Proceedings of Meetings on Acoustics

Volume 19, 2013 http://acousticalsociety.org/

ICA 2013 Montreal

Montreal, Canada

2 - 7 June 2013

Acoustical OceanographySession 2aAO: Seismic Oceanography

2aAO5. Mapping turbidity layers using a combination of high resolution seismicoceanographic and physical oceanographic dataEkaterina A. Vsemirnova and Richard W. Hobbs*

*Corresponding author's address: Dept. Earth Sciences, Durham University, Durham, DH1 3LE, United Kingdom,r.w.hobbs@durham.ac.uk Synchronised seismic and oceanographic data were acquired during the GO (Geophysical Oceanography) project cruise in the Gulf of Cadizin April - May 2007. The small volume (117 cu-inch) mini GI-gun seismic source used during the GO experiment provided high resolutionseismic data which revealed features of the internal structure of the ocean. The seismic acquisition design gave a usable bandwidth of 50-250Hz with a vertical resolution of 1.25 m which is similar to that achieved by co-located CTD casts. We focus on the reflections observed onseismic data covering an area to the east of the Portimao canyon in water depths of 600-800 m. To test the hypothesis that measurablereflections can be generated by suspended sediment we perform forward modelling of seismic response based on the temperature, salinity andsediment concentration derived from light attenuation measurements, available from CTD casts. Modelling based solely on temperature andsalinity data show that thermohaline structure does not always explain reflections in water column, but these discrepancies are resolved whenthe contribution from suspended sediment is included.

Published by the Acoustical Society of America through the American Institute of Physics

E. Vsemirnova and R. Hobbs

© 2013 Acoustical Society of America [DOI: 10.1121/1.4799124]Received 22 Jan 2013; published 2 Jun 2013Proceedings of Meetings on Acoustics, Vol. 19, 005012 (2013) Page 1

Introduction

Turbidity layers are some of the largest sediment-laden underflows that occur in ocean basins. They are typically defined as relatively dilute flows in which particles are dominantly supported by fluid turbulence with sediment volume concentrations of <~10 % [Sumner et al., 2009]. Turbidity layers play an important role in transporting fluvial, littoral and shelf sediments into deep ocean environments. They may be sourced from sediment-laden river flow cascading down submarine canyons, slope failure, or by the remobilization of unconsolidated sediment by strong currents. Experimental studies on turbidity layers are available [e.g. Middleton, 1966; Sumner et al., 2009], however natural turbidity layers and other sediment laden transient currents are hard to observe and study, due to their irregular occurrence and often destructive nature [Hay, 1987]. Recent research has shown that using a combination of seismic oceanographic and physical oceanographic data it is possible to map turbidity layers [Vsemirnova et al., 2012]. Some near sea-bed reflections recorded on seismic data which are not explained by thermohaline structure are thought to be directly observed turbidity layers.

Area of Interest

A combined seismic and oceanographic dataset was acquired in the Gulf of Cadiz (SW Iberian peninsula) in April-May 2007. The warm and salty Mediterranean Outflow Water (MOW) spills out of the Strait of Gibraltar, sinks and flows westwards along the continental slope of the south Iberian margin, forming the Mediterranean Undercurrent (MU) [Ambar et al., 2002]. The target for the GO experiment was to examine the MU in the Gulf of Cadiz at the point where it achieves neutral buoyancy and forms a distinct layer in the North Atlantic. The region was chosen for three reasons: extensive legacy datasets of both oceanographic and seismic data; variety of oceanographic data; variety of oceanographic features including margin slope, rough sea-bed topography and large eddy currents; and the strong oceanographic signature of the Mediterranean water [Hobbs, 2009].

Data Sets

The GO project provided an integrated physical oceanography and seismic imaging survey with coincident and collocated XBT/XCTD sampling to examine thermohaline fine structure. Moorings were deployed on the margin to the east of the Portimao Canyon investigate turbidity layers and mixing [Alvarado-Bustos et al., 2009] and, for this study, the seismic data were obtained with the small volume mini GI-gun seismic source [Hobbs et al., 2009, Geli et al., 2009].

