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Atlantis II Dye Experiment

Tracer tests involve releasing some sort of dye at a source and tracking its dispersion over time. In industry, they are often used to find leaks. In environmental studies, the tracer test is an excellent method of mapping subsurface water flow.

Factors that affect subsurface water flow are varying flow media (different kinds of rocks, strata, fractures, etc.) and chemical interactions between solutes and the media. It is difficult to directly observe these factors, so the tracer test is a key tool for geologists and environmental engineers.

There are at least six types of commonly used tracer tests:
  • Single well: yields information about specific flows
  • Multi well: often used to study convergent flow
  • Multi-level: determines the vertical flow
  • Insertion into solid core rock: used to study fracture transport
  • Insertion into porous rock: used to study fast transport
  • Shallow insertion: used to study capture zones

    • Simulation of tracer distribution after injection into a heterogeneous medium.

    (source http://www.sandia.gov/eesector/gs/gh/tracer.html)

    According to our mentor, Dr. Hart, "a tracer release experiment would be very feasible as a way to learn about water movement. I've never understood why no one has ever done this yet!" Although there are enormous difficulties with conducting a tracer experiment, the potential data yield is equally great. Some such problems involve finding downflow zones (so that the dye will penetrate rather than return to the surface), interpreting results indicative of extremely complex currents, and establishing a method to trace the dye. Also, how can the dye be injected? What can be learned by releasing the dye above the seafloor?

    In order to use the minimal amount of dye possible, visible detection (just "seeing" the dye) will not be an option. Fluorometers, installed in AUV's and in a sensor array, will be able to detect concentrations of the dye in desired areas. Detection limits for fluorometers can go all the way down to 20 picograms of dye per gram of water.

    Procedure

  • Before dye can be released, two things have to happen: a preliminary survey of the area to determine potential injection and release sites, and the setup of the sensor array. Since the main focus of the tracer test is to gain information about diffuse hydrothermal fluid flow, a variety of sites will have to be used.

  • Find Downflow Zone-this is an area that is ideal for dye injection below the surface. If we hit an upflow zone, or worse yet, a zone without much activity, the dye probably won't achieve a desired distribution
  • The dye will be both injected into the ground and released from point sources 50m above the seafloor. The dye that will be used is Rhodamine WT, a commercially available, nontoxic fluorescent dye that comes in a variety of colors that can be distinguished by a fluorometer. Since large volumes of water are involved, the liquid form of the tracer should be used at a ratio of about 1 pound per 40,000 gallons

    • Example dye injection unit

    (source http://www.sandia.gov/eesector/gs/gh/tracer.html)

  • Since tracer tests require significant amounts of time to carry out (about 60 days for most dye to diffuse, depending on flow rates), the types of injected tracer tests have to be prioritized. Since diffuse hydrothermal flow is the main subject of study, multilevel insertion will probably yield the most valuable information.
  • Releasing the dye at a point source will be much simpler then injecting it. However, the problem will be detecting it-the dye will diffuse much more quickly. To solve this, remote AUV's will be used more than the sensor arrays since they can cover more area.

    • Sources of Error

  • Unrecorded currents
  • Poor injection sites (upflow zones)
  • Disturbance of water flow due to AUV, MV activities
  • Dye pH variance (the intensity of dye can vary with pH. The solution to this problem is to couple pH readings with fluorometer readings, establish a curve in the lab -similar to Gas Chromatograph analysis using standards to establish the curve-, and use that to interpret results.