📚 Echo Sounding | 回声测深
Echo sounding is a method of using reflected sound waves to determine the distance to an object or to map the profile of a surface, most commonly the seabed.
回声测深是一种利用反射声波确定物体距离或绘制表面轮廓（最常见的是海底）的方法。
It plays a vital role in marine navigation, geological surveys of the ocean floor, and in locating underwater objects such as wrecks or dense shoals of fish.
它在海洋导航、海底地质勘测以及定位沉船或密集鱼群等水下目标方面起着至关重要的作用。
1. Basic Principle of Echo Sounding | 回声测深的基本原理
An echo sounder transmits a short pulse of sound, often ultrasound, vertically downwards from a transducer mounted on the hull of a vessel.
回声测深仪从安装在船体上的换能器垂直向下发射一个短促的声脉冲（通常是超声波）。
The pulse travels through the water, reflects off the seabed or any object with a different acoustic impedance, and the reflected echo is detected by the same or a separate transducer.
该脉冲在水中传播，从海床或任何具有不同声阻抗的物体反射，反射回波由同一个或单独的换能器检测。
e the depth d, the depth is given by d = v Δt / 2, where v is the speed of sound in water.
精确测量发射和接收之间的时间间隔 Δt。由于声波传播的总距离是深度 d 的两倍，深度可由 d = v Δt / 2 给出，其中 v 是声波在水中的速度。
2. The Role of Ultrasound | 超声波的作用
Frequencies used in echo sounding typically lie in the ultrasonic range, from about 10 kHz to several hundred kHz, well above the audible limit of human hearing.
回声测深所使用的频率通常在超声范围内，从大约10 kHz到几百 kHz，远高于人耳可听范围。
gher resolution, enabling the detection of smaller features on the seabed or individual fish.
之所以首选超声波，是因为较短的波长（λ = v / f）可提供更高的分辨率，能够探测到海底较小的特征或单条鱼。
Furthermore, ultrasonic beams can be made more directional, reducing the spread of energy and improving the signal‑to‑noise ratio.
此外，超声波束可以做得更具定向性，从而减少能量扩散并提高信噪比。
3. The Echo Sounder System | 回声测深仪系统
A typical echo sounder consists of a transmitter that generates electrical pulses, a transducer that converts them into acoustic pulses and vice versa, a receiver amplifier, and a display or data processor.
典型的回声测深仪由产生电脉冲的发射器、将电脉冲转换为声脉冲（反之亦然）的换能器、接收放大器以及显示器或数据处理器组成。
The transducer often uses piezoelectric crystals that vibrate at the chosen frequency when a voltage is applied, producing the outgoing sound wave.
换能器通常使用压电晶体，在施加电压时该晶体会以选定频率振动，从而产生出射声波。
When the returning echo hits the transducer, the same crystals generate a tiny voltage that is amplified and timed.
当返回的回波撞击换能器时，相同的晶体会产生微小的电压，该电压被放大并计时。
4. The Echo Equation | 回波方程
The fundamental relationship in echo sounding is straightforward. If the speed of sound in water is v and the time between transmission and reception is t, the depth d is:
回声测深的基本关系很简单。如果水中声速为 v，发射和接收之间的时间为 t，则深度 d 为：
d = ½ v t
In some texts the equation is rearranged to calculate the range to an object at an angle, but for vertical soundings the factor of one‑half always appears because the signal covers the distance twice.
在一些文献中，该方程被重新排列以计算与有一定角度目标的距离，但对于垂直测深，因子 ½ 始终出现，因为信号覆盖了双倍的路程。
ge R, then R = v t / 2 and the true horizontal distance requires trigonometric correction.
如果物体不在船的正下方，而是处于倾斜距离 R 处，那么 R = v t / 2，而真实的水平距离需要三角校正。
5. Dependence on the Speed of Sound | 声速的依赖性
The speed of sound in water is not constant; it varies with temperature, salinity, and pressure. A commonly used average value is 1500 m/s in seawater.
水中的声速并非恒定；它随温度、盐度和压力而变化。海水中常用的平均值为1500 m/s。
In practice, echo sounders are calibrated for local conditions, and modern systems apply real‑time corrections using measured temperature and salinity profiles.
实践中，回声测深仪会根据当地条件进行校准，现代系统利用测得的温度和盐度剖面进行实时修正。
A depth error of several percent can arise if an assumed speed of 1500 m/s is used in freshwater, where the speed is closer to 1450 m/s.
如果在淡水中假设声速为1500 m/s（淡水中的速度接近1450 m/s），则可能产生几个百分点的深度误差。
6. Resolution and Frequency | 分辨率与频率
The ability to distinguish two closely spaced objects depends on the pulse length and beam width. Shorter pulses give better range resolution, while higher frequencies (shorter wavelengths) improve lateral resolution.
区分两个相距很近的物体的能力取决于脉冲长度和波束宽度。较短的脉冲可提供更好的距离分辨率，而较高的频率（较短的波长）可改善横向分辨率。
The table below compares typical frequencies used in echo sounding and their effects on resolution and penetration.
下表比较了回声测深中使用的典型频率及其对分辨率和穿透力的影响。
| Frequency (kHz) | Wavelength in water (mm) | Typical use | Resolution | Penetration into sediment |
|---|---|---|---|---|
| 12 | 125 | Deep‑ocean bathymetry | Low | High |
| 50 | 30 | General navigation | Moderate | Moderate |
| 200 | 7.5 | Fish finding, high‑resolution surveys | High | Low |
Low frequencies penetrate deeper into the seabed but provide less detailed images; high frequencies reveal fine details but are absorbed more rapidly, limiting the range.
