chc-nsc 2018, victoria, bc, march 26-29 quantifying the ......conclusions: internal wave activity is...
TRANSCRIPT
Quantifying the Impact of
Internal Wave Activity on
Multibeam Bathymetry
John E. Hughes Clarke Kevin Sheehan,Vera Quinlan,
Fabio Sacchetti
CHC-NSC 2018, Victoria, BC, March 26-29
65.5m - 2425 tonnes• EM2040S• EM1002• EM302• EK60 – 18/38/120/200
RV Celtic Explorer
RV Celtic Voyager
31.4m - 340 tonnes• EM2040D• EA60 – 38/200
CE-2018
CV-2018All Celtic Sea
7 yearprogram
SeasonalStratification
Holt and Umlaut, CSR 28 (2008) 887-903
Section of temperature (line contours) and chlorophyll concentration (colours)Sharples, et a.l , 2013, Progress in Oceanography, 117, 1-8
Distance (km)
Dep
th (
m)
Jun – AugMean TPronounced
Thermocline(Velocline)
Perturbed byInternal Waves
IW gen.
IW gen.
SubsurfaceChlorophyllMaximum
The seasonal onset and decay of thermal stratificationSound Speed
CE -1702Sept./Oct.
2017
Sound Speed
CE -1701July/August
2017
A Canadian Problem too!Summertime Stratified Continental Shelves
Internal Wave Activity(~+/-10m)
Scotian Shelf - (2012) Grand Banks - (2012) Gulf of St. Lawrence - (2012)JAN DEC JAN DEC JAN DEC
10 km+ wavelength – tidal periods
500m -5 km
20m -200 m
2 hours – 300m offset
30km survey lines
Labadie Bank
Labadie Bank
Static BiasDue to vertical displacement
Slowly Changing BiasPrimarily displacement, minor slope
Rapidly Changing BiasMinor displacement, predominantly slope
10 km
Depths65- 135m
Marine Institute: 20% overlap
CHS : 100% overlap
4 consecutive lines – showing evolution of soliton packet, triggered from bank top
Initiation of Soliton Packet
This wave appears to have been triggered from the nose of a lateral intrusion
along the thermocline. The nose was crossed at 02:26:40 (July 24th, JD205).
EM2040 Vertical Curtain (200 kHz)
80-120m water depth
Initiation1.5 hours later
4 hours later
40 min. later
1000 m
1000 m
50m
100m
Second View of Packet (Wobble A)
This occurred at 04:06:26 on July 24th (JD205) as the vessel
was steaming west at ~ 7.5 knots
+/-0.75% Z anomalies
horizontal slice through water column at 40m depth
Next line : 1.5 hours later
1000 m
1000 m
50m
100m
Third View of Packet (Wobble B)
This occurred at 07:56:44 on July 24th (JD205) as the vessel was steaming back to the east
at ~ 8 knots. About 3 hours later as we had to go to the west end of the line and return.
Next line : 4.0 hours later
1000 m
1000 m
50m
100m
finitethickness
ORIGINALMODEL
Developed2017
1. Wavelength2. Amplitude3. Azimuth
+4. Thickness
* Relative to oblique path length
Step VeloclineGradational VeloclineVariationWith
Wavelength
Long*Wavelength
Short*Wavelength Multiple
Surface Normals
SingleSurface Normal
I.W. Wavelength(projected across track)
I.W. Wavelength(projected across track)
VariationWith
Obliquity
Ratio of Oblique ray-trace distance through
thermocline
A directional filter: -- only acts on
apparent wavelength,
projected across track
Abrupt Interface
Gradational Interface(finite thickness)
MultipleSurface Normals
SingleSurface Normal Pronounced
Outer-swath anomalies
AttenuatedOuter-swath anomalies
Impacton
SeafloorArtefact
Expression
8Representative
examples
SHORTWAVELENGTH
LONGWAVELENGTH
1000m
1702_D
1702_C
1701_B
1701_G
1701_D
1702_A
1702_B
1701_C
51m+/-10m
40m+/-10m
30m+/-10m
34m+/-10m
36m+/-10m
51m+/-10m
41m+/-10m
38m+/-10m
10
0
dep
th
0
10
0
dep
th
0
1000m
1000m
HorizontalWater
ColumnSlices
to identify:Depth
WavelengthAzimuth
?
?
1000m
-2m
+2m
-2m
+2m
-2m
+2m
20 m/s
11
m = 140 m, azi 065°
A ~= +/-5 m
20 m/s
15
m = 40 m, azi 075°
A ~= +/-2 m
15m 10m 5m
10
m
5m
13 m/s
5 m
= 190 m, azi 035°
A ~= +/-5 mA ~= +/-2 m
= 19 m, azi 088°
13 m/s
5 m
= 20 m, azi 070°
A ~= +/-1 m ?
Shortest Wavelengths developed on steepest
veloclines – worst morphologic degadation
Conclusions:Internal Wave Activity is a major concern for seasonally stratified shelf seas
(Celtic Sea – North Sea – Scotian Shelf – Grand Banks – Gulf of St. Lawrence)
• 5 km + wavelength canbe addressed by MVP
• 0.5 to 5 km requires means of tracing thermocline acoustically -see Jose Cordero Ros – next presentation
• < 0.5km wavelength are tricky to identify
but are the prime cause of false morphology.
Numerical modeling of Internal Wave azimuth, wavelength, amplitude and layer thickness well describes observed relief.
Can infer Internal Wave Activity and Dimensions1. Wavelength2. Amplitude3. Azimuth4. Thickness
A
dz
Recommendations:
• Hydrographers need the tools/training torecognize these features
• Avoid the oceanographic conditions.
• Adapt survey to respond
Marine Institute: 20% overlap
CHS : 100% overlap
2018 operationsApril-May
BEFORE/DURINGthermocline development