UT Data Representation: A-Scan, B-Scan, and C-Scan

This article summarizes the principles, structures, and applications of the three scanning methods used to visualize data collected by Ultrasonic Testing (UT) equipment from an engineer’s perspective.

1. Overview of Scan Modes

UT equipment visualizes echo signals received by the transducer in three formats—A-scan, B-scan, and C-scan—based on the coordinate axes configuration and how data is accumulated [cite: 259]. These three modes are not mutually exclusive; in PAUT systems, it is common to display A, B, and C-scans simultaneously from the same dataset [cite: 260].

• Core Relationship: B-scans and C-scans are constructed by spatially accumulating and reconstructing A-scan data [cite: 261]. Thus, the quality of the raw A-scan data determines the reliability of all higher-level representations [cite: 261].

2. A-Scan

2-1. Definition

A one-dimensional waveform plot representing the received echo signal amplitude on the Y-axis and the ultrasonic pulse travel time (Time of Flight, ToF) on the X-axis [cite: 264]. It is the most fundamental representation of UT data, captured at a single position of the transducer [cite: 265].

2-2. Axis Configuration and Interpretation

• X-axis (Horizontal): Ultrasonic propagation time $\rightarrow$ converted to depth (mm) using the material’s sound velocity [cite: 267]
• Y-axis (Vertical): Echo signal amplitude $\rightarrow$ used for estimating reflector size and reflectivity strength [cite: 268]
• Peak: Echoes generated from individual reflective interfaces (defects, backwall, etc.) [cite: 269]. Peak location determines the depth, while peak height indicates reflection intensity [cite: 269].

2-3. Depth Calculation

$$\text{Defect Depth} = \frac{\text{Time of Flight} \times \text{Sound Velocity}}{2}$$

• Sound velocity in steel: approx. $5,900 \text{ m/s}$ for longitudinal waves / approx. $3,230 \text{ m/s}$ for shear waves [cite: 272].
• Accurate calculations require precise sound velocity calibration for the specific material [cite: 272].

2-4. Key Gate Configuration

In an A-scan, a gate is positioned over a specific time range so that only echoes within that interval are evaluated [cite: 274]. Event marks in B-scan and C-scan are generated based on the echoes within this gate [cite: 274].

• Gate Start & Width: Defines the inspection depth range [cite: 275]
• Threshold: The reference amplitude level used to distinguish noise from actual defect echoes [cite: 276]
• Evaluation Method: ① Simple threshold exceedance (go/no-go or 0/1 binary), ② color or grayscale representation proportional to amplitude [cite: 277]

2-5. Application Context

• Used as the primary display for real-time waveform monitoring in manual field inspection [cite: 279].
• Utilized as the reference waveform when establishing sensitivity calibration curves (DAC, TCG, etc.) [cite: 280].
• In PAUT, an independent A-scan is generated for each focal law [cite: 281].

3. B-Scan

3-1. Definition

A two-dimensional graphic presentation in a rectangular coordinate system, displaying a cross-sectional view of the test specimen by combining the transducer travel direction (X-axis) with the ultrasound propagation time or depth (Y-axis) [cite: 284]. As the transducer moves along a scan line, A-scan data at successive positions are accumulated sequentially to construct the cross-sectional image [cite: 285].

3-2. Axis Configuration

• X-axis: Transducer travel distance (scan direction) $\rightarrow$ position along the length of the test specimen [cite: 287]
• Y-axis: Ultrasonic travel time $\rightarrow$ defect depth within the material [cite: 288]
• Event Mark: Displays echoes within the gate at each X position as 0/1 binary marks or color-coded values proportional to amplitude [cite: 289]

3-3. Event Representation Methods

Each pixel (event mark) in a B-scan represents the echo inside the A-scan gate at that specific transducer position using one of two methods [cite: 291].

