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Geology

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Paper: Burial Recording Mines: a valid technique to study bedform migration and storm impact above the sea-floor, Sonia Papili, Thomas Wever, Yves Dupont, Marine and River Dune Dynamics- MARID IV= 15&16 April 2013- Bruges, Belgium

Background

Burial Recording Mine, also referred to as Burial Recognition Mine (BRMs), technology was first used in 1974 by the military to understand how mines were buried and shifted on the sea floor over time.  Since then they have evolved into a useful tool for scientists investigating the evolution of geomorphologic structures in the sediment and how they influence the position and burial of objects lying on the seafloor.

Experiment

Two BRMs were deployed in a shallow, dynamic environment 12 kilometers from the port of Zeeburggee in the Wandelaar region near the main navigation channels on the Belgian part of the North Sea from September 2008 to January 2009.    This region is dredged annually to remove around 1.4 tons of sand and mud and is near three permanent hydro-meteorological stations. There is a strong current and tidal range, and a sandy bottom that hosts both small features like mega ripples (ripples >10 centimeters high or 1 meter in length) and dunes (up to 2 meters in height).  The location is ideal because the researchers were able to study the substrate characteristics and morphology as well as acquire hydrological (wave height, period, and direction, and current velocity) and meteorological (wind velocity and tidal) data from nearby stations, giving them a well-rounded picture of controls on the evolution and dynamics of the Wandelaar region.

The BRMs used in this study were cylindrical tanks (Figure 1) that sit on the sea floor and record variation of sediment presence around the object in time.   The BRMs used in this study were 1.7 meters by .47 meters and weighed 1100lbs (500kg).  They were equipped with a total of 72 bridge sensors split evenly between three sections of the tank into evenly spaced rings (Figure 2).   The presence of sediment was recorded by detecting whether a light bridge could be made between the emitter and receiver pair of a sensor, the idea being that if sediment is present it blocks the sensor.   Measurements were recorded in 15 minute intervals on one BRM and 60 minute intervals on the second; in the three month period of the study 10,000 measurements were made. The pitch and roll of the instrument (how much it’s tilted, how much it’s turned) and percent volume buried are recorded by the BRM.

Burial Recording Mine

Figure 1: Burial Recording Mine

visual description of pitch and roll

Figure 2: visual description of pitch and roll

 

Results

Over the three-month recording period three distinct stages of BRM position and burial were defined and the transition between them can be marked by two major storm events.  The BRMs started off in a neutral position with 20% burial.  After the first and second storm events the neutral position had been compromised and the degree of burial was much greater (near 80% on average). During a high-energy storm the seafloor is scoured and the sediment supporting the BMR was eroded causing a change in pitch and roll.  When a storm ends there is a sudden decrease in energy resulting in the settling of sediment and eventual burial of objects.  The position of the BRMs was relatively constant between the events such that the storm directly affects the position of the object until the next event.

The burial of the object is affected by the degree of deposition at the termination of a storm; however burial does vary with the current and tides independent from storm events.   For example, mega ripples were quickly redistributed by the storms but had a short (couple of days) recovery after the area re-established normal hydrodynamic conditions.   The mega ripple evolution can also be linked to seven cycles of spring-neap tidal transitions: during the neap tides the ripples were less subject to turbulence resulting in minimal sediment erosion and deposition (there was <5% burial variation), while during spring tides when more local turbulence was present there were periods of complete burial.

Conclusion:

In conclusion the position (pitch and roll) of an object on the seafloor depends on the degree of scouring of sediment that occurs during storm events.   The degree of burial depends on the currents, tides, and wave energy in the region and can vary constantly.   BRMs are a very effective tool for obtaining near real-time information about small-scale dynamics of shallow sea beds under different scenarios.  The information gained from this study can be applied to the construction of bridges or jetties, the placement of moorings and buoys, and the modeling of changes in position and burial of any object placed on the sea floor, either intentionally or accidentally.

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