AIRGUNS

⚠ (:toc:) <:vspace> !GUNS USED ---- [[http://www.bolt-technology.com/pages/products_longlife.htm | Attach:bolt-1500.png"Bolt mod. LL 1500 Airgun"]] ''Bolt mod. LL 1500 Airgun. Click to visit manufacturer's web site.'' <:vspace> We are using four ea [[http://www.bolt-technology.com/images/PDF_1500LLSpecSheet.pdf | Bolt Long Life 1500]] guns. Each has a volume of 1200 inch'^3^'. <:vspace> * [[http://www.bolt-technology.com/images/PDF_LongLifeGun.pdf | Bolt Long Life Airgun Brochure]] <:vspace> This source is needed for refraction surveys with several hundred km offset distances. We simply want to put as much low-frequency energy into the ground as possible. <:vspace> !ACOUSTIC SIGNATURE ---- The source signature is, of course, a "disaster" for the mini-streamer we also employ, due to the large "bubble pulse" oscillations these guns produce. To get an idea of how bubble pulses behave, look at the high-speed video recording of a sparker firing - the source mechanism is different (an electrical discharge) but the bubble pulse phenomenon is similar to what we experience with airguns. <:vspace> [[http://www.idac.rm.cnr.it/laboratory_files/idac02c.mpg | Attach:sparker-bubbles.png"Click image to see sparker bubble oscillation film"]] ''Click image to see sparker bubble oscillation film. From [[http://www.idac.rm.cnr.it/lab_UA.php | CNR - Institute of Acoustics, "O.M. Corbino"]]'' <:vspace> Attach:airgun-bubbles1.jpg"Illustration of airgun bubble expansion and contraction" ''Figure illustrating airgun bubble expansion and contraction.'' <:vspace> Another figure on the same subject: <:vspace> [[http://www.ldeo.columbia.edu/~johnd/airgun_tutor/bubble.html | Attach:bubble.gif"Click to visit short article re. airgun bubbles"]] ''Figure from http://www.ldeo.columbia.edu/~johnd/airgun_tutor/bubble.html'' <:vspace> To reduce the bubble effect you would compose an array of airguns of different sizes. In that case, the bubble pulses interact destructively and the resulting pulse would have a much better shape. This method is used in '''reflection seismics''' (i.e. seismic vessel towing one or several streamers). Our OBS survey is, however, an example of '''refraction seismics''' where total low-frequency output matters more then nice acoustic signature - the main purpose is (as mentioned) to produce acoustic ernergy that can travel far distances. <:vspace> !AIRGUN MAINTENANCE ---- As an aid in airgun maintenance, Stig Monsen made this nice figure: <:vspace> Attach:1500_kanon_2.jpg"Parts used during Bolt 1500 LL Gun maintenance." ''Parts used during Bolt 1500 LL Gun maintenance.'' <:vspace> <:vspace> !PICTURES ---- <:vspace> Attach:p6.jpg"30.May 2009" ''In top of the picture one of the airguns rests after line 1 has been completed. The bulky section on the right is the replaceable chamber; this one has a volume of 1200 cubic inch. They're fed with compressed air at 140 Bar. In comparison, that's the same pressure you'll find at 1400 m water depth. Pressure of such values must be treated with utmost respect.'' <:vspace>