• 0/1 (Binary) Method: A mark is plotted if the echo inside the gate exceeds the threshold; otherwise, it remains blank [cite: 292]. This method is suitable for simple go/no-go evaluations [cite: 292].
• Amplitude Proportional Method: Represents the echo amplitude using a color palette (or grayscale), which is advantageous for evaluating the distribution of reflectivity strength [cite: 293].

3-4. Key Applications

• Determining the cross-sectional location of root, sidewall, or cap defects in welds [cite: 295]
• Estimating the through-wall height of defects [cite: 296]
• Combining with PAUT S-scans (sectorial scans) to obtain multi-angle cross-sectional views [cite: 297]
• Profiling pipe wall thickness distribution [cite: 298]

3-5. Limitations

A B-scan only provides a 2D cross-section along the transducer path and depth [cite: 300]. It does not contain information along the width direction (perpendicular to the scan path); therefore, a C-scan is required to evaluate the planar area of defects [cite: 300].

4. C-Scan

4-1. Definition

A two-dimensional plan view (top view) displaying defect echoes relative to the surface of the test specimen [cite: 303]. It is generated by raster scanning the transducer along two axes (X and Y) and mapping the gated echo information to the planar coordinates [cite: 304]. It can be understood as a defect distribution map looking down from above the specimen [cite: 305].

4-2. Axis Configuration

• X-axis: Transducer travel direction 1 (scan/index axis) [cite: 307]
• Y-axis: Transducer travel direction 2 (perpendicular axis, raster/step axis) [cite: 308]
• Pixel Value: Gated echo amplitude or Time of Flight (ToF) $\rightarrow$ represented as a color map [cite: 309]

4-3. Event Representation Methods

The same two methods as B-scan are applied [cite: 311].

• 0/1 (Binary) Method: Plots marks where the threshold is exceeded to map the distribution of defect presence [cite: 312]
• Amplitude Proportional Method: Displays the amplitude levels with a color palette to visualize the reflectivity strength distribution [cite: 313]

4-4. Depth Information and D-Scan

A standard C-scan plots only the peak amplitude within the gate and does not inherently contain depth information [cite: 315]. To display depth details, a D-scan is used, which maps the Time of Flight (ToF) of the gated echo to color values [cite: 316].

In modern digital UT systems, data digitization and storage allow post-processing to easily switch between C-scan and D-scan views from the same dataset, or to generate multiple C-scans for distinct depth intervals (gates) [cite: 317].

4-5. Applied Techniques

• Pulse-Echo Technique: Applied when only single-sided access is available [cite: 319].
• Through-Transmission Technique: Transmitter and receiver transducers are placed on opposite sides, which is highly effective for detecting internal delamination in composites [cite: 320].

4-6. Key Applications

• Assessing delamination distribution in composites (CFRP, GFRP) [cite: 322]
• Mapping results from large-area automated scanning systems (immersion tanks, squirters) [cite: 323]
• Quantifying defect area and generating accept/reject maps [cite: 324]
• Visualizing overall corrosion distribution (remaining wall thickness mapping) [cite: 325]

5. Comparison of A-Scan, B-Scan, and C-Scan

6. Application in PAUT Systems

PAUT (Phased Array Ultrasonic Testing) systems typically display A, B, and C-scans simultaneously from a single scan dataset, often complemented by an S-scan (Sectorial Scan, a fan-shaped cross-sectional view over a range of angles) [cite: 329].

• A-Scan: Monitors signal quality by displaying real-time waveforms for the selected focal law (element combination) [cite: 330].
• B-Scan: Uses the cross-sectional view along the scan path to locate root or sidewall lack of fusion in welds [cite: 331].
• C-Scan: Illustrates the echo amplitude distribution across the entire scanned area to verify inspection coverage and report defect distribution [cite: 332].
• S-Scan: Displays multi-angle A-scans in a fan-shaped coordinate view, which is advantageous for obtaining cross-sectional images of complex weld geometries [cite: 333].

After completing a scan, stored PAUT data can undergo offline post-processing to adjust gates, switch views, and measure defect sizes [cite: 334]. This capability is one of the key advantages of PAUT over conventional single-crystal UT [cite: 335].