Overwater geophysics and surveying

[dpsprink]

Hi all,

I have recently purchased 5 RS2 units for use on an overwater geophysics research project. One unit will act as a base station and the other 4 as rovers that will be attached to a sensor “streamer” being towed behind a boat along a river. The streamer will extend approximately 500-600 feet behind the boat and the 4 RS2s will be used to collect position data at those locations on the streamer so that we can know where are sensors are during the survey. Basically we need to resolve the “shape” of the sensor line during the surveys so that we can position the sensors that are spaced along the streamer…thus the RS2s.

I welcome any and all advice/recommendations dos/don’ts etc that the community feels will make my life easier on the data collection / processing side.

Some initial questions I have:

Will we have any issues with sending the RTK corrections over the radios as long as we are within ~8km of the base station? The river we are working on has relatively high terraces /canyon walls on either side occasionally, but the base station will be positioned at a local topographic highspot.

Has anyone used a similar setup (i.e. 1 base with 4 rovers)…any issues I should know?

Any recommendations on waterproof housings or floatation devices for attaching the rovers to our streamer cable?

Any recommendations on a datalogger?

Thanks in advance!

[jjb (juan borda)]

You will be using the RS2 for a bathymetry survey? If yes, then the contraption on which these units will be placed should be sturdy in construction with fixed geometry. I have used at most 3 rovers (not RS2) and they were placed in a pre-defined L shaped metal brace with 3 rovers on the 3 L points with the depth sensor placed perpendicular to this L brace.
The 3 rovers would give you a mathematical plane on which to reference correctly your depth sensor position. 2 rovers would not give you an absolute depth sensor position.
The math to get sensor position would be tedious if you do not have any 3rd party software. We used an in-house program that took only fixed positions of the 3 rovers to compute the sensor position. Any fix from less than 3 rovers were discarded.

The depth sensor that we used has its own internal SD card data logger.

We did not use RTK for this setup. We processed in PPK. For a similar scenario of a river channel with high slopes, I would suggest that you use a 2 base/3 rover setup & processed in PPK. The 2 bases I would place at clear sky locations at both ends of the river section or within 5-8km range. This way as the boat traverses the river,you would always get 2 baselines to the rovers.

[dpsprink]

Hi Juan,
No, the units will be used for positioning our sensor streamer ( a cable with sensors towed behind a boat). We are towing a 500 foot long cable with sensors spaced at various locations along the line and need to know where the sensors are in space/time (the cable will be moving like a snake as we move up/down river).

We need the 4 rovers to accurately position the streamer, but that’s a good idea to setup a 2nd base station. Maybe I’ll just have to purchase one more unit.

Thanks for your reply!

[Pascal_P (Pascal)]

I often work on geophysical survey with ref station, but offshore. So not the same constraint as in river, but long range radio telemetry with high height antenna is a must for RTK. Now we are more and more using satellite correction, from Trimble.
If you are working in Europe or other well cell phone covered places, you can use 3G/4G connection for RTK transmission. Then small radio modem to get positions from the RS2 to your navigation computer.

[TB_RTK]

May i ask what kind of transducer/sonar you used?

[FJDT]

Its sounds as if you are either towing a seismic or a resistivity streamer?

When towing downstream you will struggle to keep that streamer in a straight line behind you unless you travel faster than the river flow. - A drogue will help.

If your streamer is heavier than water it will sink during turns - make wide tear-drop shaped turns to keep the speed of the streamer up to stop it sinking/

Floats for the receivers might be made from a pair of tubular fenders with a plywood board - a bit like a toy catamaran. Keep the centre of gravity low with lead to prevent the whole thing from turning over. The Emlid’s are only IP rated so it’s all at your risk

[dpsprink]

Hi,

We typically motor up-river and back trawl slowly down-river with the current to keep the streamer as straight as possible. We definitely use a tail buoy to add drag to line keeping the streamer straighter than it would be otherwise. A drogue is maybe a better solution. We also have lots of foam flotation attached all along the streamer to keep the line afloat. I like the catamaran idea for floating the receivers. I am currently trying to build some waterproof housings that will ultimately be attached to whatever floatation we go with for the receivers themselves. We definitely can’t afford to lose any to the river gods.

[FJDT]

Yes I’m thinking of something along the lines of the boomer catamarans you see for SBP - though not necessarily with all the shiny stainless (=$$$), plywood will get you through a short survey.

Normally hydrographic software will allow positioning and layback, or multiple receivers, although there might not be streamer models.

I use Peli cases or Otterboxes a lot and fill with a re-entryable (is that even English?) gel. You pour it in, it sets and if you ever need to fiddle with your connections you tear the jelly open, work on it and pat it back. The Gel then self seals again. It is quite expensive here in the UK but works.

[dpsprink]

Great advice! Thank you