1. Introduction

  Ultrasonic testing is a critical method for evaluating the diameters and verticality of boreholes and diaphragm walls. The Tensense Ultrasonic Borehole Monitor utilizes high-frequency sound waves to measure the diameters, verticality, sediment thickness of borehole and diaphragm wall, as well as to shape the borehole walls. However, the accuracy of the test results heavily relies on the proper configuration of two key parameters: Gain (G) and Beta (b). These parameters directly influence the clarity of ultrasonic waveforms, enabling the distinction between valid signals (e.g., primary echoes) and noise (e.g., secondary reflections). This article provides a technical guide to adjusting G and b parameters, emphasizing their roles, correct calibration methods, and consequences of improper settings.

1.  Understanding Gain Parameters: G vs. b

2.1 Gamma Gain (G)

Definition:

  Gamma Gain (G) is a nonlinear amplification parameter that enhances weak signals disproportionately to strong ones. It applies an exponential adjustment to the received waveform, prioritizing the visibility of low-amplitude echoes (e.g., deep defects) without saturating high-amplitude signals (e.g., near-surface reflections).

Function:

• Amplifies faint echoes from deep regions or small defects.
• Preserves dynamic range by avoiding over-amplification of dominant signals.

Technical Basis:
The output signal S_out  is derived from the input signal S_in using:

S_out=S_in^γ(γ=G value)

A higher G value (γ>1) increases contrast for weak signals but risks amplifying noise.

2.2 Beta Gain (b)

Definition:
Beta Gain (b) is a dynamic compensation parameter that adjusts amplification linearly with depth or time. It compensates for ultrasonic energy loss due to material attenuation (e.g., scattering in concrete).

Function:

• Balances signal strength across depths (e.g., stronger amplification for deeper regions).
• Prevents near-field saturation and far-field signal obscurity.

Technical Basis:
The gain curve is often defined as:

Gain(t)=b₀+b_1*t(t=wave travel time)

Here, b₁ controls the slope of gain increase over time.
  In practical operations with the Tensense Ultrasonic Borehole Monitor, Beta Gain (b) parameters typically require minimal adjustment unless significant attenuation is observed.

3.  Correct Gain Configuration

3.1 Initial Setup and Waveform Analysis

Before testing, ensure the probe is properly coupled to the borehole. Acquire a baseline waveform (Figure 1). A qualified waveform should display:

Primary Echoes (A, B, C, D): Clear, high-amplitude peaks corresponding to valid reflections at interfaces (e.g., pile boundaries).

Secondary Echoes (A’, B’, C’, D’): Lower-amplitude noise caused by multiple reflections or structural reverberations.

Figure 1: Qualified waveform with distinct primary (A-D) and secondary (A’-D’) echoes

And at some situations, there also maybe third echoes, fourth echoes….

3.2 Fine-Tuning Gamma Gain (G)

Enhance Weak Signals:

Apply moderate G values (1–9) to amplify deep or faint echoes without distorting primary peaks.

Noise Suppression:

If secondary echoes (A’–D’) become prominent, reduce G to prevent noise amplification. 

Example Workflow:  

1. Start with G=1and acquire a waveform.  
2. If primary echoes (A–D) are indistinct, incrementally increase G by 1until echoes are clear.  
3. If secondary echoes (A’-D’) emerge, reduce G.

Result: A well-configured waveform (Figure 2) shows sharp primary echoes and suppressed noise.

Figure 2: Well-configured waveform with distinct primary echoes (A–D) and suppressed noise

4.  Consequences of Incorrect Gain Settings

4.1 Over-Amplification (High G or b)

Symptoms:

• Saturated near-field signals.
• Merged primary and secondary echoes, making defect identification impossible(Figure 3).

  Figure 3: Failed waveform with indistinguishable echoes

4.2 Under-Amplification (Low G or b)

Symptoms:

• Weak or absent deep signals (e.g., missing defects below 10m).
• Flat color map in imaging (Figure 4), indicating poor contrast.

Figure 4: Uninterpretable image due to low gain

5.  Best Practices for Reliable Testing

Calibration with Reference Data:

  Use test piles with known defects to optimize G and b.

Iterative Adjustment:

  Adjust b first to balance depth-wise signals, then refine G for contrast.

Noise Monitoring:

  If secondary echoes dominate, reduce G or apply signal filtering.

6.  Conclusion

  Proper configuration of G and b parameters is pivotal for accurate ultrasonic testing. Gamma Gain (G) enhances critical details in weak signals, while Beta Gain (b) ensures uniform signal strength across depths. Misconfigured gains lead to ambiguous waveforms and unreliable reports. By following the guidelines above, engineers can achieve clear, interpretable results, ensuring the structural integrity of boreholes and diaphragm walls.