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New highly updated hardware
released on October 22nd 2018.
(cutting edge of electronic
technology).
12. HARDWARE PRESENTATION.
Introduction
What is more important in this methodology, as in any physical
experiment, is the data acquisition. Incorrect data will end up into
false conclusions. The entire hardware setup was constructed from
common in use hardware modules (computer, modem, ADDA card), while
the signal preconditioning unit is a direct application of typical
integrated circuits (ICs) available in the electronic market,
allover the world.
A general lay out of the monitoring site hardware is as follows:
12.1. Block diagram of the monitoring site hardware.
The entire monitoring site consists of the following main units:
a. The receiving dipoles (A). Two dipoles are used,
sharing a common ground electrode.
b. The preconditioning unit (B). This unit incorporates
the low-pass filters, the band-pass filters, the gain amplifiers and
the power supporting UPS.
c. The desktop computer (C). This unit consists of a
typical desktop computer, fitted, with an Analog to Digital –
Digital to Analog (ADDA) card of 12bits conversion.
d. The communications modem (D).
The interconnection of these units is shown in the following figure
(12.1.1).
Fig. 12.1.1.
Flow-chart of the composition of monitoring site
hardware. The arrows indicate the signal flow-path.
In details each unit (A, B, C, D) is prescribed as follows:
12.2. Section A. Receiving dipoles.
The receiving dipoles consist of: a) the actual electrodes which
are put in the ground, and utilize the electrical contact with it
and b) the wires that connect the electrodes with the
signal-preconditioning unit.
12.2.1. Electrodes used.
Two kinds of electrodes are used with equal results. The first one
is the well known “copper – copper sulfate” non-polarizing
electrode, while the second one is the metal casing of existing in
the area, for a long period of time, boreholes.
12.2.1.a. Non-polarizing electrodes.
The structure of this electrode is shown in the following figure
(12.2.1.a.1).
Fig. 12.2.1.a.1.
Copper – copper sulfate electrode composition.
A plastic bowl, of about 35cm in diameter, is filled with copper
sulfate. In this bowl, a copper disk of 20cm, in diameter, is fitted
and its connection to the external circuitry is made via a
connecting cable. Electrical connection with the ground is made
through the lower surface of the plastic bowl. The copper sulfate
remains “wet”, due to ground’s moisture, for a long period (it has
been used continuously for three years in ATH and for more than one
year in HIO) without any noticeable malfunctioning at all.
Typical “contact resistance Rc”, measured for this type of
electrode is:
Rc = 150 Ohm.
(12.1)
while the potential, developed, between the ground and the electrode
is of a few mV.
12.2.1.b. Borehole metal casing.
This type of electrodes consists of the metal casing of already
existing boreholes in the area, where the monitoring station will be
installed. The depth extent of the boreholes is variable, but any
length from 10 to 100meters is suitable. The problem that was faced
with boreholes was that these are not usually located in convenient
places, in order to form a typical NS – EW array. Moreover, the
distance between them was either small or large thus, prohibiting
the installation of a regular, of equal distant dipoles, array.
A typical electrode of “borehole type” is shown in the following
figure (12.2.1.b.1).
Fig. 12.2.1.b.1.
Typical Borehole metal casing electrode is presented.
Typical “contact resistance Rc”, measured, for this type of
electrode is:
Rc = 10 Ohm
(12.2)
An objection which arose for the use of such type of electrodes, is
that a rather high parasitic, potential develops between the metal
casing and the ground, because of the presence of electrolytes in
the ground. Although this is correct, what actually happens is that
this parasitic potential is cancelled out, in its majority, when a
second electrode, of the same type, is used to form the overall
receiving, electric dipole. This is shown in the following equation:
Assume that BH1 electrode develops a parasitic potential P1 and BH2
electrode develops a parasitic potential P2. Thus, the observed
potential difference dP between the two electrodes is:
dP = P2 - P1
(12.3)
Since BH1 and BH2 are located in the same ground environment, then
P2 almost equals P1 and therefore, the observed dP is expected to be
very small.
Another objection which arose for the use of such type of
electrodes is that, this exhibit a large amount of drifting, due to
change of contact resistance, temperature, water level etc.
The opposite point of view is that, the borehole metal casing that
has remained in the ground for a long time period (usually for some
years) exhibits very stable behavior, as it has been observed in PYR
and VOL monitoring sites.