低频能更深地穿透海底，但提供的图像细节较少；高频能揭示精细细节，但吸收更快，限制了作用距离。
7. Pulse Length and Bandwidth | 脉冲长度与带宽
se of duration τ corresponds to a length L = v τ in water, so the resolution Δd ≈ v τ / 2.
距离分辨率约等于发射脉冲空间长度的一半。持续时间为 τ 的脉冲对应于水中的长度 L = v τ，因此分辨率 Δd ≈ v τ / 2。
with v ≈ 1500 m/s, the pulse length is 0.15 m, giving a range resolution of about 7.5 cm.
对于一个发射0.1 ms脉冲的200 kHz系统，τ = 1 × 10⁻⁴ s，取 v ≈ 1500 m/s，脉冲长度为0.15 m，距离分辨率约为7.5 cm。
Shorter pulses require a wider bandwidth, which can increase noise; modern systems often use chirp pulses (frequency‑modulated signals) to combine good resolution with high signal energy.
更短的脉冲要求更宽的带宽，这会增加噪声；现代系统通常使用啁啾脉冲（调频信号），以在良好分辨率与高信号能量之间取得折衷。
8. Interpreting the Echo Trace | 解读回波轨迹
The output of an echo sounder is traditionally displayed as an echogram, a two‑dimensional record with time (or depth) on the vertical axis and horizontal position on the other axis.
回声测深仪的输出传统上显示为回声图，一种二维记录，垂直轴表示时间（或深度），另一轴表示水平位置。
A strong, sharp echo indicates a hard, flat seabed; a weak, spread‑out echo suggests a soft, muddy bottom. Layered structures in the sub‑bottom can also be imaged with low‑frequency sounders.
强且尖锐的回波表示坚硬平坦的海床；弱而分散的回波表明松软的泥质底部。低频测深仪还可以对次底层的层状结构进行成像。
Fish schools appear as distinct mid‑water echoes, often with a characteristic “cloud” shape on the display.
鱼群表现为明显的中水层回波，在显示器上通常呈特有的“云团”形状。
9. Common Sources of Error | 常见的误差来源
Errors in echo sounding can arise from inaccurate knowledge of the sound speed profile, especially if a thermocline (a sharp temperature gradient) bends the sound beam away from the vertical.
回声测深的误差可能源于对声速剖面了解不准确，特别是当温跃层（陡峭的温度梯度）使声波束偏离垂直方向时。
Multiple reflections between the vessel’s hull and the seabed can produce ghost echoes that confuse the detection of the true bottom. Bubbles from wave action or marine life also scatter and attenuate the signal.
船体与海底之间的多次反射会产生假回波，干扰对真实底部的探测。波浪作用或海洋生物产生的气泡也会散射并衰减信号。
Sloping seabeds make the return echo weaker and less well defined, because the reflected energy is not directed straight back to the transducer.
倾斜的海床会使返回的回波较弱且不太清晰，因为反射能量没有直接返回到换能器。
10. Worked Example | 计算示例
between transmitting a pulse and receiving the echo is 0.40 s. Calculate the water depth.
一艘勘测船使用工作频率为30 kHz的回声测深仪。在海水中 v = 1530 m/s，发射脉冲到接收回波的时间为0.40 s。计算水深。
d = ½ v t = 0.5 × 1530 m/s × 0.40 s = 306 m
m/s, and the measured time is still 0.40 s, the new calculated depth would be 296 m, illustrating the need for sound‑speed calibration.
水深为306 m。如果同样的设备用于淡水，其中 v = 1480 m/s，且测得的时间仍为0.40 s，则新计算深度为296 m，这说明了声速校准的必要性。
11. Modern Developments: Multibeam and Sidescan Systems | 现代发展：多波束与侧扫系统
Single‑beam echo sounders measure depth directly beneath the vessel. In contrast, multibeam systems use an array of transducers to transmit a fan‑shaped acoustic pulse and simultaneously record echoes from many points across the swath.
单波束回声测深仪测量船舶正下方的深度。相比之下，多波束系统使用换能器阵列发射扇形声脉冲，并同时记录来自条带内许多点的回波。
This allows rapid mapping of wide corridors of the seabed, producing high‑resolution bathymetric maps essential for modern oceanography.
这使得能够快速绘制宽阔海底走廊的地图，生成高分辨率的海底地形图，对于现代海洋学至关重要。
Sidescan sonar is a related technique that provides detailed images of the seafloor texture and objects protruding above it, often used in archaeological searches.
侧扫声纳是一种相关技术，可提供海底纹理和突出物体的详细图像，常用于考古搜索。
12. Summary | 总结
Echo sounding is a direct application of the reflection of sound at impedance boundaries, allowing accurate distance measurements underwater.
回声测深是声波在阻抗界面反射的直接应用，能够在水下进行精确的距离测量。
The key physics concepts are the pulse‑ relationship d = ½ v t, the dependency of sound speed on medium properties, and the trade‑off between frequency, resolution, and range.
关键的物理概念是脉冲回波计时、关系式 d = ½ v t、声速对介质性质的依赖性，以及频率、分辨率和作用距离之间的权衡。
Understanding echo sounding not only supports marine navigation but also reinforces wave principles and practical measurement techniques required by the A‑Level syllabus.
理解回声测深不仅有助于海洋导航，而且强化了A‑Level 课程大纲所要求的波动原理和实际测量技术。
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