Geophysical Systems

CRA has been using marine magnetometers, sidescan sonars, and sub-bottom profilers since the company’s inception in 1987.

The sidescan sonar system of choice is the Edgetech Model 4125, which is a very portable system and easy to use from small boats, yet is a very high end unit. Data is acquired using the Edgetech software and then processed in Chesapeake Technologies SonarWhizMap software. The latter offers a very sophisticated processing and mosaicking capability together with target recognition and reporting. The results from the system are typically made into geo-referenced tiff images for import into CAD where interpretation layers are added for a final product. The enclosed graphics show the results from a survey of a dock where a large amount of debris can be seen lying on the bottom.

 

 

 

 

 

 

 

The magnetometer owned and operated by CRA is the Marine Magnetics Explorer unit, which again, is designed for relatively shallow work from small boats. This unit employs an Overhauser sensor that does not need calibration and which offers an absolute accuracy of 0.2nT and a sensitivity of 0.02nT. The unit records in software running on a notebook PC together with GPS information, which allows a layback and offset corrected ascii x,y,mag data string to be output to a file. This file is then read into Golden Software’s “Surfer” gridding program to produce 2D and 3D color contours of the magnetic field in the work area, which is then brought into CAD for interpretation and final charting. uses a Datasonics or Edgetech Chirp system for higher resolution surveys and an Applied Acoustics Boomer/Sparker system for greater penetration. Either system records into a CODA/Octopus topside processing unit, which also incorporates GPS data. Processing and interpretation are done in Chesapeake Technologies SonarWhizMap, and then the results brought into CAD for final charting.

 

 

 

 

 

 

 

The choice of sub-bottom profiler depends largely on the objective of the survey. Pingers and Chirps are higher frequency, higher resolution, but lower penetration. Boomers and then sparkers go lower in frequency and higher in power. The former are used to see unconsolidated sediment over rock or to find pipelines. The latter are more geological tools to see further down into the sedimentary sequence.

 

 

 

 

 

 

 

CRA typically uses a Datasonics or Edgetech Chirp system for higher resolution surveys and an Applied Acoustics Boomer/Sparker system for greater penetration. Either system records into a CODA/Octopus topside processing unit, which also incorporates GPS data. Processing and interpretation are done in Chesapeake Technologies SonarWhizMap, and then the results brought into CAD for final charting.

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