Seismic Reflection

The seismic line GO-HR-008 (Figure 1) was acquired to map the structure of water column. The acquisition used a 117 cu in air-gun source optimized for high frequencies (bandwidth 50–250 Hz, centered around 150 Hz) and a 600 m long streamer with 72-channel hydrophone receiver array (group spacing of 6.25 m) towed at ~2m depth.

The acquisition configuration produces a detailed image of the water column and reveals bands of reflectivity at 750m which correlates with the boundary of MU as it flows along the slope (Figure 1) The processing sequence was specifically designed for water column and included Butterworth filtering, spatial binning, removal of the direct arrival, NMO with water velocity 1510 m/s, stack and migration [Geli et al., 2009].

Discontinuous reflections can be traced as thin layers at depths of 300-500 m, on the western slope and along the top of the margin. These will be investigated further using forward modelling routine.

Physical Oceanography Datasets

In this study a 25 cm Alphatraka transmissometer was interfaced with SeaBird CTD for a series casts above Iberian margin.

The CTD data (Figure 2 a, b) shows no significant changes in either temperature or salinity at the depth of 300-500 m and in the proximity of the seabed. However, there is evidence of a change in suspended sediment from transmissometer readings at those depths (Figure 2c).

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FIGURE 1. Processed high-resolution section over the moorings area showing top of MU water as it flows along the slope (reflections at ~ 800-1000 ms/600-900 m depth). Picture courtesy of Dr. D. Klaeschen (IFM-GEOMAR, Kiel, Germany).

The addition of suspended sediment will change the bulk average sound-speed and density which we propose cause the changes in impedance necessary to produce the observed seismic reflections which we can test through modelling.

Modelling

To test our hypothesis that measurable reflections can be caused by suspended sediment we perform forward modelling of seismic response based on the temperature, salinity and light beam transmission measurements from CTD cast (Figure 2 a, b and c). Initially, we compute the background sound-speed and density profiles using the UNESCO equations of state [Fofonoff and Millard, 1983] assuming no suspended sediment (Figure 2 d, e; blue line). From these profiles we can calculate the vertical derivatives [Ruddick et al., 2009] which, when combined to form the impedance contrast and convolved with the seismic source wavelet, will produce a seismogram similar to that observed experimentally.

The modelling routine is then repeated but this time including the effect of the sediment load (Figure 2 c) with the depth dependent density and sound-speed modified using the equation � � � � � � � �,1 sedimentwatermod zzz ����� ��� (1)

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Proceedings of Meetings on Acoustics, Vol. 19, 005012 (2013) Page 3

were � is the fraction of sediment by volume and � represents either the sound-speed or density [Vsemirnova et al., 2012]. We assume the sediments on the slope would be largely composed of quartz with bulk values of

1sm6000 � and 3mkg2700 � for sound-speed and density respectively.

(a) (b) (c)

(d) (e) FIGURE 2. (a) temperature, (b) salinity and (c) beam transmission readings from the CTD cast coinciding with location of seismic section on the Figure1. The transmission readings were despiked and smoothed prior to further calculations. (d) corresponding sound-speed profile without (blue) and with (red) addition of suspended sediment (assumed to be quartz). (e) the vertical gradient of sound-speed again without (blue) and with (red) suspended sediment. The effects of intrusions are highlighted by arrows.

The fraction of sediment (� ) is quantified from beam transmission measurements. The data for beam

transmission were corrected for factory and field calibrations. Beam transmission was converted to beam attenuation coefficients using equation [Gardner et al., 2005]:

,100

ln1�

� �

�

� ���

Trr

c (2)

were c is the beam attenuation coefficient, r is beam path length, and Tr is beam transmission readings. The amount of dissolved salt in the water does not affect the transmission of light through it. Transmission through particle-free fresh water and salt water is the same [Richardson & Gardner, 1997]. So measurements of beam transmission are not affected by changes in ocean thermohaline fine structure, but in order to determine the

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attenuation specific to particulate matter, the attenuation due to water ( 1water m364.0 ��c for SeaBird

transmissometers) must be subtracted from the beam attenuation coefficient values: .watersediment ccc �� (3)

Because during GO cruise no dry particulate matter measurements was collected, we used calibrations obtained

during the United States Joint Global Ocean Flux Study (JGOFS) program [Gardner et al., 2005] and assessed results using typical nephelometer profiles for Atlantic Ocean [Biscaye & Eittreim, 1977].