Moreover, borehole metal casing is more or less at temperature
equilibrium with the deeper, underground formations, due to its
high, thermal conductivity. The fact that differential measurements
are taken, with any pair of boreholes forming an electric dipole,
guarantees data free of any drift that takes place, simultaneously,
in both borehole electrodes.
12.2.2. Cables layout.
These consist of coaxial cable (RG59) of which the shield is
grounded at the central (CE) electrode, while the inner electrode
carries the signal. The length of the dipoles depends on local
conditions and the presence of installation facilities. In HIO
monitoring site, local conditions permitted a length of almost 200m
long dipoles to be installed at NS and EW directions, using
non-polarizing electrodes, due to the absence of boreholes. In PYR
monitoring site, 160m long dipoles are used, at almost NS and EW
directions, using as electrodes the metal casing of existing
boreholes. In ATH monitoring site, 20m dipoles are used only, with
non-polarizing electrodes. The dipoles directions of ATH monitoring
site are SW-NE, SE-NW.
Experience, obtained, from the results so far, shows that
precursory, seismic electric signals can be successfully received,
even with small length dipoles, provided that, an effective signal
to noise separation scheme, is used. It is characteristic, that a
dipole of 120m located in Volos, Greece, at a distance of 650Km far
from the corresponding epicentral area, detected the precursory
electrical signals, generated, by the earthquake in IZMIT (17th
August, 1999, M=7.5R).
Fig. 12.2.2.1.
Typical cables lay out that form the receiving dipoles.
L1 and L2 are the lengths of the dipoles while φ1 and φ2
are the corresponding deviations from true NS and EW
directions.
Problems, met, during the operation of the network are mainly the
animal’s attack to the wires. Goats and ships like very much the
plastic cover of the wires.
Some times, strong winds fatigue and break the wires, thus,
producing gaps of data for some hours. Finally, some careless field
workers, accidentally, cut the wires during fieldwork.
Consequently, each monitoring site must be supervised by a
technician, who will be responsible to repair any damage (mainly in
cables), reported, by the central, monitoring, research group.
12.3. Section B. Signal pre-conditioning unit.
This is the “heart” of the monitoring hardware. The following
figure (12.3.1) will show its importance.
Fig. 12.3.1.
Typical “wrong” installation of a monitoring site is
shown. A ground current loop is generated, because of
the different grounding location of the used dipoles and
the main hardware.
In most cases, the field dipoles are located in a certain distance,
far from the place where the rest of the registering hardware was
installed. This creates a serious problem and induces severe errors
in the measurements. The common electrode CE is grounded directly at
its location, while, at the same time, it is grounded at GE, where
the main hardware was grounded, through the grounding line of the
mains AC supply lines.
The result of these two grounding locations is the generation of a
“ground current loop”, which “distorts”, locally, the form of the
Earth’s electric field that is registered by the two dipoles. The
latter, results into erroneous, data obtained and consequently, in
wrong, azimuthal direction calculations, too.
Therefore, it is of primary importance to solve this technical
problem before commencing into any further data registration and
processing. The problem of the galvanic isolation of the grounding
was solved by using separate, floating power supplies and optical
isolators as follows:
12.3.1. Power supply.
The power supply of the preconditioning unit consists of four
distinct, floating power supplies P1, P2, P3, P4 consisting of: the
secondary windings of a main transformer (primary 220VAC) of 19 VAC,
the rectifying unit and a voltage regulator of 13.5V. These separate
power supplies continuously charge four 12V batteries of 2Ah each.
Therefore, this unit serves as a UPS system for the preconditioning
unit. The power supply is shown in the following figure (12.3.1.1).
Fig. 12.3.1.1.
The used power supply and UPS for the preconditioning
unit.
12.3.2. Front end.
From this section and on, all operational amplifiers are considered
as powered by + / - 9V and by-passed in the traditional way.
The front end of the preconditioning unit (fig. 12.3.2.1) serves
two purposes. The first one is to protect the monitoring site
hardware from any lighting discharge that could destroy the entire
system, while the second one is to balance out any parasitic,
stable, potential, developed at the electrodes.
Lightning and overload input protection is achieved through R1 and
the pair of zener diodes Z, connected back to back. The combination
of R1-Z forms a clipping system, having set the clipping level at
+/- 7 volts, which is within the working dynamic range of the
Op-741.
