Galparsoro, I.

González, M.

Hernández, C.

Uriarte, A.

UPCommons

Seafloor cartography and characterisation of the basque country inner continental shelf

UPCommons. Portal del coneixement obert de la UPC

INSTRUMENTATION VIEWPOINTSessions73SEAFLOOR CARTOGRAPHY AND CHARACTERISATION OF THE BASQUE COUNTRY INNER CONTINENTAL SHELFGalparsoro I.; C. Hernández; M. González & A. UriarteAZTI-Tecnalia. Marine Research Division. Herrera kaia portualdea z/g. 20110 Pasaia, Gipuzkoa. Spain.Email: igalpar-soro@pas.azti.esKeywords: Earthquake, SW Iberian Margin, turbidite, Synchroneity, interdisciplinary studie Introductionigh quality information about geomorphology and seafl oor charac-teristics of the Basque continental shelf is scarce. Since the fi rst bathy-metric charts were performed by the Spanish Marine Hydrographic Institute in the late sixties, little research has been done. Only in the early nineties, seafl oor characterisation was conducted to locate and characterise sand disposal places for beach regeneration [1], [2]. Due to a growing interest in cartography of the traditional fi shing sites, studies were carried out in 2000 for the Agricultural and Fisheries Department of the Basque Government [3]. These studies show an initial approach to seafl oor classifi cation using the Acoustic Seafl oor Discrimination System, RoxAnnTM and a sidescan sonar.In 2003 the Biodiversity Direction of the Environmental and Territory Management Department of the Basque Country agreed to establish a permanent observatory of the Natura 2000 Network in the Basque Country in order to guarantee fulfi lment of its objectives and its for-mulation. This situation highlighted the necessity of cartographic information from the continental shelf and the identifi cation and characterisation of the most signifi cant marine habitats in the Basque Country. To solve these requirements, a three year project started in 2005 with the principal aim of generating seafl oor cartography and seafl oor characterisation of the continental shelf, the defi nition and delimitation of marine habitats and the identifi cation of the main species of fl ora and fauna associated to each habitat type.The Basque continental shelf (NE of Spain) borders to the East with France (1º46’50’’ W) and to the West with the autonomic community of Cantabria (3º9’13’’ W). The total surface of the study area is approxi-mately 936 km2 (between 5 m and 100 m water depth) and the total length of the coast is 150 km. Bathyemtric dataBathymetric and seafl oor backscatter information has been acquired using high resolution multibeam SeaBat 7125 and SeaBat 8125 sys-tems, having both equipments similar characteristics. Most of the work has being carried out with the latest SeaBat 7125 model. Its op-eration frequency is 400 kHz, produces 256 beams in 128º angle swath and up to 50 swaths per second. Beam width is 0,5º along track and 1º across track producing very small footprints which produces very high horizontal resolution Digital Elevation Models (DEM). Instead of interpolating between a series of survey tracks, dense soundings can be collected from the entire seafl oor and virtually no interpolation is required to produce the details of the physiographic surface [4]. Its vertical resolution is 6 mm and it is fully operative in a range of depths between 0 m and 100 m. All these characteristics make the system fulfi l the IHO’s special order requirements [5]. Apart from the multibeam echosounder, the system is composed of a gyrocompass and motion sensor Octans III, a Trimble DGPS, a sound velocity pro-fi ler and a surface sound velocity sensor.Bathymetric data were acquired and processed using specifi c soft-ware PDS2000. Tide correction was applied using the nearest tidal gage (six stations along the Basque coast) and 1 m resolution sea-fl oor DEM was produced for all the continental shelf (higher reso-lution DEMs are possible to produce in shallow waters if required). Finally, the DEM was exported into ESRI grid format and integrated into a Geographic Information System (GIS) for further topographic features interpretation and analysis.Main seafl oor featuresThe Cantabrian continental shelf is characterised by its narrowness. In the Basque Country, it ranges from 7 km off  Cape Matxitxako, to 20 km off  the estuary of the river Oria. The Cantabrian (Basque) inner shelf is covered by a quasi continuous belt of rocks, which constitutes an extension of the rocks of the continental cliff s. The continuity of this rocky substrate is interrupted regularly by the presence of river mouths, beaches, etc., further off shore the shelf