FIGURE 3. Normalized amplitudes of reflections within MOW. Blue line assumes that there is no suspended sediments whereas red line includes affects of suspended sediment. En enhanced reflectivity at 200-600 ms corresponds well to that observed on seismic section in Figure1.

Results

The result (Figure 3, red curve) shows normalized amplitudes for synthetic seismic trace calculated over profile with the addition of the suspended sediments. A band of strong reflectivity can be observed from 700 ms to 800 ms two way traveltime, which equates to depths of 525-600 m. This correlates well with the temperature and salinity change between MOW surface and undercurrent waters. En enhanced reflectivity observed in the upper layer for profile with addition of the sediments.

FIGURE 4. From [Vsemirnova et al., 2012]. Detail of seismic section focused on the near-bed reflections between 65 and 82 km of FAST profile which traverses the whole width of Faroe-Shetland Trough.

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The difference to the sound-speed and density profiles caused by the addition of calculated the suspended sediment are small (Figure 2 d, example for sound-speed). However, the absolute values of density and sound-speed are not important as the seismic reflection is only sensitive to the change over depths that are shorter than the wavelength of the seismic signal. So the reflectivity potential is revealed when gradients for sound-speed and density are computed (Figure 2 e). The MU interface is very close to the sea-bed in GO-HR-008 section, so it is difficult to observe near sea-bed reflections, but the addition of the sediments probably contribute to reflectivity at the depth. Recent research [Vsemirnova et al., 2012] investigated combined seismic and oceanographic dataset and found that the impedance contrast in water column is strong enough to be seen on seismic sections. They focused on near-bed reflections observed on FAST profiles [England et al., 2005], (Figure 4).

Conclusions

Our investigations here and in another study in the Faroe-Shetland channel [Vsemirnova et al., 2012] show that an impedance contrast in water column caused by turbidity layers is strong enough to be seen in seismic sections. This provides a new way to visualize this type of current to assess its dimensions and lateral structure.

ACKNOWLEDGMENTS

GO data acquisition was funded by the 6th Framework Program for Research and Development of the European Community (grant FP6-NEST-15603). Final plots were made using Matlab, Seismic Unix [Cohen & Stockwell, 2010] and Generic Mapping Tools (GMT).

REFERENCES

Alvarado-Bustos, R.; Huthnance, J.; Krahmann, G.; Hobbs, R. (2008) “Mixing in the Gulf of Cadiz using XBT data”, In: ALSO/AUG/TOS Ocean sciences meeting 2008: from the watershed to the global ocean, Orlando, Florida, USA.

Ambar, I., N. Serra, M.J. Brogueira, G. Cabecadas, F. Abrantes, P. Freitas, P. Goncalves, & N. Gonzalez (2002). “Physical, chemical and sedimentological aspects of the Mediterranean outflow off Iberia”, Deep Dea Res., Part II, 49, pp. 4163-4177.

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England, R.W., McBridge, J.H., & Hobbs, R.W. (2005). “The role of Mesozoic rifting in the opening of NE Atlantic: evidence from deep seismic profiling across the Faroe-Shetland Trough”, J.Geol.Soc., 162, pp. 661-673.

Fofonoff, P., & R.C. Millard Jr. (1983). “Algorithms for computation of fundamental properties of seawater”, Unesco Technical Papers in Marine Science, 44, Unesco.

Gardner,W.D., J. Gundersen, & M.J. Richardson (2005). “Beam Attenuation Coefficient, Light Scattering, Fluorescence protocols”, Texas A&M University, U.S. JGOFS, (http://usjgofs.whoi.edu/PI-NOTES/arabian/Gardner-beamcp.html).

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