Identification and quantification of noise sources in marine towed active electromagnetic data
View/ Open
Date
03/07/2017Author
Tcheheumeni Djanni, Axel Laurel
Metadata
Abstract
The towed streamer controlled source electromagnetic (CSEM) system collects data
faster than the conventional static node-based CSEM system. However, the towed
streamer CSEM is typically much noisier than the conventional static node-based
CSEM. Identifying and quantifying various sources of noise is important for the development
of future robust electromagnetic streamer system. This is the problem I
address in this thesis. I achieve this in three parts.
First, I examine the idea that the towed streamer suffers from noise induced by its
motion through the Earth’s magnetic field according to Faraday’s law of induction. I
derive expressions for the motionally-induced noise for the cases of a horizontal streamer
parallel to the acquisition vessel’s path and a curved streamer caused by a constant
cross-current. These expressions demonstrate that the motionally-induced noise is sensitive
to the magnitude of the feather angle at the head and at the tail of the streamer,
and to the vertical and lateral motion of the streamer. The key finding is that no
motionally-induced noise is generated when the streamer is horizontal and moving in
a constant magnetic field. By contrast, when the streamer shape is curved because
of cross-currents, motionally-induced noise is generated if the velocity of the streamer
varies over time.
Second, I analyse and compare the noise recorded using the first generation of towed
streamer with the noise recorded using a static ocean bottom cable (OBC) CSEM. I
find out that within the frequency range of interest, 0.01–1 Hz the towed streamer noise
is 20 dB greater (factor of 10) than the noise recorded with the OBC CSEM. I show
also that the motion of the telluric cable between the pair of electrodes in the towed
streamer is responsible for this difference in amplitude between the two systems. In the
frequency ranges, 0.03–0.1 Hz and 0.03–0.2 Hz, the motionally-induced noise is shown to
be uncorrelated across all channels. However, within the frequency band 0.1–0.3 Hz, the
motionally-induced noise correlation gradually increases and becomes well correlated
at about 0.2 Hz. This correlated noise could be caused by ocean swell from surface
waves, water flowing around the streamer or cross-currents.
Finally, to identify and quantify the contribution of several distinct sources of noise,
and to describe the mechanisms generating each source of noise, I co-designed a prototype
towed streamer CSEM. I carried out an experiment with the prototype streamer
suspended 1 m below the water surface in the controlled environment of the Edinburgh
wave tank located in King’s building campus (the University of Edinburgh). I then
subjected the streamer to flow running at velocities of 0–1ms−1 along its length and
to waves propagating in the same direction, at 45°, and perpendicular relative to the
streamer direction.