A deep insight into the subsurface
By Henrik Grob and Sjard Stratmann | Finally! After 6 ½ weeks of waiting it was time for marine seismics. The aim of the seismics on board were pre-site investigations for an IODP (International Ocean Discovery Program) proposal . But what if anything is seimics?
Seismics enable us to get a deep insight into the subsurface. Alike parasound, it is an imaging process and visualizes cross-sections of subsurface structures up to several kilometers depth. Therefore we obtain a seismic image.
Basic idea of applied seismics is generating artificially seismic waves (earthquakewaves), which penetrate deep into the bedrock and get reflected at geological layers. Hence, seismic source and receiver are required.
Here on board we use a so-called „air gun“. The air gun generates the required seismic waves with the aid of compressed air and emits them. This procedure is repeated every several seconds. A streamer – a long hose containing „microphones“, which are called hydrophones – is used as receiver and records the reflected signals. In our case, the streamer is 600 m long and is reeled on a several tons heavy winch. Both air gun and streamer are towed behind the Polarstern. This means for us go to deck, roll up your clean sleeves, take helmet and safety vest and bring with support of crew and geologists the gadgets into the water.
This is the onset of our –Jürgen Gossler, Henrik Grob and Sjard Stratmann– work in the seismic lab. Systems have to be monitored, set parameters have to be checked, if they are still valid and we have to be sure that the gadget rear (aft) doing well outside.
Simultaneously, marine mammal observations are done by our colleagues of the crew and scientists during our seismic measurements to guarantee that the mammals will not be disturbed. In case a marine mammal should approach the vessel, all systems will be shut down, immediately.
So, a cross profile of 20 nautical miles length per profile is measured for the IODP proposal. A further very important part of our work contains processing the measured data for enhancing the quality of the seismic image by suppressing noise e.g. from the vessel or other physical noise effects. Processing begins while ending a profile to get a first impression of the measured data as far as possible.
