2. Identification and quantification
of noise sources in marine towed
active electromagnetic data
Axel Laurel Tcheheumeni Djanni
Thesis presented for the degree
Doctor of Philosophy
School of GeoSciences
2016
3. 2
Identification and quantification
of noise sources in marine towed
active electromagnetic data
Ph.D. Thesis
01/05/2012–06/06/2016
Submission 30/06/2016
Viva Voce –/–/2016
Final Version –/–/2016
University of Edinburgh
School of GeoSciences
Axel Laurel Tcheheumeni Djanni
tcheheumeni@gmail.com
M.Sc. Geophysics Exploration 2012
The University of Pau et des Pays de l’Addour
Supervisors
Prof. Anton Ziolkowski
Royal Academy of Engineering
Research Professor of Petroleum Geoscience
Dr. David Wright
Research Fellow
4. Abstract
The towed controlled source electromagnetic (CSEM) system collects data faster than
the conventional node-based system. However, the towed receiver cable is typically
much noisier than the stationary nodes. Understanding the noise generation mechanism
is important for the development of future electromagnetic streamer cables. This is the
problem I address in this thesis.
I achieve this in three parts. First, I examine the idea that the towed streamer CSEM
suffers from noise induced by its motion in 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 and a curved streamer in a constant, and time-varying mag-
netic field. These expressions demonstrate that the motionally-induced noise is sensit-
ive 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 towed streamer CSEM with
the noise recorded using a static ocean bottom cable (OBC) CSEM system. The main
findings are that within the frequency range of interest, 0.01-1 Hz, the towed streamer
CSEM noise is 20 dB greater than the noise recorded with the OBC-based CSEM. I show
also that the motion of the telluric cable between the pair of electrodes in the towed
streamer CSEM 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 0.1–0.3 Hz frequency
band, 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 these noises, we designed a prototype towed
CSEM streamer. We carried out an experiment with the prototype CSEM streamer
suspended 1 m below the water surface in the Edinburgh FloWave tank. The streamer
was then subjected to flow running at velocities of 0–1 m s−1 along its length and to
waves propagating in the same direction, at 45◦, and perpendicular relative to the
streamer direction.
5. 4 0 | Abstract
From the data obtained in the FloWave experiment, I identify, quantify and characterise
two separate sources of noise: the motionally-induced electric field noise due to flow rates
(referred to as flow noise) and the motionally-induced electric field noise due to wave
motion (referred to as wave motion noise). I show that the motion of the streamer in
response to an increase of flow rate increases the noise level. However, most of the flow
noise occurred from 0 m s−1 to 0.5 m s−1. Above 0.5 m s−1, the flow noise increases is
modest. The initial large flow noise difference observed when the flow increases from
0 m s−1 to 0.5 m s−1 is caused by local hydrodynamic effects due to flow over the
electrode surfaces. The wave motion noise is 12 dB above the flow noise at the wave
frequency of 0.29 Hz. I show that this wave motion noise is due to the motion of the
telluric cable in response to wave motion. In addition, I estimate the wave motion noise
following Faraday’s law of induction, and compare it with the measured electric field
noise. The key findings are that the estimated wave motion noise is correlated with the
measured electric field noise. Moreover, the measured electric field noise is generally
greater than the estimated wave motion noise by a factor of about 3. This discrepancy
is likely caused by additional noise in the telluric cable between the electrodes and the
amplifier.
The results of my investigations contribute to the understanding of the relation between
the motion of the streamer when towed in the sea and the electric field noise recorded.
The main source of additional noise in the towed streamer CSEM system seems to
be the motion of the telluric cable in the Earth’s magnetic field. From these results,
I conclude that the wave motion noise could be reduced in three ways: use of rigid
telluric cables, tow the streamer deeper, and increase the cable and the telluric cable
tension to reduce the effect of ocean swells.
