Download B. Measurement of Seismic Wave

Seismic Wave Characteristic of Ground Surface
Layer Produced by Pulsed Discharges in
Closed Water Domain
Chengyan Ren, Rongyao Fu, Ailong Fan
Xuzhe Xu, Yaohong Sun, Ping Yan, Tao Shao
Institute of Electrical Engineering, Chinese Academy of
Sciences, Beijing 100190, China
University of Chinese Academy of Sciences,
Beijing 100049, China
[email protected]
Institute of Electrical Engineering, Chinese Academy of
Sciences, Beijing 100190, China
Key Laboratory of Power Electronics and Electric Drive,
Chinese Academy of Sciences, Beijing 100190, China
[email protected]
Abstract—The technology of portable spark source,
independent of surrounding water environment, needs to be
developed for some special situations. The transfer efficiency
from electric energy to seismic wave energy is the most important
factor in this technology. The electrode filled with water was
firstly designed to develop the pulsed discharge experiment. The
seismic wave generated by pulsed discharge was measured by
seismic wave detector. The relation of seismic wave amplitude
and discharge parameter was observed. The electrode component
is machined by stainless steel and other materials which have
similar acoustic impedance with salt water to enhance the
transfer efficiency from electric energy to acoustic energy. The
piston-like electrodes were also designed to enhance the transfer
efficiency from electric energy to mechanical energy, in which
some components could be active up and down due to the
impulsive force produced by pulsed discharge. The seismic wave
in different electrode construction was measured and analyzed.
The experiment result indicates that the amplitude of seismic
wave is largest in piston-like electrodes, which also indirectly
indicates that the energy of pulsed discharge in closed electrode
filled with water is transferred to the ground surface layer
mainly by mechanical shock.
phenomena and production mechanism of pulsed discharge in
water, such as the characteristics of pressure, electric,
discharge plasma, bubble, optical and sound radiation [4-5].
Keywords—pulsed discharge; seismic wave; transfer efficiency;
electrode construction component
I. INTRODUCTION
The technology of pulsed discharge in water has been used
in many industrial applications based on the electro-hydraulic
effects, such as seismic exploration, electro-hydraulic forming,
extracorporeal shock wave lithotripsy and rock fragmentation
[1-3]. The high-voltage capacitor is charged in direct current
voltage firstly, and then discharges instantly in water by
switcher and transmission cable. At last the discharge plasma
channel is formed in water. The discharge channel will obtain
huge energy in very short time due to the short discharge time
(μs-ms) and high discharge current (104 A), which produces the
high pressure and high density plasma in water body. The
plasma channel expands fast in huge pressure and the shock
wave is formed in water. Many applications have been
exploited based on the mechanical effect of shock wave. The
extensive study has been developed about the experiment
This work was supported by R&D of Key Instruments and Technologies
for Deep Resources Prospecting (National R&D Projects for Key Scientific
Instruments) under contract ZDYZ2012-1-06, the National Natural Science
Foundation of China under contracts 51477163 and 51277171, the Science
and Technology Project of State Grid Corporation of China.
The pulsed discharge in water is also widely used in
seismic exploration. The sparkers used in the sea or on the land
have formed series of products, which is based on the pulsed
discharge in expansive water domain, such as in the sea or in
the deep well. But the conventional sparker is not applicable in
some places short of water, especially in the mountain area.
However, most of metallogenic belt of metal ores are in the
orogenic belt, and the surface and subsurface geological
conditions are very complicated [6-7]. The conventional
seismic exploration techniques (such as dynamite source,
vibrator, air gun and the sparker, etc) can not meet the
requirements of the seismic exploration in these areas. The
phased array sparker technology can make the seismic wave
focus on one direction. When the target stratum reflects, the
reflection waves will just point to the surface geophone array.
Thus the signal-to-noise ratio can be increased, and the high
resolution seismic profile can be obtained. A phased array
sparker was developed to verify the feasibility of this concept.
The discharge electrode, namely energy transducer, is key
component in phased array sparker.
The discharge electrode filled with water was designed to
develop pulsed discharge experiment in closed water domain,
which means the narrower water space closed in the discharge
electrode. The seismic wave generated by pulsed discharge was
measured by seismic wave detector. The electrodes in different
structures were also designed to enhance the transfer efficiency
from electric energy to mechanical energy.
II. EXPERIMENT SETUP AND MEASUREMENT
A. Pulsed Power Module
The phased array sparker system consists of a diesel
generator, a charging system, multiple identical pulsed power
modules, multiple transducers, a phase shift controller and a
seismograph [7]. The seismic waves produced by multiple
transducers were focused on one point by controlling the
discharge time sequence using phase shift controller. The
geophone array and seismograph were used to measure the
seismic waves of ground surface layer by pulsed discharges.
Geophone array
1m
Discharge electrode
Fig. 3. Picture of discharge experiment.
Fig. 1. Schematic of one pulsed power module.
Each pulsed power module mainly consists of an isolating
diode D, a capacitor C (MKMJ5-50, 50 μF) with a stored
energy of 625 J, a discharging switch SCR (KP8 800-65), a
one-way diode D1 and an inductor L, as shown in Fig.1.
