One could argue that seismic reflection exploration is the most valuable geophysical survey methods. That because every single barrel of oil coming up is located by reflection surveys. Enormous amount of money have been spent on evolution of the method. Only space race and warfare have been using such money on RD. Earth-science has profited a lot from that.
Here is a good reference on basic reflection principles: http://pioneer.netserv.chula.ac.th/~tthanop/Seismic%20Prospecting%20PDF/Lecture%2 01%20Basic%20Concepts%20of%20Seismic%20reflection%20Survey.pdf
And here on marine seismic: http://www.iagc.org/attachments/contentmanagers/4352/Marine%20Seismic%20Operations%20Overview.pdf
The method is mainly used in sediment geology where the oil is and in those cases the top of Metamorphic or Igneous crust considered as „bottom“ like in this survey west of El Hierro: http://www.sciencedirect.com/science?_ob=MiamiImageURL&_cid=271781&_user=713833&_pii=S0025322797001308&_check=y&_origin=mlkt&_zone=rslt_list_item&_coverDate=1998-04-30&wchp=dGLbVlS-zSkzk&md5=576f58486ee576c3896f57bda6f80e86/1-s2.0-S0025322797001308-main.pdf (NB limited access.)
But there have been reflection surveys down to moho, like close to NewZealand: http://earth.usc.edu/~okaya/nz/pdfs/mortimer_sight_nzjgg2002.pdf
Oceanographic vessel (O/V) Sarmiento de Gamboa (SM from now on) is the newest vessel of the UTM, launched 2006.
The Sarmiento de Gamboa
The Marine Technology Unit (UTM) operates within the Mediterranean Centre for Marine and Environmental Research (CMIMA) and belongs to the Department of Natural Resources of the Higher Council for Scientific Research (Consejo Superior de Investigaciones Científicas, CSIC from now on).
SM is a multipurpose research vessel but with main weight on seismic exploration. The equipment is state of art although it is only fit for 2D seismic.
The equipment used to perform seismic studies is divided into three groups according to the function performed: Energy sources, which provide a pulse of acoustic energy, acquisition equipment, responsible for capturing and recording the reflected signals and processing systems, to analyze and represent the seismic signals: http://www.utm.csic.es/doc/bo_sarmiento_de_gamboa/equipamiento/Configuraci%C3%B3n%20S%C3%ADsmica%20SdG.pdf
Acoustic energy source use compressed air guns, mainly from Bolt: http://www.bolt-technology.com/pdfs/LongLifeGun.pdf They can be connected in arrays giving higher output and some control over pulse shape and bubble pulse. The energy is controlled by change of volume and varying air pressure. The tradeoff here is more power for less resolution. Air gun is often towed some tens of meters behind the ship at a dept. of ca 10 m.
The sensors used for seismic in water are so called hydrophones, which are pressure sensitive crystals (piezo) giving voltage out by pressure change. Each channel has 10-20 sensors 1-2 metersapart, connected in parallel. That gives increased sensitivity but also some directivity, enhancing signals at perpendicular direction but cancelling unwanted signals along the cable. The hydrophone strings are fed into a tube called streamer ~ 50 mmdia. Along with connecting wire and filled, before they used oil, gel but now solid foam. The strings are made up in sections ca 150m long. SM uses Sercel Sentinel ® up to 480 channels, select sections 6 kilometerslength. Solid streamer. In the case of el Hierro I saw somewhere they were using ca 2 kmand then probably 120 channels. The acquisition systems is also Sercel, SEAL 408XL up to 2000 channels: http://www.sercel.com/Products/marine/systems/seal.php
Just to mention, Modern 3D oilfield seismic are done by running 8-12 streamers like those parallell behind the same ship. Even two such ships with shooting vessel in the middel!
Seismic vessel with streamers Copyright IAGC
El Hierro project
It seems from news that SM was alredy underway doing a project on the west Africa continental shelf. There seems to be increased interests in this area, maybe due to oil smell ??? First step is usually government run 2D seismic, and then results bid out to oil companies which bid for the right to to 3D seismic and then maybe test drilling. A report on some scouting here: http://sbc-msu.com/assets/images/publications/Hadler_Jacobsen_et_al_2006.pdf
El Hierro got 2 days worth of profiling, most at north but some at the south. Both transverse and radials. One would expect some problems doing such work on a living volcano. One would be the seismic tremor (harmonic or not) but hopfully its freqency is below that of interest and can be filtered away. Other problem might be from basaltic lava streams possible in the sediments around the island but they tend to be rather opac to seismic energy. Anyway the data processing takes months so we‘ll not know for a while.
Other aspects of marine seismic is use of wide angle reflection and refraction. That takes either a second ship but usually a combination of OBS‘es (http://en.wikipedia.org/wiki/Ocean-bottom_seismometer) and land stations. On El Hierro there are now ca 5 digital seismic stations that have certainly received a lot of data. SM syncronages their airguns to GPS time signal as the seismometers. Given the traverses and radials from SM where each shotpoints location is known to 2 meters and time to the millisecond are KNOWN WE HAVE ALL THE MATERIAL FOR THE BEST TOMOGRAPHY WE WILL EVER GET. We have S-waves as well though they are not generated or carried in water, at hitting the bottom the P-waves generate the S-waves we need. Well they have, not we.
Reference on marine seismic: http://woodshole.er.usgs.gov/operations/sfmapping/seismic.htm
Trial by fire for R/V Ramón Margalef
The Role of Ramon Margalef in the monitoring of the El Hierro eruption has been very important.
It is remarkable that the ship is really in it’s virgin trip.
In addition to very important sampling, the bathymetry has been the main course. The tool for that is the newest, most technical of Kongsberg Simrad multibeam sonar, the 710.
Another multibeam sonar on board is the ME 70, a very important tool for fishery control.
I am not an expert on the subject, just interested former geotech, dynamite shot master and borehole logger. Please feel free to criticize.