is covered by sandy sediments; these, in turn, isolate the exposed rocky areas of the sea-bed. Exposure to wave energy appears to determine the distribution of sediments. The coast consists of a succession of capes and embay-ments, which have been developed by the continuous action of the waves and rivers within the region. The resistance to erosion of the diff erent materials has controlled the actual morphology.Rocky seafl oorFlysch type formation of rocky substrata is recognisable in subtidal areas. A shallow and high roughness bedrock belt, associated to coastal topography is dominant. It presents approx. a 10% slope, af-ter an infl exion point approximately at 32 – 37 m water depth. Further off shore the platform again extends with a milder slope (oscillating between 1,5 % and 2 %). In this area, seafl oor roughness is lower and sand patches are common between rock strata.Rippled scour depressionsRipples appear distributed along all the study area. They are well de-veloped in subtidal sandy pocket beaches or in sandy areas sheltered by rocky outcrops. The crests appear well developed and they main-tain continuity during tens of meters. They present low undulation with occasional bifurcation, refl ecting their wave origin. Crests are oriented mainly in NO – SE according to predominant wave direction. Rippled fi elds backscatter signal indicate that they are composed by coarser sand than the seabed around.Sea-level change evidencesPrevious studies indicate that, during the last glacial maximum, sea-level was lower than 120 m than present. The main physiographic features that evidence eustatic changes are paleochannels and abrassion platforms. The use of only multibeam data is not enough to describe paleochannels because most of them are covered by re-cent material. In cases where hydrodynamic processes are energetic enough or places where erosion processes are dominant paleochan-nels are possible to identify. In general terms, they present S - N ori-entation. Actual estuaries associated paleochannels are the largest ones identifi ed but other smaller channels can be also distinguished. They have continuity in land with small creeks, which indicates that in former times their water discharge was signifi cantly higher.ConclusionsDue to the high resolution of multibeam system, new seabed features can be now be identifi ed and described. This project will produce base knowledge and the results obtained will be of special interest for integrated coastal management, decision makers, habitat map-ping, description of seabed processes, and studies of climatic change and related impacts.M7INSTRUMENTATION VIEWPOINTSessions74Bibliography[1] Iberinsa. 1990. “Estudio geofísico marino entre la frontera Francesa y Ondárroa”. Ministerio de Obras Públicas y Urbanismo. [2] Iberinsa. 1992. “Estudio geofísico de la Costa de Bizkaia”. Ministerio de Obras Públicas, Transporte y Medio Ambiente. Dirección General de Costas. [3] I. Galparsoro. 2005. “Elaboración de una cartografía sedimentológica detal-lada y de mapas de biotopos marinos del litoral vasco.” Informe inédito de AZTI-Tecnalia para la Viceconsejería de Desarrollo Agrario y Pesquero, Depto. de Agricultura, Pesca y Alimentación, Gobierno Vasco. 28 págs. [4] J. V. Gardner, P. Dartnell, L. A. Mayer and J. E. Hughes Clarke. 2003. “Geomor-phology, acoustic backscatter, and processes in Santa Monica Bay from multi-beam mapping.” Marine Environmental Research. Integrated Assessment of an Urban Water Body: Santa Monica Bay, California 56 (1-2): 15-46.[5] IHO. 1998. “Normas de la OHI para levantamientos Hidrográfi cos”. Publi-cación especial Nº 44. 4ª Ed. Publicado por el Bureau Hidrográfi co Interna-cional. Mónaco. USING CONTROLLED SOURCE EM METHODS TO CONSTRAIN PHYSICAL PROPERTIES OF THE UPPERMOST SEAFLOOR: AN EXAMPLE OF INSTRUMENTATION AND A CASE STUDY FROM A GAS POCK-MARK OFFSHORE IRELANDX. Garcia (1), R.L. Evans (2), X. Monteys (3)(1) Dublin Institute for Advanced Studies,5 Merrion Sq., Dublin 2, IrelandTel: +353-1-6621333 email: xavi@cp.dias.ie(2) Woods Hole Oceanographic Institution, Woods Hole, MA, USA.(3) Geological Survey of Ireland, Dublin, Ireland1. IntroductionThe shallow section of continental shelf is a key interface between Earth and the ocean. This part of the seafl oor provides a record of sedimentary history through the Holocene that can be interpreted in terms of changes in sediment supply and re-working. Important chemical fl uxes pass through shallow sediments and into the ocean, including groundwater in coastal settings and fl uxes of methane in deeper water.  