M. Van Ruymbeke (1), F. Beauducel (2), and A. Somerhausen (1)(1) Observatoire Royal de Belgique, Avenue Circulaire, 3, B-1180 Bruxelles, Belgique
Abstract. The forecasting of natural hazards requires a multi-parameter approach to incur an increased understanding of the processes involved in nature and thus aid to decrease the risk of false alarms. For very complex interactions concerning fluid-flow modulation induced by tectonic activities (volcanic, geothermal and seismic areas, land-slide zones...), the diversity of the sensors is so broad that the use of existing technology provides a difficult barrier for scientists with a limited knowledge in metrology. To address this requirement, an Environmental Data Acquisition System EDAS was developed at the Royal Observatory of Belgium, this system is conceptually based on the standardisation of geodynamically based instrumentation and provides for the implementation of resistive and capacitive transducers by means of sensor interfaces which operate using standard supplies and standard output signals. The core base of this standardised data acquisition system are the DAS and ÁDAS monitoring devices which produce standard data files which follow a standard sequence to contain date, time and the recorded values in ASCII columns. These files can in turn be viewed and analysed by utilising the software explicitly created for EDAS files.Keywords: Geodynamics, transducer, seismology, volcanology, tides, multi-parameters system
With the improvement of accuracy in tidal instrumentation, devices like gravimeters, tiltmeters and strainmeters, now exhibit various effects of environmental origins previously negligible but now clearly seen in the residues of signals. The EDAS equipment is the answer to the request of European geophysicists to monitor the environment's parameters in geophysical stations (Walferdange 1993, Aussoie 1994, and Lanzarote 1995). After a period dedicated to the introduction of electronics in existing instruments, installed in the Underground Laboratory of Walferdange (Gr. Duchy of Luxembourg) [Flick et al., 1991], a prototype system was developed in collaboration with the State Seismological Bureau of China [Cai Weixin et al., 1991] dedicated to the monitoring of ground deformations. At the request of R.Vieira this instrumentation was installed in the three main laboratories on the island of Lanzarote (Canarian archipelago, Spain) [Vieira et al., 1995], as difficulties were being experienced due to local environmental conditions, in comparison to the tidal laboratories which due to their location at sufficient depths, allow for thermal waves existing at the surface to be neglected. Similar equipment (EDAS) was also experimented on during the solar eclipse that took place in the south of Brazil in 1994. On this occasion, a network of more than thirty instruments was deployed in two days around a tidal gravimetric station to observe pressure, moisture, light, temperature, tilt and gravity.
The probes designed at the Royal Observatory of Belgium are adapted to the property of their concerned geophysical parameter. It is required to consider the medium where instruments will be installed as it is an intrinsic part of the transducer. The following chart (Table 1) presents examples of this statement. In order to increase the precision of the sensor, evaluated by the signal to noise ratio, we systematically select passive transducers that need a minimum in energy transfer from the surrounding medium. The effect of the sensor on the medium is thus minimised. Only resistive, inductive and capacitive (R, L, C) sensors are considered. Optical sensors are based on on/off detection and are adaptable to EDAS electronics but this topic is not addressed in this paper. The types of probes concerned with this conceptual approach are listed in Table 2. The probes are classified into two main categories: transducers without geometrical changes and transducers with geometrical deformations. The transducer's qualification is carried out in a laboratory where the admittance of various parameters that could influence the output signal is established. For on-site operation, a procedure allows to ensure that each sensor is functioning within its range properly and a further quick testing confirms that the instruments will provide the same transfer function than in the laboratory. High-resolution probes having a limited range often need a re-centring procedure that could modify the transfer function. The first type of probes mentioned is characterised by minute dimensions while the second type can be as large as needed.
Table 1. List of multi-parameter probes developed in the EDAS Concept at the Royal Observatory of Belgium.
|Geodynamical Categories (1)||Geophysical Parameters||Type of Instruments within the EDAS Standard (2)|
|Aquifers (C,F,G,T)||Water level||Nivocap probe|
|Air Temperature||EDAS Thermometer|
|Earth Tides (C,G,T)||Gravity field||Feedback control for gravimeter|
|Tilt||Horizontal Pendulum, Vertical Pendulum, Water tube|
|Strain||Horizontal Strainmeter, Vertical Strainmeter|
|Geothermal zones (C,F,T)||Gravity field||Feedback control for gravimeter|
|Oceanic tides||Oceanic level||Tide Gauges with tube and with aneroid cell
Scale-type Tide Gauge
|Seismic zones (F,G,T)||Tilt||Horizontal Pendulum, Vertical Pendulum, Water tube|
|Volcanic zone (C,G,T)||Gravity field||Feedback control for gravimeter|
Table 2. Description of various instruments within EDAS Concepts.
