September 12 After each big earthquake, the Earth rings like a bell. The sudden, extremely rapid movement of large blocks of rock within the Earth’s crust during a quake hits the rest of our planet like a clapper striking the body of a bell. As a consequence, the whole structure begins to hum. Inside the Earth, these vibrations are called normal modes or free oscillations and the may shake the whole planet for hours if not days. Researchers have found two types of this kind of ringing: The spheroidal mode changes the actual shape of the Earth while the toroidal mode twists the body of the Earth back and forth (see figure 1). Unlike strong earthquake waves, such free oscillations cannot be felt by humans. The reason: The periods of these vibrations are very long. They reach from 100 seconds for the shortest period up to 54 minutes. Figure 1 – The Earth’s two normal modes Such periods are well below the pass-band of modern broad band seismometers. Nevertheless, if the installation is done well, say in a properly insulated seismic observatory built on an outcrop of hard rock, free oscillations have been recorded around the globe by many Guralp seismometers. However, these free oscillations have been rarely – if at all – registered by seismometers located on the ocean bottom (OBS). Usually, the strong noise generated by the ocean waves masks the weak amplitudes of the bell-like vibrations. A very unique recording of free oscillations has now been found in the data collected by a very broadband Guralp OBS (CMG-3T, 360 sec – 100 Hz) after the huge magnitude 8.6 earthquake off the coast of Sumatra on April 11th 2012. The OBS is located in 2500 meters water depth at the bottom of the Mediterranean off the coast of Toulon, France. The sensor was installed (see figure 2) in November 2010 and has been operating flawlessly ever since. Figure 2 – CMG-3T OBS instrument deployed in the Mediterranean Figure 3 (courtesy of Anne Dechamps from the Laboratoire Geoazur, Nice, France) shows the spectrum of the free oscillations captured by the OBS (blue line, labelled ASEAF) as compared to the seismic land-station SSB (red line). The nomenclature describes the various modes of...
Learn MoreMay 12 Guralp’s new Seismic Array for Downhole Investigations, CMG-SADI, has passed its first deployment with flying colours. Ordered by a Turkish company to be used in the exploration for coal, the tool was recently installed at 825 m depth in a borehole in Western Turkey. GSL cooperated with our regional distributor, Sentez Earth and Structure Engineering Ltd., during the deployment and the intensive testing. The downhole tool, CMG-SADI, can be used for classic Vertical Seismic Profiling (VSP), but also in a continuous sampling mode for ambient noise studies. The tool is very versatile: It currently has 24 elements but it can easily be extended with up to 128 independent seismometers. Passive geophones can be used as well as our newly developed miniature broadband feedback sensors, which have a flat frequency response to acceleration between 10 sec and up to 300 Hz. Each element has its own downhole digitiser with sampling rates down to 0.25 msec. The synchronisation accuracy between the digitisers is better than 3 mircoseconds. With an outer diameter of only 50 mm, it fits into almost any borehole (see image). During the test phase, CMG-SADI was operated in VSP-Mode with seismic energy being generated either by Vibroseis sweeps (20-200 Hz, sine wave sweeps) or by shots. The picture below shows all traces recorded at the same geophone for all shots over the line. The data shown are completely unfiltered and no processing corrections have been applied. Click the image for a larger version. We also have developed a new data-gathering and quick-view software for CMG-SADI. The software, called “VSP Viewer”, is based on our successful SCREAM software for regular data acquisition. The image below shows a screen shot of VSP Viewer with highlights of some features of the software. Click the image for a larger version. For more information, email...
Learn MoreApril 10 Guralp Systems will be attending three major scientific conferences on Earth Science in Europe and the United States. During these meetings, we will be exhibiting our latest products and discuss full technical solutions with customers. We will also contribute technical papers to the various symposia. We will be attending Memphis in the American state of Tennessee, where we will attend the annual meeting of the Earthquake Engineering Research Institute (EERI). The meeting will be held 10-13 April 2012 together with the National Earthquake Conference. The theme of the conference is “Learning from the Past to Protect the Future”. It coincides with the 200th anniversary of the New Madrid earthquakes, some of the strongest temblors ever to hit the United States. From the Mississippi River we move to the Pacific Ocean, where we will be present at the annual meeting of the Seismological Society of America (SSA), held in San Diego 17-19 April 2012. Besides exhibiting we will present technical papers on new ways of testing seismic instruments and on our new three component miniature broadband sensor. Just a week later we will attend the General Assembly of the European Geosciences Union (EGU). As usual, this conference will be held in the Austrian capital Vienna and it is scheduled for 22-27 April 2012. GSL has been a regular exhibitor with EGU for more than a decade. For more information, email...
Learn MoreGuralp Systems’ position as the world leader in developing, manufacturing and delivering seismic systems for applications on the seafloor got another boost last week. Neptune Canada ordered two additional CMG-1T Ocean Bottom Seismometers (OBS). Once this order is fulfilled, Neptune Canada will operate a total of six GSL broadband OBS in its high profile, multidisciplinary network off the coast of British Columbia. In addition Neptune Canada has ordered from Guralp Systems digitizers and data acquisition modules for the short period seismic component of its network. Map of the Neptune Canada OBS Network Neptune Canada is the world’s first regional scale cabled seafloor network which last month celebrated two years of continuous online data delivery. It operates a seafloor network of more than 125 sensors for various scientific purposes. They are connected to many nodes along a several hundred kilometer long deep sea cable loop. The cable, which originates and ends in the coastal town of Port Alberni, delivers live data from these sensors to scientists at the University of Victoria and beyond. Currently, Guralp Systems is the exclusive purveyor of broadband OBS sensors for this network. Delivering CMG-1T OBS broadband sensors is only one prong of our overall strategy for seismic seafloor systems. We also deliver raw instruments to system integrators, who build their own housing for the sensors and design their own OBS systems, integrate several types of our own sensors into in-house designed and built titanium based deep water housings, sell complete sensors to instrument pools, build and deploy our own complete portable OBS systems, and deliver turn-key cabled OBS systems, such as networks in the Sea of Marmara (Turkey) and along the US Pacific coast. CMG-1T OBS Installation For more information, first contact your local distributor, email sales@guralp.com, call +44 118 981 9056 or fill in the form...
Learn More
