Hydrographic Survey with single beam echosounder
Echo Sounding Sonar uses the time interval between a series of soundings and echoes (pick one) for several purposes like range finding (survey) , fish finding or imaging. The bathy logger is a single beam , single frequency echo sounder made for conducting survey grade bathymetric surveys. Over 90% of bathymetric surveys in the world are still done with single beam echo sounders. Survey Echo sounders have a narrower beam (cone) and cannot see fish in the water column. We just want to track the bottom and interpret the returns differently then a fish finder. In pic two you can see the difference of the sonar beam angle.
The sonar cone is similar to a upside down ice cream cone. A single beam echo sounder will take the first return it gets within this cone, so slight movement of the cone won’t affect getting the first return directly below the boat. Also note if you survey near a Dam face or along a pier the cone may hit the wall face and not the bottom, so remove any bad sounding from those sections. Typically surveys are conducted as slow as possible 2-4 mph and working the survey back and forth similar to the lines on a football field. Also going in the perpendicular direction will form a grid and really tie the survey in nicely. Fish finders will generally have a wider beam width while survey echo sounders will have a narrower. Also the standard survey single beam echo sounder should have a 200 Khz transducer. A 200 Khz single beam echo sounder is powerful enough to ensure strong returns. This is a required spec for the USACE.
For shallow water surveys , those under 100M deep the GPS is Typically put on top of the pole with the bl200 echosounder transducer at the bottom, this will eliminate any x,y offset. Traditionally Hydrographic Surveyors input a transducer draft ( depth below surface) and add this to the soundings offset in Hypack or other programs. With a RTK type GPS and software you will enter the rod offset similar to doing a traditional land survey, except this is from the bottom of the sonar transducer. Most GPS field software’s today will apply this depth below transducer to the rod offset and you will log bottom elevation in real time. Most Land Surveyors compare this to doing a continuous topo on a quad, as you move along the depths will be applied to the rod offset, pretty simple and easy for the user.
As the surveyor you control the amount of points you log. In the Hydrographic community they are used to collecting a lot of points. The Land Surveyors are not typically used to this and space soundings out quite a bit. River crossings may not require a lot of points, but pre and post dredging and bridge scouring surveys it would be beneficial to have dense data to create an accurate map of the bottom. The bl200 allows for up to 10 soundings per second. In Hypack or Hydromagic this is normal, with Survey data collectors normally 2 / sec.
200kHz Survey single beam echo sounders, any make, need approx 1.5 feet below transducer or you will experience what is know as “Double or Triple ” returns. It will actually read a depth 2-3 times higher then it should. We suggest doing Rod Shots along any shallow shoreline. Also try using a USV -unmanned surface vessel like the BathyCat RC survey boat which has an extremely short draft and transducer depth.
Before you start any Hydrographic Survey Job you should always get a SV or Sound Velocity of the water you are working in. The easiest most accurate method is a SVP or sound velocity profiler. These are expensive so the next alternative is to use the thermometer and the Temp Charts provided with the bl200 single beam echosounder. Remember get the thermometer down as far as you can toward the middle of the water column and wait for it to level off in temp for more accuracy.