Fig. 12.3.2.1. Lightning protection and parasitic
potential balance out circuit is shown.
The parasitic potential is
balanced out through the R3 – R4 system. In practice, an opposite,
in sign and level, potential in respect of the parasitic one, is fed
into the Op.-741 through the potential divider R3 – R4. The
front-end unit is powered with +/-9V dc, obtained, through a voltage
regulator, powered, with 13.5V from P1 and P2. The double stage
regulation of the power supply of the Op.-741 serves as a protection
against any excessive discharge of batteries, when a very long
period of time power failure takes place.
12.3.3. Optoisolator.
This unit serves the main purpose of galvanic isolation. To this
end, the A7800 optoisolator is used. The detailed circuitry is shown
in the following figure (12.3.3.1). Bypassing of power of Op. 741
and A7800 is applied, as usual. Care must be taken for the A7800,
for keeping the used bypassing capacitors leads as short as
possible.
At the input, the signal fed from the previous section is lowered
about 50 times for preventing saturation of the A7800 since it is a
low-level potential optoisolator. The original signal level is
restored back through the next Op.741.
Fig. 12.3.3.1.
Detailed, optical isolation circuitry, used, in the
preconditioning unit.
The main feature of this unit is that the first part of the A7800
is powered with 5Vdc, obtained from P2, while the rest of the
circuitry is powered from P3 and P4. Consequently, there is no
galvanic connection between the front end (12.3.2) and the rest of
the hardware.
In case that a lightning strikes nearby and the clipping circuitry
fails to operate because it has already been “evaporated”, then the
damage will be limited to just one Op.741 and one A700. The rest of
the hardware will remain safe. So far, within the 4 years period of
operation, no such damage took place in any of the operating,
monitoring stations.
12.3.4. Low-pass, band-pass filters.
Two filters process the optoisolated signal. The first one is a
low-pass active filter while the second one is a pass-band active
filter (Graeme and Tobey 1971; Mitra and Sanjit 1971; Moschytz
1975).
12.3.4.a. Low pass active filter.
The low-pass filter is implemented in figure (12.3.4.a.1). The
upper band limit is set to 30 seconds. In other words, signals with
periods shorter than 30 seconds are rejected.
Fig. 12.3.4.a.1. Low-pass filter implementation is
shown. Upper band limit is set to 30 seconds.
12.3.4.b. Band pass active filter.
The band-pass filter is implemented in figure (12.3.4.b.1). The
center band period is set at 24 hours.
Fig. 12. 3.4.b.1.
Band-pass filter implementation. Center band period is
set at 24 hours.
The purpose of this filter is to extract, from the raw data, the
daily, tidal, lithospheric oscillating signals, which are generated
by the tidal waves. The same type of oscillating signals can be
obtained from the low-pass filtered data, through the use of an FFT
band-pass filter. The difference between the two methods is that the
band-pass filter generates real-time oscillating signals, in
contrast to the FFT method that is applied “a posteriori” on a data
set. In practice, since the evolution of a preseismic signal lasts
for a few days, there is no practical “predictive” difference
between the two methods which are used.
12.3.5. Signal amplification.
This part of hardware (fig. 12.3.5.1) serves a specific reason
only. It has been added so that the resolution of the registered
signal increases beyond the 12 bit capabilities of the ADDA card.
Fig. 12.3.5.1.
Signal amplifier with selectable gain (1, 5, 10).
Its operation is as follows:
Let us assume that a 10mV signal enters the system. Moreover, a
+/-5V ADDA 12Bit card is used for digital conversion.
Then, each step of conversion (resolution) equals:
5000mV / 4096 (12bit) = 1.22 mV.
The 10 mV signal will correspond to 10mV/1.22mV = 8.2 steps of
conversion.
Next, the 10 mV signal is amplified 10 times and becomes 100 mV.
This signal corre-sponds to 82 steps of conversion. During the final
processing of the signal, this is divided by 10 so that the original
signal amplitude is obtained.
Consequently, the resolution of the entire system (signal
amplification – ADDA card – signal amplitude restoration) becomes
0.122mV/conversion step.
In practice, the 12bit ADDA card, through the use of this
amplifier, behaves as a 15bit ADDA card! In simple words, the
dynamic range of the ADDA card is much better exploitated by
increasing the low level signal amplitude, as long as it remains in
the dynamic range of the ADDA card.
