The Trouble with Sub-bottom Profiling !

Generally sub-bottom profiling (SBP) systems are single-channel data loggers with single a channel hydrophone array input or transducer used to acquire shallow high resolution reflection seismic profiles.   A variety of Sub-bottom profiling systems are available, these systems function at differing transmit frequencies and this has an effect on the resultant resolution and the depth of acoustic penetration below the seabed.

Lower frequency sound sources produce deeper acoustic penetration below the seabed however, as a consequence of low transmit frequency a lower resultant resolution is achieved.   Higher frequency systems achieve less penetration but produce higher resolution data.

The sub-bottom profiling penetration depth is related to the transmit frequency, source energy and the nature of seabed geology, for example, layers of course sands or near seabed gas blanking will often severely limit penetration and with only one receiving channel available low amplitude reflected signals in these zones may be lost within the ambient noise created by the survey vessel its self and or the sea state.

Listed below are a number of most commonly used sub-bottom profiling equipment and their approximate operational characteristics:

 

 

 

 

There are a number of disadvantages in using digital streamer systems for ultra high resolution (UHR) data acquisition such as the purchase cost, physical size and the added cost and time required for post acquisition processing.  However, multi-channel solutions offer the ability to significantly increase the signal to noise ratio and recover low amplitude signals buried within the ambient noise by stacking and processing data.  The survey industry today has moved towards digital multi-channel streamers which can provide better reliability than multi-channel analogue streamers of an earlier design.

Digital streamers are effectively passive hydrophone array channels digitised at the head of each section. this technology was welcomed when analogue multi-channel streamers became longer and channel counts became greater in number, physically the inter-connect plugs and sockets were running out of space!  In addition to this, the longer the streamers became the greater the signal loss and the lower the signal to noise ratio became.

Now, taking all this into account when we consider that a UHR multi-channel streamer may only have say 24 to 48 very short channel group lengths, typically 3.125 or 1.576 meters long, the need to digitise signals within the streamer because of inter-connect pin space and signal loss is no longer absolutely necessary.

So, an "analogue" i.e. passive hydrophone streamer array construction with simple miniature buried pre-amplifiers for each channel greatly reduces the cost of construction, this also removes the digitising modules associated with digital streamers thereby removing the "point" weight of these modules at the head of each section giving rise to a more stable and balanced streamer!         

The ProSeis x16 further reduces the costs of acquisition by employing these purpose built, short group analogue streamers, digitising the signals vessel side using high specification analogue to digital conversion.  The system software acquires, stacks and processes data in near real time ready for preliminary interpretation thereby removing the need for on-board processing equipment and personnel, especially when project reporting schedules are under tight time constraints.


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