2D vs 3D Pile Tomography: Choosing the Right Approach for CSL Testing

Crosshole Sonic Logging (CSL) is widely used to evaluate the integrity of deep concrete foundations. By sending ultrasonic signals between access tubes installed inside a pile, engineers can assess changes in signal travel time and energy that may indicate areas requiring further investigation.

• 

• 

• 

Above is a CHUM CSL system software screenshot, showing a waterfall diagram on the left, with FAT (red line) and RE (Relative Energy) blue line. The orange arrow points to the severity graph. On the right is an example of a CHUM report. Center - Is a tomography dashboard with 2D and 3D representations. Right - Is a 3D Tomography simulation representing the ability to "dive" into the pile looking for defects (green), which can be rolled to be viewed from all angels.

As CSL technology has developed, engineers have gained access to more advanced ways to visualize this information. 2D and 3D pile tomography can transform CSL measurements into visual representations that make complex data easier to interpret.

For readers looking for an introduction to the underlying testing process, our Crosshole Sonic Logging (CSL) guide provides further information about CSL testing, equipment, and its role in deep foundation assessment.

In standard CSL testing, measurements are collected sequentially between pairs of access tubes to create 1D profiles showing acoustic performance versus depth. Each pair of tubes defines a single testing path, meaning the total number of profile combinations grows significantly as more tubes are installed. For example, a concrete pile equipped with 4 perimeter CSL tubes yields a total of 6 distinct tube-pair profiles (T_{1-2}, T_{2-3}, T_{3-4}, T_{4-1}, T_{1-3}, and T_{2-4}). While these individual profile line-graphs effectively flag depth zones where first arrival times delay or energy levels drop, evaluating all 6 profiles independently to pinpoint the exact cross-sectional location or physical boundaries of a defect can be challenging without advanced multi-path tomography.

What Is Pile Tomography?

Pile tomography is an advanced way of processing and visualizing measurements collected during ultrasonic pile testing.

In a CSL test, ultrasonic probes travel through water-filled access tubes embedded within the pile. The system records measurements including First Arrival Time (FAT) and Relative Energy (RE).

For more information about the underlying testing process, see Ultrasonic Pile Testing.

Tomography builds on these measurements by considering information from multiple paths and presenting it spatially.

The result can be a 2D cross-sectional image or, with suitable data and processing, a 3D representation of defects within the pile. Hence, if there are no defects, nothing will be shown.

How Does 2D Pile Tomography Work?

2D tomography provides a cross-sectional view of the information obtained from CSL testing.

Imagine looking down through a pile at a particular depth. Instead of seeing several independent CSL profiles, tomography can combine relevant measurements to create a representation of conditions across the section.

This can make it easier to identify patterns.

For example, if several measurement paths indicate unusual ultrasonic behaviour around the same region, a 2D visualization may help show that the readings are spatially related rather than isolated observations.

• 

• 

• 

The text below explains the above 2D tomography representation screenshots

There are two types of 2D tomography.

1 – Vertical 2D tomography showing the anomaly from the side of the pile or a profile (above on the right) 2 – Horizontal or Pile Slice – Which shows an anomaly at a specific depth from a top view. This also show the Effective Area (which represents the ratio between concrete and void) at a specific depth (above center). On the left is a report with a slice per specific depth, usually where an anomaly is detected on the FAT graph)

Piletest's CHUM – Cross Hole Ultrasonic Monitor supports CSL testing and advanced tomography analysis.

What Are the Advantages of 2D Tomography?

2D tomography can be useful because it:

• Provides a visual representation of CSL measurements.

• Helps identify the approximate location and 2D size of potential anomalies at a specific depth.

• Makes complex datasets easier to communicate.

• Can provide real-time visualization during testing.

• Allows engineers to examine individual pile sections.

• Can be sufficient for relatively straightforward investigations.

  
  

For many applications, 2D visualization may provide an effective balance between detailed analysis and practical interpretation.

What Is 3D Pile Tomography?

3D pile tomography extends the visualization from individual sections into a three-dimensional representation.

Instead of considering only a single cross-sectional image, 3D analysis can help engineers examine potential anomalies across multiple depths and directions, providing insight into the anomaly volume and its exact shape and location within the pile boundaries.

This becomes particularly valuable when the shape or extent of a potential anomaly is complex. See 3D tomography screenshots below

• 

• 

A 3D representation can help answer questions such as:

• Does the anomaly continue vertically?

• Does it appear across multiple tube combinations?

• Is the affected area localized or more extensive?

• How does it affect the pile strength at the given depth

• What is its approximate spatial relationship to the pile geometry?

• Does the pattern remain consistent through different depths?
  

The CHUM Cross Hole Ultrasonic Monitor provides advanced capabilities for CSL testing and tomography, supporting more detailed analysis of pile integrity data.

Before deciding to do 3D Tomography, one should first read some best practices on 3D tomography here.

2D vs 3D Tomography: Which Is Better?

There is no universal answer.

2D tomography is not simply an inferior version of 3D tomography. The two approaches can serve different purposes.

2D tomography can be highly useful when the investigation focuses on a specific depth or cross-section, and the available CSL measurements provide sufficient information. 2D tomography can be a first step, performed in real-time at the construction site, for review of the anomaly, and can drive the decision to collect more data for 3D Tomography, which is calculated offline.

3D tomography becomes more attractive when engineers need to understand the potential spatial extent of an anomaly within a deep pile foundation, and obtain additional supporting data to inform the mitigation decision regarding the anomaly in question.

The best choice depends on the project requirements, pile geometry, number and arrangement of access tubes, quality of the collected data, and the engineering question being investigated.

When Is 2D Tomography Enough?