In this paper, we outline how measurements of electrical resistivity are able to contribute to an understanding of the shallow seafl oor. Resistivity provides a fi rst order measure of seafl oor porosity in sedi-mentary settings, allowing facies maps to be constructed, or changes in lithology to be identifi ed. In unconsolidated sediments poros-ity is a key parameter to understanding fl uid transport. Resistivity is sensitive to the salinity of pore-fl uid, allowing identifi cation of fresh groundwater. And fi nally, in areas of active gas seepage, there are often changes in pore-fl uid salinity and temperature that resistivity measurements can identify, while accumulations of massive gas hy-drate are thought to cause an increase in seafl oor resistivity [1]. 3. The towed EM system The towed-electromagnetic (EM) system discussed in this paper consists of three main components, the deck electronics, a transmit-ter, and the receiver string. The seafl oor components of the system (transmitter and receivers) form a 40 m-long array which is towed in contact with the seafl oor at speeds of 1-2 knots. The EM transmitter, a horizontal magnetic dipole, generates harmon-ic magnetic fi elds over a range of frequencies (~200 Hz – 200 kHz), and the three receivers, tuned to measure these magnetic fi elds, are towed at fi xed distances behind (4m, 12.6m and 40m). At a given fre-quency the strength of magnetic fi elds decays away from the trans-mitter as a function of the conductivity of the seafl oor (i.e., according to the skin depth), decaying more rapidly in more conductive media. The sensitivity of the magnetic dipole-dipole system, along with the physics of the propagation of the fi elds through the seafl oor was pre-sented in [2]. Further details of the system are given in [3].Because the system maintains a fi xed distance between source and receiver, it can be regarded as a mapping tool. In order to build up a map of sub-seafl oor structure only relatively sparse coverage is needed [4]. The resulting maps provide superior spatial coverage than conventional coring techniques and can measure porosities in regions where coring techniques fail to recover samples, but more importantly provides a means of interpolating between discrete core locations.  Finally, the method provides estimates of physical prop-erties where seismic refl ection profi les are contaminated by strong bottom multiples or the presence of biogenic gas [5]. The system is, however, perfectly complementary to seismic methods and is best used in concert with high resolution seismic refl ection techniques which defi ne the stratal geometry while the EM data defi ne the phys-ical properties [7].3. The Malin sea experimentIn 2006, we ran the system across an area of gas seepage in the Malin shelf area, off shore Ireland. Within the survey area, which generally has very fl at bathymetry, more than 220 pockmarks, related to gas transport through the seafl oor, are distributed in clusters around the main structural lineaments. Seismic data show disturbances in the sediments beneath pockmarks to depths of about 80m. The objec-tive of the project is to use a combined acoustic and electromagnetic geophysical approach to study the near-seabed composition in a known shallow gas bearing area. The combination of these various methods should enable us correlate the main geophysical signatures and the geological properties of the seabed, providing a unique tool for geohazard identifi cation, seabed classifi cation, fl uid fl ow migra-tion paths and sediment porosity. As an example of the type of data collected, we show EM coverage across a major depression and pock mark fi eld in Figure 1. The data are presented as apparent porosities using Archie’s law to convert half-space inversions of the raw data on each receiver – this is how data appear in real-time on board the ship. Porosities across the re-gion are remarkably consistent, with very little variation. However, there are subtle changes observable as the system enters a pock-mark. The transition into the basin is marked by an increase in porosi-ty, most likely representing looser unconsolidated pelagic sediments. More detailed examination of the porosity profi le shows a complex structure as the system enters a small pockmark. The 40m and 13m receivers show opposite behaviours, with data on the 40m receiver dropping slightly in value while the 13m receiver records slightly higher values. M7

Seafloor cartography and characterisation of the basque country inner continental shelf

Abstract

Similar works

Full text

Available Versions

UPCommons

UPCommons. Portal del coneixement obert de la UPC