2021年经济学人 海神的召唤(1)（在线收听）

Oceanic cartography

绘制海洋地图

Full fathom five

海神的召唤

How to map the seabed from the sky

如何从空中绘制海底地图

An alien seeking a name for the third planet from the sun might reasonably plump for "Sea" or "Ocean", rather than "Earth". Two-thirds of its surface is covered by salt water, and its predominant colour, viewed from far away in space, is blue. What underlies all this brine, though, remains surprisingly mysterious to the planet's ape-descended inhabitants. As recently as 2019, for example, researchers found several thousand new underwater mountains, known as seamounts, by measuring the effects of their gravity on the ocean's surface. More such discoveries almost certainly await.

外星人在为太阳外的第三颗行星命名时可能会选择“Sea”或“Ocean”，而不是“Earth”。因为它的表面有三分之二被海水覆盖，从太空远处望过去它的主色是蓝色。然而，对于生活在这个星球上由猿类进化而来的居民来说，这些海水的背后仍然是个谜。例如，就在2019年，研究人员通过测量海洋表面重力的影响发现了数千座新的水下山脉，即所谓的海山。几乎可以肯定的是，有更多这样的发现在向我们招手。

One important reason for ignorance about the seabed is the lack of a tool that can easily map its topography from an aircraft flying above the water. That, though, is about to change. Researchers at Stanford University, led by Amin Arbabian, an electrical engineer, have developed what they call the Photoacoustic Airborne Sonar System, PASS. This makes it possible to scan the ocean floor rapidly, from a helicopter, rather than relying on a slow-moving ship or submarine.

海床被忽视的一个重要原因是：人们缺乏一种可以在水上飞行的飞机上绘制出海床地形的工具。不过，这种情况即将改变。在电子工程师阿敏·阿巴比亚的带领下，斯坦福大学的研究人员已经开发出了他们所谓的光声机载声纳系统PASS，使得从直升机上快速扫描海底而非依靠缓慢移动的船只或潜艇成为可能。

The problem to be solved is that sound waves, in the form of sonar, are the only reasonable way to accomplish such mapping. Both light beams and the radio waves of radar are rapidly absorbed by water. Sound, by contrast, propagates well. What it does not do well is cross the boundary between water and air. When this happens its amplitude is diminished a millionfold. That diminution applies in both directions, so a pulse of sonar broadcast from an aircraft and reflected back to it from the sea floor would have a trillionth of its original amplitude. Expecting to detect such a reflection would be a fool's errand.

需要解决的问题是，声纳形式的声波是完成这种测绘的唯一合理方式，光束和雷达的无线电波都会被水迅速吸收，相比之下声音传播的效果会更好，它的缺点是无法很好地跨越水和空气之间的边界，这种情况下它的振幅会减小一百万倍，这种减小会发生在来往两个过程中，所以从飞机发出的声纳脉冲再从海底反射回飞机时其振幅只有原来的万亿分之一，因此期望探测到这样的反射是徒劳的。

PASS partly overcomes the air-sea boundary problem by circumventing the first of those crossings. It does so by generating the sonar pulses not on board the aircraft but rather in the water itself, using intense bursts of laser light fired at the water's surface. These heat the water, causing rapid expansion. That generates a sound wave which propagates to the sea floor, whence it is reflected back to the surface. Only then does it cross the energysapping interface between water and air. Though that still weakens the signal a lot, the other part of PASS, a device called a CMUT, is sensitive enough to detect it.

PASS通过绕过第一个交叉点克服了海空的边界问题。它并不是在飞机上产生声纳脉冲，而是用强烈的激光射向水面从而在水里产生声纳脉冲，使得海水变热，迅速膨胀。之后产生声波，声波传播到海底，然后又反射回海面，只有这样声波才能穿过水和空气之间的能量消耗边界。尽管信号仍会被大大削弱，但PASS的另外一部分(一种叫做CMUT的设备)已经足够灵敏，完全可以检测到信号。