Socon | Ultrasonics | underground cavity survey, underground storrage cavity, underground storrage cavern

Ultrasonics as an aid for measuring distance

The echo systems for surveying caverns are, from a historical point of view, derived from the echo sounding equipment used in fishing boots. Maritime echo sounding was applied to determine the clearance between the boat and the sea floor (navigability) or to locate shoals of fish.

Nowadays there are still sonar tools in operation throughout the world that work on this basis. Solely the design of the tool has been modified and the ultrasonic emitter further developed so that it can be rotated and tilted. In the original echo sounding method the echo values were plotted along the course traveled. When using sonar tools the echo values are plotted in a plane around the survey axis. The values are plotted at angle increments calculated to be equivalent to each step the incremental motor makes. These systems are ideal for measuring trends. Every reflection increases the density of the structural information displayed.

Einsatz des Echolots zur Ermittlung der Bodenfreiheit unter Booten und zur Ortung von Fischschwärmen.

In previous years the reflections were displayed by black lines marked on strips of paper or by an oscilloscope. Nowadays output is via computer. In order to be able to adequately consider the physical conditions that are important for cavern surveys, surveying systems are needed that satisfy elaborate requirements. These systems are not in a position to do this. When trying to cut costs the systems can be made cheaper and less complicated, but the results they provide are equivalent to the results obtained in the early years, that is they good only for estimating the clearance under fishing boats or locating shoals of fish. They enable comments to be made that indicate only certain trends, such as "exists" or "does not exist", and "critical" or "not critical".

Use of ultrasonics in distance measuring equipment:

Distance measurements made with sonar tools are based on an ultrasonic sensor system that emits a burst of high frequency oscillations which generally excites a piezoceramic unit to mechanically vibrate, and this in turn creates acoustic waves in the cavern medium. These acoustic waves are reflected from the area exposed to the sonic beam.

The ultrasonic emitter emits pulse bursts that are reflected from the target surface. Depending on the surface structure of the target, the acoustic waves are reflected to a certain extent diffusely in the cavern.

The wavelength ? of an sonic oscillation is dependent on the transmitted frequency f and the acoustic velocity v prevailing in the medium.
The time between the individual bursts is used to receive the reflection responses. Depending on the surface structure of the target, the acoustic waves are reflected more or less concentrated and partly diffused into the cavern. The acoustic waves reflected at the target (echoes) return directly and/or indirectly to their point of origin.

The time an acoustic pulse needs to travel from the emitter to the receiver (which is also generally the emitter) is referred to as the two-way traveltime and is made up of the outward and return journey, i.e. twice the distance. An ultrasonic sensor system therefore does not measure distances, but rather just the traveltime that an acoustic pulse needs to travel to the target and back again. Half of the two-way traveltime multiplied by the acoustic velocity provides the required value of the distance.

The quality of a traveltime measurement is affected by numerous physical parameters. Provided the general physical conditions are adequately taken into consideration it is possible to a large extent to minimize potential error sources and measurement errors.

A critical physical factor is the angle of beam spread of the ultrasonic lobe.

The graphic shows the ultrasonic lobe and the acoustic pressure distribution when a plane wavefront strikes an aperture of diameter d (corresponds to the emission characteristics of an ultrasonic transducer with an equivalent diameter). ? equals half the angle of beam spread.

The angle of beam spread (?) in the following formula is dependent on the wavelength (?), the diameter (d) of the ultrasonic transducer and the parameter (k):

α = 2γ = 2 * arcsin (k * λ / d)

The factor k defines a point on the ultrasonic lobe at which the amplitude of the acoustic level has decreased by a specific amount. Usually a value of 0.51 is used for k, so that the point is defined as the one at which the amplitude of the acoustic level has decreased by -6dB, i.e. by 50%.

"The k factor can be misused as a very good company parameter for optimizing the concentration of the sonar beam when advertising in brochures. All that needs to be done is select a small enough value for k."

Diagram of the ultrasonic lobe and the area exposed to the ultrasonic beam at different distances.

The above graphic illustrates that the wall area exposed to the sonic beam varies in size depending on the distance to the target, provided the emission conditions remain constant.

The specifications of the angle of beam spread are true only if the measuring signal strikes the cavern wall also between the points defined above. In order to ensure that the indicated angle of beam spread is maintained it is necessary to have optimum control at every individual measuring point.
If this is not the case (in other sonar systems the parameters for a 360° scan are constant) the angle of beam spread is significantly higher!

In order to be able to determine correct and reliable distance values it is absolutely essential that the energy and sensitivity are adjusted to the reflection conditions for every single survey point.

All SOCON systems are adjusted to the general physical conditions with the help of a computer. Microprocessors request the type and the serial number of all the important sensors and subsequently make the appropriate calibration tables available.

The ultrasonic sensors are adjusted and optimized for every job they are to perform, if necessary survey point for survey point.

The angle of beam spread becomes smaller and therefore more concentrated:
with increasing diameter of the sonic source
with decreasing wavelength
with increasing frequency
with decreasing acoustic velocity

Sonar in caverns

SOCON SONAR CONTROL
Kavernenvermessung GmbH

The World-wide leading engineering company specializing in the surveillance of underground caverns and voids.

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SOCON Sonar Control - Kavernenvermessung GmbH
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