D CSL Testing Provide?

Traditional CSL results are often displayed as profiles showing how measurements change with depth.

This information can help identify the depths at which signal characteristics differ from average, indicating an anomaly.

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The above shows (on the right) the CHUM CSL system waterfall diagram with FAT (red line) and RE (blue line), and a severity graph that combines the two (orange arrow). The center diagram illustrates how severity is set according to FAT and RE changes. The left shows the CHUM CSL test report format, presenting 3 profiles out of 6 of the 4 CSL tubes pile.

For example, an increase in arrival time (FAT) or a reduction in relative energy (RE) may indicate an area requiring further investigation.

However, a single CSL profile primarily describes conditions along a particular path between two access tubes.

This means engineers may know the anomaly severity and its associated specific depth and tube pair, but additional measurements may be required to better understand its location, shape, and volume of the anomaly within the cross-section.

This is where tomography becomes particularly valuable.

How Does CSL Data Become 2D Tomography?

2D pile tomography combines CSL measurements from multiple paths to create an image or cross-sectional representation of the anomaly in the pile.

Instead of looking at each ultrasonic profile independently, tomography allows related measurements to be visualized spatially.

For example, if several CSL paths show abnormal signal behavior at approximately the same depth, the information can be represented in a cross-sectional image to help indicate where the affected area may be located.

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Above (center and left) are screenshots of CHUM CSL 2D tomography, and on the right is a 2D tomography report presenting a 2D pile slice at depth that represent anomalies in the FAT graph.

Piletest's CHUM – Cross Hole Ultrasonic Monitor supports real-time 2D tomography during CSL testing.

What Does 3D Pile Tomography Add?

3D tomography extends 2D tomography by creating a three-dimensional representation of the pile's internal anomalies.

Rather than reviewing one cross-sectional slice at a time, engineers can examine the data spatially through the depth of the foundation element.

This can make complex anomalies easier to interpret, particularly when their geometry is not adequately represented by a single 2D section.

The CHUM system supports advanced tomography capabilities, including 3D analysis and reconstruction, giving testing professionals additional ways to examine complex CSL datasets.

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Above left is the CHUM Tomography dashboard, and at the center is a video presenting the ability to "dive into the pile in search of anomalies and the ability to see them from all angles.

When Should 3D Tomography Be Considered?

3D tomography can be particularly useful when:

• There are 3 or more CSL tubes in the pile.

• An anomaly appears across multiple CSL profiles.

• The geometry or extent of a potential defect is unclear.

• A complex pile requires more detailed visualization.

• Engineers need to compare measurements from several access-tube combinations.

• Additional spatial information could improve interpretation.

• The project requires a more comprehensive visualization for technical review.
  

For relatively straightforward conditions, conventional CSL profiles or 2D tomography may provide sufficient information.

The appropriate level of analysis should therefore be determined by the pile geometry, available access tubes, testing objectives, and engineering requirements.

Before deciding on 3D Tomography, it is important to check the best practices of pile tomography here.

2D vs 3D Pile Tomography: What Is the Difference?

The simplest way to understand the difference is to think of 2D tomography as sectional imaging and 3D tomography as spatial reconstruction.

2D tomography can provide a useful cross-sectional view of an area of interest. It can help show where a potential anomaly may be concentrated within a particular section.

3D tomography adds another dimension, allowing the potential feature to be assessed through the depth and across multiple sections.

Neither approach should be considered a replacement for engineering interpretation. Tomography is a visualization and analysis tool that helps professionals interpret CSL measurements more effectively.

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From 1D FAT+RE profile analysis >> to 2D tomography slice representation >> to 3D Tomography

What Can CSL Tomography Reveal About Pile Defects?

CSL testing can identify changes associated with potential anomalies such as poor-quality concrete, voids, honeycombing, segregation, and other irregularities.

Tomography can then help visualize the spatial distribution of these changes.

However, CSL tomography does not automatically provide a definitive diagnosis of every concrete defect. Abnormal ultrasonic measurements should be interpreted in context, considering construction records, pile geometry, access-tube configuration, and other available evidence.

For broader information on foundation testing, readers can also explore Piletest's pile integrity testing products.

Choosing the Right Tomography Visualization Method

Different processing approaches can offer different ways of interpreting the same underlying CSL dataset.

Modern tomography analysis can include approaches such as fuzzy logic, parametric analysis, and 3D reconstruction. You can read about these 3D tomography techniques here

Each approach can have advantages depending on the data quality, pile geometry, and objective of the investigation.

The key question should therefore be:

What information does the engineer need to make a better technical decision?

If the objective is to identify a relatively localized area of concern, 2D visualization may be appropriate. If the objective is to understand the spatial extent and geometry of a more complex anomaly, 3D tomography may provide additional diagnostic value.

Why CSL Tomography Matters for Deep Foundations

Deep foundation elements are often inaccessible after construction. Engineers therefore need reliable non-destructive testing methods that can provide meaningful information about the concrete without excavating the entire pile.

CSL testing provides a way to investigate the internal condition of concrete using ultrasonic measurements, while tomography adds a visual dimension to the interpretation.

For large drilled piles and complex foundation projects, this combination can help testing professionals move from raw measurements to a clearer understanding of potential anomalies.

The CHUM Cross Hole Ultrasonic Monitor combines CSL data collection and advanced tomography analysis, providing a practical solution for professionals involved in deep foundation testing.

Frequently Asked Questions About CSL Tomography

What does CSL tomography show?

CSL tomography uses ultrasonic measurements collected during Crosshole Sonic Logging to create 2D or 3D visual representations that can help identify and locate areas of potentially abnormal concrete conditions.

What is the difference between CSL and CSL tomography?

CSL measures ultrasonic characteristics between access tubes at different depths. CSL tomography uses multiple measurements to provide a spatial visualization of the data.

What is 3D tomography for pile foundations?

3D pile tomography is an advanced visualization and analysis approach that represents CSL data in three dimensions, helping engineers investigate the spatial extent and geometry of potential anomalies.

When should 3D tomography be used?

3D tomography may be valuable when a potential anomaly is complex, appears across multiple CSL profiles, or requires additional spatial information for interpretation.

When should 3D tomography NOT be used?

If the CSL test presents issues at several meters from the top of the pile, or a soft bottom, then 3D tomography should not be used, as there are limitations in collecting enough good data for tomography at these pile locations – more on best practices for 3D tomography here.

Can CSL tomography detect pile defects?

CSL can identify abnormal ultrasonic responses that may be associated with defects or poor-quality concrete. Tomography helps visualize these anomalies and give them shape and volume, but final diagnosis should be based on qualified engineering interpretation and supporting evidence.

Does CHUM support 2D and 3D tomography?

Yes. Piletest's CHUM system supports 2D and 3D tomography capabilities for CSL data analysis.

Read more about 2D vs 3D here

Conclusion

CSL tomography represents an important development in the interpretation of Crosshole Sonic Logging data. By moving beyond individual CSL profiles and incorporating 2D and 3D visualization, engineers can gain a clearer picture of where potential anomalies occur within deep foundation elements.

For straightforward investigations, traditional CSL analysis or 2D tomography may provide the information required. For more complex conditions, 3D pile tomography can provide additional spatial insight into potential defect geometry and distribution of potential defects.

As deep foundation projects become larger and more technically demanding, combining reliable ultrasonic data acquisition with advanced visualization can give testing professionals a stronger basis for engineering investigation and decision-making.