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ECHO SOUNDER - FREQUENTLY ASKED QUESTIONS
The following is a brief summary of very common questions asked by customers purchasing an echo sounder for a survey application. Many of the points usually raised are not based on scientific fact but are a form of folklore which has surrounded the mystery of measuring depth within water using ultrasound. To exacerbate the situation many of these myths have been wrongly incorporated into survey equipment specifications, an example being the requirement for depths up to 200m around a country with maximum survey depth of 100m, the 200m have been wrongly converted from 200 feet when the specification was written in metric form.
Is Dual Frequency required ?
Dual frequency echo sounders were originally designed for use by sea going vessels to give reliable depths in deep water situations (low frequency) and more accurate navigation within shallow areas (high frequency). Low frequency is of little use in shallow hydrographic surveys
Physical accuracy is outside IHO specifications
Power consumption prohibits true portable use
Minimum depth possible outside survey requirements
Can mud thickness can be measured with Dual Frequency ?
The residual difference between low and high frequency shown on an echogram gives the impression that mud thickness can be measured. In fact the trace does give an impression of soft sediment, however, in most sounders this is just the difference in reflected energy as a result of simple penetration of higher power low frequency signals plotted against the low power high frequency returns.
To measure mud thickness and avoid litigation on wrong results the surveyor should use equipment specifically designed for geophysical measurements such as a sub-bottom profiler, penetrometer, seismograph or a simple bottom sample grab.
Is a Barcheck required ?
Many older technology echo sounders needed to "Warm up" before they became stable, in addition to their internal frequency/timing circuits varying with ambient conditions the physical parameters that effect speed of sound in water also varied with location. The accepted solution was a ''Barcheck'' where a large plate was lowered in a specified sequence where the depth measured by the echo sounder to the plate was compared with an absolute stave or tape measurement. This method ensured that all variable parameters were included in the calibration, as a gross check the Barcheck was normally also performed at the start and end of a survey.
There are several problems with this method ....
The location of the barcheck is only relevant to the water column at that particular barcheck location and time.
Older analogue echo sounders gave the user facilities to ''fiddle'' with many parameters during the survey, modern digital sounders do not expose settings such as gain and threshold.
The older instruments do not record changes to instrument settings during the survey.
Narrow beam sounders with bottom detection algorithms can misdetect the moving plate.
Modern digital electronic timing components are very accurate and stable.
There is still a requirement for calibration of sound velocity but if required this should be measured using a calibrated Sound Velocity Probe lowered through the water column to build a Velocity Profile.
A simple equivalent of the Barcheck is to accurately visit several locations within the survey area which have a known elevation (normally derived from GPS and a weighted tape) and these points used as a reference throughout the duration of the survey.
Should Pitch/Roll angle be applied to the depth ?
Many surveyors assume that sonar is like a laser being shone through the water and that the distance measured should be assumed is a hypotenuse measurement to be trigonometrically corrected by any Pitch/Roll angle that has been measured.
The best physical analogy of the echo sounder beam would be a torch light beam that is shone over an area, within that area there is a small piece of mirror that reflects the light at that point, sonar is very similar except that the reflected point is normally the closest point within the beam.
What is a narrow beam transducer ?
The properties of a transducer are normally a function of its physical size/shape and its resonant frequency. The beam pattern of a given transducer is normally presented as a radial distribution pattern versus output power applied. This generally means that the spread (beamwidth) of the transmitted ultrasound increases with amplitude. Most modern echo sounders use a Digital Signal Processing (DSP) technique to reduce the power/gain of its transmitted signal and thus maintain the minimum beam width for a given transducer.
The advantage of a narrow beamwidth survey transducer is the ability to "see" into narrow valley shapes thus gaining a more representative definition of the bottom surface being surveyed. This is contrary to the navigation use of an echo sounder which has a reasonably wide beamwidth where the returned signal within the beam is the ''shoalest'' or shallowest point within the beam, obviously of more interest for hull clearance requirements.
Is my Fishfinder OK for survey use ?
Modern Fishfinder type navigation echo sounders use quite sophisticated DSP techniques to show the bottom surface, bottom type and fish in the water column. As with the beamwidth discussion the electronics of a 'leisure' Fishfinder are not designed for a survey application. In particular the depth values are either heavily averaged to show a smooth transition in numeric depth values or are optimized to show the shallowest depth seen in the particular beam area. Similarly survey echo sounders make efforts to remove anomalies such as fish swim bladder reflections from the measured data.
Do I need more pings per second ?
Assuming the pings all return good values the advantage of more pings per second is that the survey boat can travel faster and therefore cover much larger areas in a shorter time. The assumption that more pings provide higher quality dense survey data is not so, density is a function of boat speed and is still constrained by the limitations of transducer beamwidth as discussed above. More pings can also create more noise with the high degree of insonaration in the water and particularly in shallow water situations with multipath reflections.
What is Latency ?
Latency is the time difference between a position and depth being recorded, the topic is extremely complex and can included some of the following sources of error ...
GPS position correction source time differences
Time of flight of ultrasound in water
Serial transmission of data to, from and through computer systems
Physical mounting of antennas and transducers
In general the survey software attempts to minimize the error by recording a precise timestamp on each piece of data recorded, the sum of all latency sources can then be calculated by post-process adjustment of the data using a ''Patch Test'' algorithm. The degree of latency is dynamic so is always a function directly related to speed of the boat when data acquired, faster the boat then more potential latency.
What effects sound velocity
The speed sound travels through water is directly proportional to the density of the water, the parameters which change the density are ....
Turbidity - the amount of sediment in suspension within the water.
Salinity - The amount of substance (normally salt) dissolved in the water.
Temperature - The temperature of the water sample.
Pressure - Sum of water depth and barometric pressure.
All of the above parameters can vary considerably in any particular water column but the assumption made with a single beam echo sounder is ....
the sound travels near vertically so does not suffer any refraction.
the sound travels there and back so is the average of all condition changes.