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