In the following figures are presented examples of recordings which
were made by using this technique. The first figure (12.3.5.2)
represents a typical 12Bit ADDA card. Its resolution is 1mV. In 4mV
recording amplitude it corresponds to 4 sample amplitude levels.
Fig. 12.3.5.2. Typical, digital recording made by a
12Bit ADDA card. Resolution = 1mV, recording amplitude =
4 mV p-p.
Next example (fig. 12.3.5.3) represents the digital recording made
by a 12bit ADDA card after the pre-amplification of the original
signal for 10 times. The increase of the achieved resolution is
self-evident, between figure (12.3.5.2) and figure (12.3.5.3).
Fig. 12.3.5.3. Digital recording made by a 12Bits
ADDA card and the use of an amplifier of Gain = 10.
Resolution = 0.1mV. Recording amplitude = 1mV p-p.
A close-up view (fig. 12.3.5.4) of the previous presentation (fig.
12.3.5.3), which indicates an improved resolution of 0.2 mV, is
shown next.
Fig. 12.3.5.4. Amplitude potential difference
observed, between two random consecutive potential
samples. Resolution = 0.2 mV.
Fig. 12.3.5.5. Best resolution, observed, of 0.01 mV
between two consecutive potential samples which present
the least observable potential difference.
Finally, a generalized presentation, concerning the performed
galvanic isolation in the preconditioning unit, is given (fig.
12.3.5.6).
Fig. 12.3.5.6. Block diagram, representing the
galvanic isolation scheme, used, in the preconditioning
unit.
The dipoles, the front end and the first half of the optoisolator
use, as ground, the common electrode CE, while the rest of the
hardware uses a second common ground which does not affect the
potential measurements at all. The electrical signals are
transferred from the front end to the rest of the hardware, through
the optoisolator, with no galvanic connection at all.
12.4. Section C. Desktop computer.
The desktop computer unit consists of: the main computer itself,
the ADDA card and the communications software. We preferred this
solution, to build the monitoring site, because it is a cheap one,
since it does not require expensive, registering hardware.
Old-fashioned computers are found almost at no cost.
12.4.1. Analog to digital conversion card.
The Decision Computer International Group 12BIT ADDA card is used,
as an analog to digital conversion card. The card is readjusted from
its default potential input of +/- 9V to +/- 5V. The ISA slot type
was selected, since this is the most common slot, found in the
majority of the old-fashioned computers.
12.4.2. Desktop computer.
The desktop computer, which is used, can be of any type capable of
cooperating with ADDA cards. What is important, of its features, is
the CPU frequency. Data samples of 30000 – 40000 samples / minute
were achieved with frequencies of 100 – 133 MHz.
The operating system of Microsoft windows (Windows 98SE or later)
was found to be very useful for implementing all the necessary
programming, required. Moreover it allows a large variety of
communication software to be installed and convert the computer into
a host server.
Before the start of the operation of any computer, care must be
taken so that:
- Any messages, at the start up of the operating
system, must be turned off. Otherwise, at any power interrupt and
the subsequent startup, the operator should answer all these
questions, asked, by the operating system. The main idea is to
automatically restart up the computer, after any power failure, in
“blind mode”.
- Summer time facility must be turned off to avoid time
errors, due to automatic change from winter time to summer time
clock and vice versa.
- A problem, sometimes, met, is “clock drifting”. It is
caused, either by low battery on the motherboard, or low quality
motherboard. In any case, observed, typical “clock drift”, is of the
order of 1-2 minutes per week that is adjustable each time (daily)
the host computer is visited, to download the data files.
12. 5. Section D. Modem.
Communication with the central offices is utilized by an external,
typical modem and dialup operation, through public telephone
network.
The entire setup of the monitoring site is shown in the following
figure (12.5.1).
Fig. 12.5.1. The entire setup is shown of the
monitoring site.
What is important in this setup is the fact, that two separate UPS
are used. The first one (UPS-1) provides power to the
preconditioning unit. Its operation on power failure can last for a
few days since the current consumption of this unit is very low (a
few mA). The second (UPS-2) provides with power the computer
motherboard and its peripherals. Its duration depends on the
batteries, which are used by the UPS-2. It has been found that its
operation lasts for 3.5 hours with a 15Ah battery.
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