NDT in nature: the sand scorpion as a master of acoustic analysis
7. March 2017
Animal sensing: the desert scorpion uses sound waves for “material testing”.
Animals are not interested in testing forgings, castings or welds. Yet food is ultimately “material” too: it has to be found, located and assessed. In a sense, many animals therefore also carry out a form of material testing.
A particularly fascinating example is the sand scorpion, known among ultrasonic testing technicians as Smeringurus mesaensis. In addition to powerful pincers and a formidable sting, this animal has a remarkable sensory system. Its eight legs act as extremely sensitive sound detectors, making it a master of vibration analysis in desert sand.
Sound waves in sand: two different types of signal
When an animal moves near the scorpion, it generates mechanical waves in the sand. Two different types of wave occur. Compression waves propagate parallel to the sand surface at a speed of around 110 metres per second. Rayleigh waves also form and travel through the loose sand at around 40 metres per second. Their motion resembles that of water waves: crests and troughs form as the grains of sand move up and down accordingly.
A highly sensitive sensor array of eight legs
The scorpion, which is around eight centimetres long and practically blind, positions itself in the loose desert sand so that its eight legs form a circle approximately five centimetres in diameter. In this posture, it waits for vibrations to arrive through the ground.
The sensors in its legs are exceptionally sensitive. The scorpion can detect even a single grain of sand, weighing around 0.2 milligrams, falling to the ground about 50 centimetres away. It identifies the direction and distance of the disturbance almost simultaneously.
Measuring distance through differences in arrival time
When potential prey moves near the scorpion, the faster compression waves reach it first. The Rayleigh waves follow a short time later.
The scorpion distinguishes these two types of wave using two characteristics:
- the different directions in which the grains of sand beneath its legs move
- the time difference of around seven milliseconds between the two wave types
From this time delay, the scorpion can infer the distance to the sound source, an astonishing achievement for an animal without sight.
Determining direction with a biological sensor array
The circular arrangement of the eight legs works much like a technical sensor array. The leg closest to the sound source detects the signal first. The other legs receive it with a minimal time delay, depending on their angle to the source.
The scorpion can evaluate time differences of fractions of a millisecond. This allows it to determine the direction of its potential prey very precisely.
Hunting strategy = UT1, UT2 or UT3?
As the prey moves, its distance and direction constantly change, so the scorpion adjusts its own position too. It moves briefly, pauses and takes another “measurement”. In this way, it reconstructs its prey’s path step by step and can strike accurately.
The sand scorpion’s hunting strategy relies on remarkably complex physical and mathematical relationships. For materials testing technicians, this almost inevitably raises a tongue-in-cheek question: what qualification does the scorpion actually hold? UT1? UT2? Or even UT3?
The answer is simple: none of them. It simply knows how.
W.S. Werkstoff Service – expertise in ultrasonic testing
You can read how ultrasonic testing works in this blog article: www.werkstoff-service.de/en/blog/materials-testing-how-does-ultrasonic-testing-work/.
Further information on UT qualification levels is available here: www.werkstoff-service.de/en/blog/qualification-levels-in-ndt-tasks-and-responsibilities-at-levels-1-3/.