Discharging coaxial cable (SYV50-9) is about 15 m, and the
discharging current up to 15 kA. The voltage applied to
capacitor C is measured by a Lecroy high voltage probe
(Model: PMK-14k), which has a division ratio of 1000: 1. The
discharge current is measured by a coaxial current divider with
the resistance of 5 mΩ. A Tektronix oscilloscope (Model: DPO
2024) is used to record the voltage and current signals. It has a
bandwidth of 200 MHz and a time resolution of 1 GS/s.
B. Measurement of Seismic Wave
The wired distributed seismograph, offered by the Institute
of Geology and Geophysics, Chinese Academy of Sciences,
was used to measure the seismic wave. It mainly consists of the
power station, the data acquisition station (namely geophone
array), the crosslink unit (used for data conversion) and the
seismic data acquisition software, which is shown in Fig.2. The
power station was used to provide power supply for the
crosslink unit. The data collected were transmitted to the
computer by network line.
seismic software
Data acquisition station
Crosslink unit
Fig. 2. Measurement system of seismic wave.
III. DISCHARGE EXPERIMENT AND MEASUREMENT RESULTS
The picture of discharge experiment is shown in Fig.3,
where two discharge electrodes were used. The discharge
experiment was mainly done using one electrode in this paper.
The geophone array was used to measure the seismic wave of
ground surface layer, including six geophones. The distance
from the discharge electrode to the first geophone is 1 m, and
the distance between two geophones is also 1 m. The cable was
used to connect the pulsed power module with the discharge
electrode.
High voltage
electrode
External cap
Ground
electrode
Internal
insulation
High voltage
electrode
Internal
insulation
Ground
electrode
Fixed part
Upper
cylinder
Bottom
cylinder
Insulation
sleeve
Insulation
sleeve
(a) Fixed electrode
(b) Piston-like electrode
Fig. 4. Discharge electrodes of different structures.
The construction of discharge electrodes is shown in Fig.4.
Two kinds of electrodes in different structures were used in our
experiment. The high voltage output of pulsed power module is
connected to the high voltage electrode, and the ground
terminal is connected with the ground electrode. The discharge
electrode is a closed cylinder, and its internal cavity is filled
with salt water. The first kind of electrode (Fig.4a) is fixed, and
the electrode component could not move when the discharge
occurs. The second kind of electrode (Fig.4b) is piston-like,
and the upper components could be active up and down due to
the impulsive force produced by pulsed discharge. Its
maximum travel is determined by the fixed part in piston-like
electrode. To ensure the upper portion movable, it matches the
bottom one relatively loose, so the static friction between them
is not big.
The high voltage electrode consists of multiple centre
conductors of cables, and the diameter of centre conductor is 1
mm, the thickness of insulation layer outside of centre
conductor is about 1 mm. The existence of insulation layer
could reduce the erosion of high voltage electrode. The ground
electrode is integral to the upper cylinder and the bottom
cylinder, and they are all made of stainless steel. The internal
insulation and the insulation sleeve are made of epoxy glass
fiber. The discharge distance is adjustable from 0-10cm. One
end of high voltage electrode is put in the salt water, and the
proportion of salt and water is 30 g/L, its conductivity is about
40-60 mS/cm.
21600
1 V/div
4
1 V/div
2
21650
2
3
3
t (ms)
4
1
1
25 μs/div
200 A/div
Arc discharge
There are two kinds of discharge modes in our experiment,
corona discharge and arc discharge, shown in Fig.5. The curve
1-3 are current waveforms and the curve 4 is the triggering
signal waveform. The three current waveforms were the
measurement results in three different elements in the array.
The time delay between triggering signal and corona
discharge is quite short, besides, the starting time of multiple
corona discharges is in complete accord. But the time delay in
arc discharge is long and the starting time varies. To assure the
synchronism of multichannel discharge in phased array sparker,
the corona discharge was chosen in our experiment.
The typical current waveform of corona discharge is shown
in Fig.6. CH1 is the discharge voltage waveform, CH2 is the
discharge current waveform, and CH3 is the triggering signal
waveform. The discharge voltage and current are respectively
4.8 kV and 780A. The high voltage electrode consists of 70
centre conductors of cables, and the discharge distance is about
4 cm. The typical seismic wave waveforms produced by
corona discharge are shown in Fig.7. The sample interval of
seismograph is 1 ms, and the record length is 30 s. The seismic
wave waveforms in Fig.7 are only a window of the 30 s record
length and there are no seismic wave in other record time. So
the whole record length was not given here.
There were six geophones in the data acquisition system,
but the data of front three channels were only given to observe
clearly the seismic wave. It can be seen that the amplitude of
the seismic wave decreases with the increasing distance
between the discharge electrode and the geophone, the initial
arrival time of seismic wave increases correspondingly. The
scale of horizontal ordinate is not marked because the
measured seismic wave amplitude is a relative value. The
seismic wave produced by pulsed discharge in closed salt water
is an oscillation pulse, decaying fast with the measurement
distance.
CH1: 2 kV/div
CH2: 200A/div
Fig. 6. Typical current waveforms of corona discharge.