2D tomography may be sufficient when:

1. The anomaly appears relatively localized.

2. The area of concern can be clearly evaluated at a particular depth.

3. CSL profiles provide consistent information across the relevant profiles.

4. The engineering team primarily needs cross-sectional visualization.

5. The anomaly does not require a complete three-dimensional representation.

In these situations, 2D visualization can provide a practical and informative way to interpret CSL measurements.

When Does 3D Tomography Provide Greater Diagnostic Value?

3D tomography can become particularly useful when the condition is more complex.

For example, suppose CSL testing identifies abnormal readings across several depths and tube combinations (profiles). Looking at each profile separately may make it difficult to understand whether these measurements represent a single continuous feature or multiple unrelated areas.

A 3D model can help bring the information together.

This does not mean that 3D tomography automatically identifies the physical cause of an anomaly. Instead, it provides an additional layer of visualization that can support engineering interpretation. Hence, it can support engineers’ decisions on the direction to mitigate the anomaly/defect.

What Can CSL Tomography Reveal About Concrete Piles?

CSL testing can help identify abnormal signal behavior associated with variations in concrete quality. Potential conditions to investigate include voids, honeycombing, segregation and other anomalies.

Tomography can help show where these abnormal responses are concentrated, and their volume.

However, interpretation must remain technically cautious.

An ultrasonic anomaly is not automatically proof of a specific defect type. Factors such as access-tube condition, signal path, pile geometry, concrete properties, and testing conditions can influence measurements.

For broader information about foundation integrity testing solutions, readers can explore the Piletest pile integrity testing products offering.

Understanding Tomography Visualization Methods

Modern pile tomography systems can use different approaches to process and represent CSL data.

These may include fuzzy logic, parametric analysis, and 3D reconstruction.

Each approach can offer a different way of examining the same underlying CSL information, depending on the data quality, pile geometry, and purpose of the analysis.

The key consideration is not simply which visualization looks most detailed, but which method provides the most useful information for the engineering investigation.

What Are the Limitations of 2D and 3D Pile Tomography?

Tomography provides valuable information, but it also has limitations.

First, the quality of the visualization depends on the quality of the underlying CSL measurements.

Second, the number and arrangement of access tubes influence the available measurement paths. There must be at least 3, preferably 4 or more CSL tubes for a good 3DT representation

Third, tomography should not be interpreted independently of engineering context.

Finally, a visual model should not create a false sense of precision. A 3D image may look highly detailed, but its interpretation still depends on ultrasonic measurements, which in turn depend on the tester's testing skills and the assumptions used to process them.

This is why qualified interpretation remains essential.

How Does CHUM Support CSL Tomography?

The CHUM Cross Hole Ultrasonic Monitor is designed for high-resolution Crosshole Sonic Logging of deep foundations and supports advanced tomography analysis.

The system provides 2D and 3D tomography capabilities, enabling CSL testing data to be visualized and analyzed within the CSL workflow.

This helps connect the overall process of ultrasonic data collection, CSL analysis, tomography, and engineering interpretation.

Why 3D Pile Tomography Matters for Modern Foundation Testing

Large infrastructure projects increasingly require detailed quality assurance for deep foundation elements.

Once a pile has been constructed, direct inspection of its internal concrete is difficult. Non-destructive testing therefore plays an important role in identifying areas that may require further investigation.

CSL provides the measurements. Tomography provides another way to understand those measurements.

When used appropriately, 2D and 3D tomography can make complex CSL datasets easier to visualize, compare, and communicate.

The result is not simply better-looking reports. The real objective is better technical understanding of potential anomalies within the pile.

By combining advanced tomography with established ultrasonic pile testing techniques, testing professionals can obtain a more comprehensive view of deep foundation integrity.

Frequently Asked Questions

What is the difference between 2D and 3D pile tomography?

2D tomography provides a cross-sectional representation of CSL data, while 3D tomography provides a three-dimensional visualization of anomalies, helping to assess their spatial extent.

Is 3D tomography better than 2D tomography?

Not necessarily. 3D tomography can provide additional spatial information, but 2D tomography may be sufficient for simpler investigations.

What does 3D pile tomography show?

3D pile tomography can visualize CSL-derived information through the depth and cross-section of a pile, helping engineers investigate the location and potential geometry of anomalous areas.

Can tomography confirm a pile defect?

Tomography can highlight abnormal ultrasonic responses, but it should not automatically be interpreted as definitive proof of a specific physical defect.

When should 3D tomography be used?

3D tomography may be particularly useful when potential anomalies extend across multiple depths, or when measurement paths and additional spatial information could improve 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.

Does CSL tomography replace traditional CSL testing?

No. Tomography is based on CSL measurements and provides an additional visualization and analysis. It complements rather than replaces the underlying CSL testing methodology.

What equipment can be used for CSL tomography?

The CHUM Cross Hole Ultrasonic Monitor is designed for high-resolution Crosshole Sonic Logging and provides 2D and 3D tomography capabilities for deep foundation testing.

Conclusion

The difference between 2D and 3D pile tomography is ultimately about how much spatial information is needed to interpret the CSL data.

2D tomography can provide an effective cross-sectional view to identify and investigate potential anomalies. 3D tomography can provide an additional level of spatial interpretation when conditions are more complex or when understanding the extent and geometry of a potential anomaly is important.

For modern deep foundation testing, the combination of high-quality CSL measurements and advanced tomography can help engineers move from isolated data points toward a more complete understanding of pile integrity.

Whether 2D or 3D analysis is appropriate, the goal remains the same: turn reliable ultrasonic testing data into useful engineering information that supports better decisions about deep foundation integrity.