Detailed 2-D imaging of the Mediterranean outflow and meddies off W Iberia from multichannel seismic data
Introduction
Low frequency water column reflections have been reported rarely in the scientific literature (Hunt et al., 1967, Gonella and Michon, 1988) and were related for the first time to fine-scale structures by Holbrook et al. (2003). These authors have shown that fine-scale O(30 m) sound velocity structure in the ocean can be imaged using standard marine seismic reflection techniques. Multichannel seismic (MCS) imaging shows thermohaline and density fine structure over much of the ocean depth (Géli et al., 2005). Although conventional industry multichannel seismic data are not as accurate as direct CTD measurements, they allow near-synoptic two-dimensional imaging of detailed two- and three-dimensional patterns with a vertical resolution approaching 10–15 m and a horizontal resolution of the order of several tens of meters (see Section 3.1), providing a highly informative snapshot of thermohaline features at a specific time (Ruddick, 2003). Acoustic imaging thus complements more accurate but far less detailed and synoptic CTD profile measurements from ships. The high horizontal coverage allows individual reflections to be tracked over great horizontal and vertical distances, including steeply-dipping features which would otherwise not be easily tracked using CTD profiles alone.
Considering that a large number of high quality multichannel marine seismic lines have been acquired in the past all around the world, there is a large source of data readily available that, when reprocessed, can help to characterize the past oceanic thermohaline structure with unprecedented detail and become a useful tool for physical oceanography. Recent work has confirmed the usefulness of using such conventional multichannel seismic reflection profiles, commonly used in the oil industry and in continental margin research, for imaging the thermohaline structure of the oceans (Holbrook et al., 2003, Nandi et al., 2004, Géli et al., 2005, Biescas et al., 2008). A new discipline, “Seismic Oceanography”, is emerging as an exciting new research tool of high potential interest for better understanding the ocean circulation.
In investigating the relationship of synthetic seismograms to CTD observations, Ruddick et al. (2009) show that seismic images are approximately images of vertical temperature gradient on the scale of the acoustic source wavelet (~ 16 m for the data shown here). This makes the interpretation of the seismic data complex because it is not measuring the commonly observed variables in the ocean, such as temperature or conductivity, but instead it provides smoothed images of their vertical gradients. Salinity gradients can contribute ~ 15% or more to the reflectivity, but since the salinity gradients are highly correlated with temperature gradients, the amplitude but not the appearance of the seismograms is affected by salinity. Thermohaline “fine structures” are well-known in the ocean (c.f., McKean, 1974), and are associated with a variety of physical phenomena: internal waves, thermohaline intrusions, double-diffusive layering, mixed water patches, vortical modes, and others. While it is the fine structures that are imaged, the water mass boundaries (where thermohaline contrasts are strongest) are outlined by seismic imaging.
Similar to the revolution caused by satellite imagery, the high-resolution, near-synoptic nature of the images shows the relationships between the fine-structure processes and the larger-scale features (fronts, eddies, etc.) and allows new insights into the causes and effects of oceanic mixing. The field is so new that we don't yet have a “catalog” of verified oceanic features that would allow us to unambiguously interpret other imaged sections by recognizing similar patterns. One way to attain this goal is to augment new MCS surveys with concurrent oceanographic observations that allow direct comparison with the images (as is currently being done within the Geophysical Oceanography (GO) project, funded by the European Union FP6 Programme). A second approach, adopted here, is the reprocessing and analysis of existing “legacy” MCS data, followed by the identification of features of interest, generation of hypotheses about their oceanic nature, and (the most critical stage) testing of those hypotheses against available oceanographic observations. While not as satisfactory as the first approach, it is vastly less expensive, and yields new oceanographic information from existing data.
In this paper we show the results of the reprocessing of a 326-km long deep multichannel seismic line acquired in the Tagus Abyssal Plain, off W Iberia, in 1993, in the scope of the Iberian Atlantic Margins (IAM) Project (Torné et al., in press, Banda et al., 1995). The processing was aimed at enhancing the reflections in the water column while preserving the amplitudes and the reflection characteristics as much as possible. Several clear reflections were revealed in the water layer, showing exciting images of the Mediterranean Undercurrent (MU), meddies, internal waves, and the interfaces of the high-salinity tongue of the Mediterranean Water (MW) with the North Atlantic Central Water. The interpretation of these features was constrained and confirmed by subsurface float measurements, Sea Level Anomalies (SLA) and Sea Surface Temperatures (SST), obtained during the period of acquisition of the seismic line. A draft version of this work was presented at a special session on Seismic Oceanography held at the 2006 AGU Ocean Sciences Meeting (Pinheiro et al., 2006).
