Figure 1. Echogram of gas flares on
the northwestern shelf of the
(HF-channel 38 kHz)
Figure 2. Echogram of
gas flare group in the continental slope inflection area.
Figure 3. Echogram of
flare on the northeast continental slope
REMOTE ACOUSTIC DIAGNOSIS OF GAS RELEASE
SOURCES ON SEABED
A.A. LYUBITSKIY
Abstract: Methods and
tools of remote acoustic diagnosis of the gas releases (mainly methane) from
seabed are presented. The methods are based upon the effects of linear and
nonlinear interaction between sound waves and gassy objects and upon the
identification of their parameters due to the characteristics of backscattered
signals. The results of field acoustic observations of gas sources in the
The phenomena of active gas release (mainly
methane) from seabed are the important object of marine environment monitoring.
It is known that gasing sources (or seeps) are widespread on the USA Pacific
shelf, in the Gulf of Mexico, Caspian Sea, Black Sea, Barents Sea, the
They
are one of mechanisms of methane saturation of the sea water and essentially
influence the sea ecosystems [3, 5]. In coastal and shelf zones the intensive
seeps ensure bubbles transport of methane into the atmosphere, which belongs to
hot bed gas components. On the other hand, there is direct correlation (in
global scale) between methane seeps amount and hydrocarbon resources in
productive geological provinces [11]. Gassing sources usually attend the
perspective deposits of oil and gas and may serve as an indicator for their
prospecting.
The
most adequate approach to the study of sea bed gassing activity in large water
areas is the use of methods and tools of remote acoustic sounding from the
board of vessel [1, 9, 14, 15].
These
methods have been designed in Institute for Radiophysics and Electronics, NAS
of Ukraine, during last decade. In this work the results of this development
are presented.
I. METHODS, ACOUSTIC INSTRUMENTS AND DATA PROCESSING
The
methods are based upon the effects of linear and nonlinear interaction between
sound waves and gas objects and upon the identification of their parameters due
to the characteristics of backscattered signals at carrier and heterodyne
frequencies.
The
multichannel hydroacoustic complex (MHC) is designed using the three-sonar
system of the research vessel “
Spectrum
central frequencies: 3.2 kHz (LF channel); 20, 38, 75, 120 kHz (HF channels);
Sounding
signals: tonal-pulsing, biharmonic and
linear frequency modulated (LFM) for LF channel;
Peak
pulse power: 1…. 20 kW (LF channels); 1…. 5kW (HF channels)
Angular
beam width: 10°(split beam for 3,2 and 38 kHz channels);
Depth
resolution: 0.1 … 5 m;
Depth
of the subsuperficial sounding of bottom sediments: 10-100 m (tonal-pulsing
signals); 60-300 m (frequency modulated signals).
Besides
sonars the complex includes: hydrophone set for receiving signals at heterodyne
frequencies, digital system for data acquisition and processing, device for
frequency modulated signals synthesis and GPS receiver. LF-channel of MHC is
used for study of gas releases as well as sub-bottom acoustic profiler of
sediments.
The
software of MHC puts into practice:
-
Management of complex operation;
-
Preliminary processing of received signals (including the matched filtering of
received modulated signals) in real time;
-
Formation of object acoustic images (echograms) and their visualization in
black-white or color palettes;
-
Tracing the vessel maneuvering according to the navigation data;
-
Determination of absolute levels of echo signals, evaluation of volume
backscattering strength for linear and nonlinear measurement modes and also
target strength of single bubbles , their sizes, rising velocity
and shrinkage rate.
Mathematical
models for interpretation of measurement results:
- Linear and nonlinear sound backscattering in gas
flares [8, 12, 13];
- Gas bubble dynamics and gas exchange processes
with ambient liquid [6, 10];
- Solving the sound backscattering inverse problem
[10].
II. FIELD ACOUSTIC OBSERVATION OF THE GASSING SOURCES
Acoustic
observation of natural gas released from seabed using the MHC have been carried
out in the northeast part and on wide areas of the northwest shelf and
continental slope of the Black Sea during the cruises of R/V “Kiev” (Nov.,
1995, Jun., 1997) and R/V “Professor Vodyanitsky” (Jul., 2001, Jul., 2004).
During the expeditions more than 400 gassing sources have been discovered and
investigated. We site some results of this investigation, which illustrate the
abilities of acoustic methods and tools. In this context acoustic observations
of a gas release are referred (following [15]) to as flares because of their
flame-like appearance on the echograms.
