On the Identification of various sources of noise in marine towed active electromagnetic system.

Date: 
Tuesday, 14 November, 2017 - 15:00 to 16:00
Location: 
NOC Southampton - Node Room (074/02) (Waterfront Campus).
Speaker: 
Dr. Axel Tcheheumeni Djanni

Tuesday, November 14, 2017 - 15:00, room 074/02
Dr. Axel Tcheheumeni Djanni*; Unviersty of Southampton (Blue Mining)
*Previously at the University of Edinburgh.

On the Identification of various sources of noise in marine towed active electromagnetic system.

Abstract:

Marine towed electromagnetic streamer, similar to the marine seismic 2D acquisition set up, is in use for efficient acquisition of controlled source electromagnetic (CSEM) data for the detection of hydrocarbons. The technique results in the possibility of cost-effective region scale surveys and offers the potential for an order of magnitude improvement in productivity. However, the method is typically much noisier than the conventional static node-based CSEM within the frequency range of interest. Hence, identifying and quantifying various sources of noise is important for the development of future robust electromagnetic EM.

The results of my investigation can be folded in two parts. First, I analyze and compare the noise recorded during a field trial using the first generation of the towed EM stream with the noise recorded using a static ocean bottom cable (OBC) CSEM. The mains findings are that the noise in the towed EM is a factor of 10 higher than the noise recorded with the OBC CSEM within the frequency range of interest (0.01 – 1 Hz). At frequencies above 1 Hz, the noise due to the towing effect (towed noise) is strongly correlated from one channel to another. The same observation is made with the OBC data. At frequencies below 1 Hz, the towing noise is uncorrelated across all channels in the frequency ranges 0.01 - 0 .03 Hz and 0.05 - 1 Hz. In contrast, noise in the OBC CSEM correlated from channel to channel. These results were unexpected for the towed EM system and this discrepancy can originate from various sources such the electrodes, ocean swell from surface waves, water flowing around the streamer, or cross-currents which cause the streamer or receivers to vibrate or deviate from its intended track.

In an attempt to understand the physical mechanism that generates the major source of noise, my supervisors and I designed a prototype EM streamer and tests were carried in the controlled environment of the new FloWave tank. From these tests, I show that the motion of the cable connecting two electrodes due to the ocean swell motion is the major source of noise in the towed EM streamer.

I will define marine node-based CSEM, compare the towed CSEM with both marine seismic 2D, and the OBC CSEM system. I will also show various sources of noise that could affect the towed EM system and suggest various ways to not record them at the first place or to reduce their impacts on the measurements.

Seminar category: 
G3 Seminars