In Section 2 we review the oceanographic water mass properties of the region, largely originating with the Mediterranean outflow. In Section 3 we describe the basic principles, data acquisition and processing of the seismic imaging. The main features imaged are described in Section 4, and the complementary data utilized are outlined in Section 5. The features are interpreted, using the images and complementary data, in Section 6. Finally in Section 7 we discuss the physical oceanographic mechanisms that create the seismic reflections that image the larger-scale features, put forth a new theory for formation of steeply-sloping reflectors and list our conclusions.
Section snippets
The Mediterranean outflow and meddy formation off W Iberia
In the northeast Atlantic, the Mediterranean Water (MW) is associated with anomalously high salinities and temperatures contributing significantly to the hydrological properties and circulation of the intermediate layers (Ambar et al., 1999). A tongue of this high-salinity Mediterranean Water was detected to the west of the mid-Atlantic ridge (e.g., Reid, 1994, Lozier et al., 1995) and it is one of the most prominent hydrographic features of the mid-depth North Atlantic (Reid, 1978). This salty
A brief introduction to the seismic reflection method
Multichannel seismic imaging is a type of echolocation in which band-limited sound emitted from air guns towed near the ship, is reflected from discontinuities in acoustic impedance (sound velocity times density), and recorded on a large number of hydrophones towed behind the ship. Returns from each reflector appear with time delay related to the source-hydrophone path, and also appear in returns from subsequent air gun “shots”. The “Common Mid-Point” method is used to “stack” these returns
Description of the main features observed on the seismic profile
The final processed section (Fig. 2) shows images of the thermohaline structure along a 326-km long, roughly E–W continuous transect across the Tagus Abyssal Plain and the adjacent continental slope (see also Fig. 3, Fig. 4, Fig. 5, Fig. 6).
As regards the vertical distribution of reflectivity, three main layers with different reflection characteristics are observed in this line (Fig. 2): (1) an upper layer, down to approximately 0.6 s (roughly 450 m) characterized by moderate to weak reflections,
Complementary data used for the interpretation
In order to interpret the observed features we did a combined analysis of data from subsurface float measurements, Sea Level Anomalies (SLA) and Sea Surface Temperatures (SST). For the subsurface float measurements, we used the compilation of 37 meddies and cyclones tracked by floats in the Atlantic in the period between 1993 and 1994, produced by Richardson et al. (2000), and used some detailed position information from float 110 from the AMUSE experiment (Bower et al., 1997) and from float 39
Interpretation
Line IAM-5 allowed detailed imaging of the complex mixing processes that occur between the high temperature and salinity MW and the N Atlantic water masses (e.g. Armi et al., 1989, Serra et al., 2005). In particular, several lens-like structures were identified, which are interpreted here as meddies and cyclones. In this section, the available evidence that supports this interpretation is presented and discussed.
Oceanographic mechanisms of reflector formation
It is evident from the images and discussion above that, as also shown by other authors (e.g. Holbrook et al., 2003, Géli et al., 2005, Biescas et al., 2008), oceanic structures such as meddies, cyclones, fronts and currents are outlined by reflection from fine-structure acoustic impedance anomalies. In this section we discuss the physical mechanisms that could create oceanic fine structures and reflectors.
Fine-scale variations in sound speed can be divided into two distinct types. The first,
Conclusions
Results from the reprocessing of a roughly E–W multichannel seismic profile that extends from the continental slope of W Iberia almost to the western limit of the Tagus Abyssal Plain reveal the vertical and lateral variations of the thermohaline structure in the water column in this area with a high detail along a continuous 326-km long section. They image the complex structure of the high-salinity tongue of the Mediterranean Water and mesoscale features.
Two well-defined lens-shaped structures
Acknowledgements
LMP thanks Louis Geli (Ifremer, Brest) for introducing him to Seismic Oceanography, and for highlighting the potential of the seismic method to image the fine thermohaline structure in the ocean. This research project is part of the European funded GO Project (Geophysical Oceanography — FP6-2003-NEST 15603) for which the authors warmly acknowledge the support and collaborations within the research team, in particular to Luis Matias for excellent suggestions concerning some of the seismic
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