Figs. 1-3
demonstrate the examples
of the gas flares observed on the northwest shelf, on continental slope
inflection and on continental slope of the
Fig.
3 illustrates the flare on continental slope near the phase boundary for pure
methane hydrate in the
The
results of seabed LF sounding indicate that gas release in this region as a
rule is controlled by disruptive disturbance. Fig. 4 demonstrates the profile
of bottom sediments on the continental slope. We may see thin layer structure
of shelf sediments and up-to-date tectonic disturbance of their continuity
towards, which the methane emission has a certain attitude.
The
echograms on Fig. 5 illustrates the geological structure of depressed syncline
with gas flare over the circular disruption of sediment continuity.
Figure 1. Echogram of gas flares on
the northwestern shelf of the
(HF-channel 38 kHz)
Figure 2. Echogram of
gas flare group in the continental slope inflection area.
Figure 3. Echogram of
flare on the northeast continental slope
Further
digital processing of echo signals from gas bubbles and their clouds allows
determining bubble-size distribution, rising velocity of bubbles versus their
equivalent radius and finally methane fluxes from the seabed.
Sizes
of bubbles and their rising velocity have been determined by direct
measurements of acoustic cross-section of single bubbles and their depth versus
observation time using bubble tracking technique [9, 10]. Sizes of bubbles are
calculated from relationship between bubble backscattering cross-section, sound
frequency, depth and equivalent bubble radius r with correction on
nonsphericalness of large bubbles.
Figs.
6 and 7 show the results of such evaluation for gas releases in the Dnepr and
the Kalanchak Paleo Delta areas. According to the obtained results the bubbles
radii for these gassing areas vary from 0.4 to 7.6 mm with most probable value
1-3 mm, and bubble size distribution satisfactorily approximate by log-normal
and exponential distribution laws.
Rising velocity spread
substantially exceeds (especially in the interval 
0.4-1.5 mm) the
measurement errors (0.8 sm/c) stipulated by the degree of bubble pollution by
surfactants which decrease bubble rising velocity by comparison to clean
bubbles. The influence of surfactants on dynamics of large bubbles (
>2.5 mm) is not significant.
The
methane flux from seabed in gas flare is determined by acoustic sounding data
as the following:
,
where
- flare square, 
- volume bubble concentration in
near-bottom layer (from acoustic measurements of volume backscattering strength
and bubble cross section), 
and 
volume and rising
velocity of bubbles with radius 
, 
- ordinate of bubble
size distribution histogram in i radii interval.
Such
calculations using field measurements data in the Dnepr and the Kalanchak Paleo
Delta areas indicate that methane fluxes in the individual flares of this
region vary from 0.03 to 360 l/min (SPT).
Effectiveness
of gas release diagnostics substantially increases with the use of non-linear
acoustic methods. These methods allow to select gas bubbles from scatterers of
other nature (zooplankton, suspension etc) and to perform independent
estimations of their concentration. The possibility of nonlinear reverberation
observation (at doubled frequency of sounding signals) was shown during field
measurements while intensive shelf flares sounding at frequencies of 3.2 and 38
kHz, and estimations of resonant bubbles concentration have been obtained [8, 9].
For
evaluation of a bubble-mediated transport of methane from seabed to the
atmosphere in the natural gas flares it is necessary to take into account the
process of bubble gas exchange with the surrounding aqueous environment using
mathematical models which describe these processes. The MHC software contains
software module for solving this problem, on the basis of the model [10] which
uses bubbles size and rising velocity acoustic measurements results as input
data. Model calculations according to the results оf our observations of gas
releases in the Black Sea showed that the amount of methane which reaches the
atmosphere is significant (> 10 %
from initial) only for shallow sources (depth < 120 m) from oxic
zone.
III. CONCLUSIONS
Remote
acoustic sounding is the effective non-invasive tool to diagnostic of gas
releases from seabed and to study their behavior in water column. The developed
acoustic methods allow solving wide range of problems including search and
mapping of gassing sources, study of their geology, determination of gassing
field sizes, gas flares height as well as estimations of gassing sources
productivity and methane fluxes from sea bed. Besides that, the results of
acoustic measurements of bubbles size and rising velocity can be used (as the
input data) for calculation of methane partial fractions dissolved in seawater
and emitted into the atmosphere on the basis of models of bubble dynamics and
their gas exchange with surrounding water at rising.
The
results of field observation confirm the effectiveness of the used models and
engineering solutions, which are the basis of the developed technology.
The
application areas of developed technologies are: marine geology, oil and gas
prospecting, the marine environment ecological control, oceanography.
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