C2
C3
Fig. 7. Typical seismic wave waveforms produced by discharge
3.5
1
7
40
70
100
3.0
2.5
Current (kA)
Fig. 5. Current waveforms of two discharge modes.
C1
Arc discharge
2.0
1.5
1.0
0.5
(a)
0.0
2
3
4
5
4
5
Voltage (kV)
2.00
1
7
40
70
100
1.75
1.50
Seismic wave
250 μs/div
200 A/div
Corona discharge
21700
1.25
1.00
0.75
0.50
0.25
(b)
0.00
2
3
Voltage (kV)
Fig. 8. Effect of high voltage electrode on results
IV. EFFECT OF ELECTRODE STRUCTURE ON SEISMIC WAVE
The effect of high voltage electrode, applied voltage and
discharge distance was studied to obtain the seismic wave in
different conditions. The discharge distance is from 1 cm to 6
cm in our experiment. The experiment result indicates that the
corona discharge is relatively stable and the arc discharge is not
apt to occur at 4 cm. The results of different high voltage
electrodes in different applied voltages are shown in Fig.8. The
high voltage electrodes respectively consist of 1, 7, 40, 70 and
100 centre conductors of cables, the applied voltage is from 2
kV to 5 kV.
The discharge current increases with the applied voltage
and the number of centre conductors from Fig.8a. The arc
discharge occurred with 100 centre conductors at 4.8 kV, and
the current of arc discharge is about 3 kA. The amplitude of
seismic wave produced by discharge increases with the applied
voltage in most conditions, but the increasing speed is
relatively slow. The effect of centre conductor number is not
very clear, but the result of 70 centre conductors is relatively
good, where the amplitude of seismic wave increases linearly
with the applied voltage. At the same time we can find the
seismic wave produced by arc discharge with 100 centre
conductors at 4.8 kV is about the same with that by corona
discharge with 70 centre conductors at 4.8 kV. Even if the
discharge current is large in arc discharge, the amplitude of
seismic wave produced by arc discharge is not always large. In
general, the experiment result with 70 centre conductors and
the discharge distance of 4 cm is relatively good in our
experiment conditions.
Only the internal insulation and high voltage electrode are
movable in the internal movable electrode. The seismic wave
produced by discharge using the movable electrode is larger
than using the fixed electrode, especially in the higher voltage
or using the external movable electrode.
6
5
4
Epoxy
Rubber
Nylon
PMMA
PE
PTFE
0.7
Seismic wave
0.6
0.5
0.4
0.3
0.2
0.1
2.5
3.0
3.5
4.0
Voltage (kV)
4.5
5.0
Fig. 9. Effect of bottom materials on results
The experiment in Fig.9 has been done with the discharge
distance of 6 cm, where the amplitude of seismic wave with
Epoxy and PTFE materials is relatively large. But a large
number of experiments indicate that the data are dispersal in
different discharge distances. When the bottom cylinder is
made of insulation materials, the bottom portion can not be
used as ground electrode, and the upper portion made of
stainless steel is ground electrode in this structure. So the
discharge path changed and the discharge energy declined
compared with the previous structure, which could have effect
on the experiment results. The amplitude of seismic wave
using the insulation bottoms is relatively lower, so the effect of
different insulation materials is not clear.
B. Movable Electrode
The previous experiment indicates that the amplitude of
seismic wave is comparatively large when the discharge
electrode has no external cap. In other words, when the
discharge occurs, part of electrode will move. In this conditions,
the seismic wave is large. To verify the idea, the piston-like
electrode in Fig.4b was designed. The experiment result is
shown in Fig.10. Three electrodes were used in the experiment,
including the external movable electrode (out, Fig.4b), the
internal movable one (in) and the fixed one (in-fixed, Fig.4a).
Seismic wave
A. Materials of Bottom Cylinder
The above experiment was done using the fixed electrode
in Fig.4a and its bottom cylinder is made of stainless steel. The
shock wave produced by pulsed discharge propagates in closed
water of discharge electrode. It will be reflected partly and the
partial energy will be lost when it reaches the bottom cylinder.
In theory, when the bottom cylinder has the similar acoustic
impedance with salt water, the propagation efficiency of shock
wave should be highest from the closed water to the ground. So
the experiments were done with different bottom materials, and
the results are shown in Fig.9.
3
out
in
in-fixed
2
1
0
2
3
Voltage (kV)
4
5
Fig. 10. Seismic wave of fixed and movable electrodes
V. CONCLUSIONS
A phased control sparker has been developed for the basic
research on the seismic exploration of the metal ores. Several
kinds of discharge electrodes were designed to obtain larger
seismic wave. The experiment results indicate that corona
discharge is more suitable in phased control sparker due to
short time delay and good synchronism. The movable
discharge electrode produces larger seismic wave than the
fixed electrode. The measured wave in our experiment is
mainly the seismic wave which propagates along the surface
layer of ground. The vibration wave produced by mechanical
shock is easy to be detected by geophone near the electrode. So
the energy transition in present experiment mainly depends on
the mechanical shock from the pulsed discharge in closed
electrode to the seismic wave on the ground surface layer.
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