Seismic methods

Marine seismic measuring methods

Marine seismic measuring methods belong to the main components in marine research for the investigation of the marine resource potential and hazard assessment, for example by earth slides, tsunamis or the release of greenhouse gas.

For bathymetric investigations, normally the ship’s built-in systems are used, for reflexion seismic and refraction seismic investigations, the measuring systems are installed directly by BGR. BGR has modern and technically mature systems for data acquisition as well as for the following steps of data processing and interpretation. Below, after a short delineation of the measuring method, the specific system-blocks for seismic data acquisition and processing are described:

Reflexion seismics

In reflexion seismic data acquisition, the seismic waves reflected in the underground at geological boundaries, are registered by a hydrophone chain and converted into electrical signals. After digitalization, the data are stored on media and processed afterwards. Running time and amplitude of the reflexion signals yield information on depth and properties of the boundary line.

The hydrophones are arrayed linearly in a streamer with a total length of 4000 m, which is dragged behind the ship in a depth of approximately 8 m.

Standard measurement configuration for reflexion seismic data acquisition. Behind the research vessel, an airgun array is dragged, serving as a seismic source. With the help of hydrophones in the dragged measuring cable (streamer) the reflected waves are recorded. Source: BGR

The seismic data recorded during marine geophysical surveys are processed in the BGR. The recording of refraction seismic data requires a greater distance between seismic source and receiver, compared to reflexion seismic data acquisition.

Example of the reflection seismic section. The x-axis denotes the horizontal distance, the y-axis denotes the two-way running time of the seismic waves. If the seismic propagation velocity is known, the running time can be converted into depth values. Source: BGR

Refraction seismics

Ocean bottom hydrophones are installed on the ocean floor and, after the measurements, taken back on board of the vessel. Running time curves up to a distance of more than 350 km are recorded. Source: BGR

The refraction seismic (wide angle seismic) method requires, unlike the steep angle method, a greater distance between signal source and receiver, in order to determine the propagation velocity of the seismic waves along boundary layers. Therefore, the seismic signals are received and recorded by ocean bottom hydrophones (OBH).

In all seismic methods, the energy reflected by the underground is applied as function of the distance over the running time. While in reflection seismics (steep angle seismics) only sub-critical reflexions (i. e. reflexion from a relatively small distance range) are observed, in refraction seismics normally the whole distance range where energy can be recorded, is covered. In practice, depending on the airgun, on the underground and location of the crust-mantle boundary, this can be more than 350 km. The running times of the most important surveys are shown in following figure with the ray path for the simplest theoretical case, the horizontal two-layer case.

Ray path and running time curve scheme for the horizontal two-layer case. Source: BGR

Bathymetrics

Bathymetric measurements Source: http://www.meeresgeo-online.de

Bathymetric measurements, unlike seismic measuring methods, do not examine the underground but the surface of the ocean floor. A fan echo sounder sweeps the ocean floor and creates a precise topographic map of the ocean floor. BGR normally uses fan echo sounder systems built in on board of the research vessels. The figure depicts the research vessel METEOR working with the STN ATLAS Hydrosweep system. The data recorded here can be stored digitally and processed afterwards. The correlation of the seismic data with wide-area bathymetric data yield additional information on the structural situation of the underground.

Detailed information on equipment and work of section "Marine Seismic Survey Techniques, Methods Development"