HR: 16:50h
AN: OS42B-03    [Abstracts]
TI: Suitability of multichannel seismic systems for imaging the internal structure of the water column
AU: * Sallares, V
EM: vsallares@cmima.csic.es
AF: Marine Technology Unit, CMIMA-CSIC, Passeig Maritim de la Barceloneta, 37-49, Barcelona, 08003, Spain
AU: Biescas, B
EM: biescas@cmima.csic.es
AF: Marine Technology Unit, CMIMA-CSIC, Passeig Maritim de la Barceloneta, 37-49, Barcelona, 08003, Spain
AU: Carbonell, R
EM: rcarbo@ija.csic.es
AF: Institute of Earth Sciences "Jaume Almera", CSIC, Lluis Sole i Sabaris, s/n, Barcelona, 08028, Spain
AU: Danobeitia, J
EM: jjdanobeitia@utm.csic.es
AF: Marine Technology Unit, CMIMA-CSIC, Passeig Maritim de la Barceloneta, 37-49, Barcelona, 08003, Spain
AU: Hobbs, R
EM: r.w.hobbs@durham.ac.uk
AF: Department of Earth Sciences University of Durham , South Road Durham DH1 3LE, Durham, United Kingdom
AB: Seismic oceanography is slowly becoming a popular tool to investigate the internal structure of the water column. The principle of this technique is that the energy generated by seismic sources is partially reflected at the boundaries between water masses with contrasting physical properties. The reflected wavefield is recorded and processed to create continuous images of these boundaries. Since the pioneer work of Holbrook was published (Holbrook et al., 2003), numerous papers have appeared showing the potential of seismic oceanography to image the ocean's fine structure with unprecedented lateral resolution (10 m), the spatial coincidence of seismic reflectivity and temperature/salinity contrasts, and the correlation between seismic reflections and internal wave spectra. Despite the relatively large amount of recent work, little has been done concerning the existing issues to adapt seismic systems to oceanographic research. In this work we present a set of basic synthetic tests to illustrate the relative significance of different parameters for imaging the oceanic fine structure using seismic methods. The parameters considered include the frequency content and energy of the source wavelet, the ambient noise level, as well as the shooting rate, signal redundancy and fold. We show that powerful (>200 dB re 1 microPa), low-frequency (20-60 Hz) sources such as those commonly used in deep seismic soundings (DSS) are, purposelessly, well-suited to image also the oceans fine structure at all depth ranges. The reason for this is that, on one hand, the acoustic impedance (i.e., reflection coefficients) associated to intra-oceanic boundaries are two orders of magnitude smaller than those associated to geological boundaries (10-3/10-1), so it is crucial to use energetic sources to overcome ambient noise regardless of the target proximity. On the other hand, the limits between water layers, in contrast to the geological ones, do not show abrupt impedance contrasts but rather smooth gradients within boundary zones of several tens of meters, so the dominant wavelength of common DSS sources (about 25-75 m) is suitable to image them. In addition, we show that for a given system layout one can define the optimal shooting rate that gives the best possible signal-to-noise ratio by taking advantage of system redundancy but at the same time allowing background seismic noise due to repeated shooting to mitigate. Holbrook, W. S., P. Paramo, S. Pearse, and R. W. Schmitt (2003), Thermohaline fine structure in an oceanographic front from seismic reflection profiling, Science, 301, 821- 824.
DE: 3025 Marine seismics (0935, 7294)
DE: 4259 Ocean acoustics
DE: 4271 Physical and chemical properties of seawater
SC: Ocean Sciences [OS]
MN: 2007 Joint Assembly