flow mow - a study of hydrothermal heat flux who: russ mcduff, fritz stahr, scott veirs, christian...
TRANSCRIPT
Flow Mow -a study of hydrothermal heat flux
• Who: Russ McDuff, Fritz Stahr, Scott Veirs, Christian Sarason - (UW); Ko-ichi Nakamura - (GSJ); Al Bradley, Dana Yoerger - (WHOI)
• What: measuring heat flux from a whole vent field
• Where: Main Endeavour Vent Field, Juan de Fuca Ridge
• Why: constrain geophysical models, improve knowledge
• When: August 2000 -- among the ROBE crowd
• With: an AUV (ABE) and good sensors
• Funded by: NSF grant OCE-9872090. Thank you!
Motivations
• Constrain geophysical models– 20% of crustal heat flux is through hydrothermal systems
– heat transfer location and quantity critical to ridge models
• Improve heat flux estimates– prior results highly uncertain and disagree
– covered different spatial scales and vent types
• New technology allows new measurements– AUVs available to cover area quickly
– accurate navigation allows more repeatable surveys
– new sensors allow better plume detection
Prior Results - Segment Scale
• 1700 ± 1100 MW Baker & Massoth, ‘87
• 3000 ± 2000 MW Rosenberg et al., ‘88
• 1000 ± 620 MW Thomson et al., ‘92
• All used neutrally buoyant plume: not clear what’s included
Prior Results - Field Scale
• 6550 ± 3000 MW; Thomson et al., ‘92
• 9390 ± 74* MW; Schultz et al., ‘92 * diffuse only
• 154 ± 84* MW; Bemis et al., ‘93 * smokers only
• 364 ± 73* MW; Ginster et al., ‘94 * smokers only
• All extrapolated so highly uncertain, especially diffuse flow
100°C
380°C
Hulk
Puffer
HeatSource
300°C
Kelley and Delaney, 2000
Plan of Attack, part 1 - Measure the buoyant part of the plume
Find best height to “mow” it using a model:
• MTT plumes (point sources)
• conserves mass, momentum, heat, salt
• 200 x 400 m horizontal plane
• sampling with “real” uncertainties
… says 75-100 m is the best!
Plan of Attack, part 2 -Team up with a robust & capable AUV
The Autonomous Benthic Explorer
Plan of Attack, part 3 - put good sensors on ABE
• Temperature - Sea Bird CTDs (ducted & pumped)
• Velocity - MAVS current meter plus vehicle tracking/velocity
• Area - use new “plume-sniffers” e.g., redox potential (Ko-ichi Nakamura)
Execute, part 1 - good background data
• Microtopography (Imagenix on ABE)
• 40- CTD stations (backgrounds, time-series, curtains)
• Currents (mooring, 5 @ 50m intervals)
Execute, part 2 - fly ABE around box
• Fly all sides of a box surrounding the vent field -- obtained one N/S/E/W sides & two further N/S sides
• Because most flux is vertical, fly as many box-tops as possible -- did 12 total
Execute, part 3 - Process Data
Heat-flux (Watts) = [A Cp () w ]
• Area from actual track length x nominal spacing
• Cp from average T, S, P for whole top
from CTD & background measurements
• Vertical velocity (w) from MAVS minus ABE or from ABE dynamic model
from CTD
• Sum for each point over whole track
Contours of ABE-48, Top 1
• heat-flux = 670 MW
4700 4800 4900 5000 5100 5200 5300
5700
5800
5900
6000
6100
6200
6300
6400
6500
Contours of ABE-50, Top 1
• heat-flux = 528 MW
4700 4800 4900 5000 5100 5200 5300
5700
5800
5900
6000
6100
6200
6300
6400
6500
ABE top150 trackline
Results (to date) -Average heat-flux = 588 ± 168 MW
• Calculated from ten tops with background = 0.063 subtracted
• 0.063 from mean of ABE-50 side-walls, represents avg. background heat (more on this later)
• background heat an issue for all prior results
More of the fine print...• Currents are highly variable in time & space• Therefore flux through sides is variable (more on
this later)
0 5 10 15 20 25-4
-2
0
2
4
6Currents during ABE top 251 - 551, Aug 19, 2000
Hor
z. v
el.,
cm
/s
Meter at 100 mab (2118 m depth) above, 50 mab (2168 m) below
0 5 10 15 20 25-4
-2
0
2
4
6
Hor
z. v
el.,
cm
/s
Time in hours from 1218 UTC (MJD 51775.12)
0 5 10 15 20 25
-5
0
5
Currents during ABE top 150 - 250, Aug 17, 2000
Hor
z. v
el.,
cm
/s
Meter at 100 mab (2118 m depth) above, 50 mab (2168 m) below
0 5 10 15 20 25
-5
0
5
Hor
z. v
el.,
cm
/s
Time in hours from 2347 UTC (MJD 51773.991)
Summary of Flow Mow ABE work
• Measured heat-flux over the Main Endeavour Vent Field more precisely than previously
• Using this technique, variations in vent-field scale heat-flux can be tracked in space and time, repeat occupations necessary
• More results coming from– finding N/S side fluxes from combined ABE, CTD and
current meter data
– combined CTD “curtains” and ABE-51 data to look at neutrally buoyant plume flux
Endeavour Segment Currents• Means from July-Oct, 2000 (74 days, hr averages)
• ~5cm/s above ridge crests, intermittently to the SW or W
• ~2cm/s within axial valley, predominantly to the N
Hodographic histograms
• Low velocities to W
• Symmetry at 100mab
• Strong shear between valley and ridge tops
Progressive vector diagrams
• Upper flow to SW, but with NW event, ~20d
• Top 3 ~uniform
• Minimal transport at ridge crest elevation
• Northerly net flow within valley
Regional hydrography, part 1
Near-field hydrography is variable on tidal time scales.
Regional hydrography, part 2
Near-field hydrography is spatially variable on tidal time scales also.
Regional hydrography, part 3
The axial valley is generally thermally contaminated, but even in the Main Field, background water is present.
Northern side of MEF
Southern side of MEF
Calculating side heat-flux
mean mean v q (MW) n (hrs)North
ABE 0.0708 0.0072 50 5Near 0.0810 0.0055 46 6Far 0.0418 0.0027 11 19
SouthABE 0.0664 -0.0088 -62 4
Near 0.0565 -0.0053 -32 9Far 0.0399 0.0083 35 13
where q = mean(i
C * vi m/s) * 1030 kg/m3 …* 3816 J/kg C * (350 m * 75 m);
Ongoing issues...
Flux out the MEF control volume can be calculated by
1) subtracting “background”, and/or
2) integrating fluxes through all sides
Background heat may be 0.063 or less.
Resolving variations in currents is critical to more accurate side wall heat flux estimates