A detailed knowledge of the morphology of the continental margin and the incoming oceanic plate is essential for understanding subduction zone processes. These are closely linked to large earthquakes, which can produce tsunamis. In addition, bathymetric data is a necessary pre-requisite for detailed tsunami wave propagation modeling, especially in the shallow water region to predict tsunami wave heights for the coastal regions of western Indonesia. The affected regions can then be evacuated. The selection of adequate locations for the Ocean Instrumentation (Ocean Bottom Unit and GPS-buoy) occurs on the basis of the bathymetric data. Thus extensive bathymetric mapping is necessary in order to be able to create an image of the ocean floor morphology.
Two main systems are available to record bathymetric data for the GITEWS. The Kongsberg-Maritime EM 120 system, installed on the German RV SONNE, is a deep-water multibeam echosounder providing accurate bathymetric mapping up to full ocean depth. The second system, a Seabeam 1050 D shallow- and medium depth water multibeam echosounder for water depths from 0 – 3000m, will be installed on the Indonesian RV Baruna Jaya IV in September 2006 to map the shallow-water regions.
Basic components of both systems are two linear transducer arrays with separate units for transmit and receive. The nominal sonar frequency is 12 kHz for the EM 120 with an angular coverage sector of up to 150° and 191 beams per ping. The Seabeam 1050 D transmits 50 kHz signals for medium depth and 180 kHz for shallow water with a swath width of 150° and 126 beams per ping. For depth measurements, 191 (EM120) resp. 126 (Seabeam 1050D) isolated depth values are obtained perpendicular to the track for each ping. Using the 2-way-travel-time and the beam angle known for each beam, and taking into account the ray bending due to refraction in the water column by sound speed variations, depth is calculated for each beam.
Data are aquired at a constant ships' speed of 10 knots to achieve sufficient bottom coverage along-track and to get data of better quality. Horizontal data resolution is better than 100 m in deep and better than 3 m in shallow water, vertical resolution is better than 0.5% of the depth. Roll- and pitch-calibration as well as water sound velocity profiles, carried out at regular intervals, ensure reliable depth determinations of the multibeam echosounder systems. Processing of the raw bathymetric data is essential to remove artifacts and outliers and therefore improve data quality. Cleaned data allows for a creation of maps with higher resolution.
The bathymetric surveys offshore western Indonesia were carefully planned in order to avoid duplicate mapping of areas mapped during previous cruises. A compilation of all bathymetric data mapped with RV SONNE along the Indonesian continental margin is shown in the figure.