|Parameter||X (1)||Y (2)||Z (3)||Max. rating||Noise Level||Utilities|
|Air temperature||B||R||O||50░C||1 m░C|
|Water temperature||S||R||F||5░C||0.1 m░C||Boreholes|
|Temperature||B||R||F||2░C||10 ÁK||High precision measurements|
|Differential temperature||B||R||F||2░C||10 ÁK||Bolometry,Thermal oscillator, Fluid Flow|
|High temperature||S||C||O||600░C||0.1 ░C||HighTEMP|
|Pressure||B||C||F,G||100 mBar||1 ÁBar|
|Water-level||S||C||O||1 - 3 m||30 - 100 Ám||Boreholes, Maregraphs|
|Inclination||B||C||F,G||1 mRad||10 nanoRad||vertical pendulum RPH, Water-tube|
|Gravimeter||B||C||M||4 cms-2||10 nms-2||Maximum Voltage Retro-action system (M.V.R.)|
|Light||B||R||-||10-4 of Full Scale||Climatology|
The EDAS concept is an answer to the problem of multiple parameter monitoring. For this purpose, some fundamental principle are applied:
A single output signal could result from a complex interaction concerning parameters of various forms. An interaction between these parameters could also exist.
These remarks have to be applied to every step of the transfer function and one must not take instrumental effects as a geophysical one.
Continuous monitoring is helpful because in physical processes many different aspects are characterised by their frequency. The energy transfer pattern is classified in the frequency domain in order to separate the different processes that have to be analysed.
Auto-correlation's confirm the periodicity. Cross-correlation between two signals reveal the admittance amplitudes of the different parts of the spectrum and the phase difference, both which assist to model the physical content requirements to be met by the physical models.
The Data Acquisition Systems (DAS and ÁDAS) developed at the Royal Observatory of Belgium have been designed so that the data treatment is made as easy as possible. In order to avoid complex filtering methods requiring high sampling rates and calculations, the DAS and ÁDAS devices are designed with counters that integrate F.M. signals on a fixed time basis. The rejection ratio is proportional to the period of the integration and is perfectly linear.
The counters have one hundred thousand points of dynamic and deliver a serial BCD value for the data acquired. These counters work permanently in parallel and the counting results are stored in memories that can be reset or not. If the counters are not reset, successive measurements are subtracted from each other and their difference is recorded.
As the full scale of the counters is set at 100000, the devices allow the recording of large dynamic signals since they are equipped with dividers for, by 2, 16 or 128 division. On the DAS, the integration period is set to 1 minute. For a 40 kHz signal which is the usual frequency of the sensor's interfaces, the values of the counting expressed in the decimal system are presented in Table 3 according to the division.
Table 3. Values of the 5-digit counting according to the division, for a 40 kHz signal and 1 minute integration period.
Division by 2
Division by 16
Division by 128
The five last data bits represent the stored value. In the case of a division by 2 or by 16, there are missing digits that correspond to the number of full scale attained by the counters.
In order to reconstitute a complete signal (without full-scale jumps), one must often work with the data coming from the signal when the divisions are by 128 and by 2. This means that an instrument must then be connected to two different channels.
The DAS was the first acquisition system that we have developed on the basis described above [Beauducel et al., 1993]. It is provided with a parallel port plug and appropriate software . Using a simple XT computer (provided with a disk drive and a hard disk or not), it is possible to let an acquisition run for more than a week (there is barely no limits if the computer is equipped with a hard disk) with the data recorded every minute and systematically transferred onto disk. Internal quartz synchronises the computer when the data acquisition system is initialised.
In a second step, the ÁDAS board was designed. This acquisition system is a four-channel stand-alone system based on a Z80 microprocessor. The integration period may be selected by software in a range going from 1 second up to 3600 seconds. The ÁDAS is equipped with static memory that can store about 215000 readings. The data is downloaded via RS-232 to a personal computer.
In the case of the ÁDAS, the counters are not reset but each time the value of a counting is lower than the previous one, the number of full scale overcountings is incremented by one. Such a method to deal with the full scale is efficient as long as the signal runs at a frequency less than 200 kHz (above EDAS maximum frequency standards).
Note that the ÁDAS is also provided with 2, 16 and 128 divisions and that Schmitt triggers are set at the inputs in order to avoid cross-talk between the input lines.
The ÁDAS can be used in three different ways:
Permanently connected to a PC just like a simple four counter data logger.
Initialised with a portable PC. The ÁDAS records the data in its memories, which needs to be downloaded at least every month if a one-minute acquisition period is selected. The ÁDAS is permanently accessible, as an ź APR ╗ file informs the user about the period of supply connection of the ÁDAS. If the data is not collected when the memories are full, the exceedent data is recorded on top of the old ones. It is possible to download either the entire memories or only the data covering a period starting when the last downloading occurred. This last procedure allows an acquisition run at a high rate of acquisition and to be collected only if an interesting event occurred in the last period covered by the memories capacity.
A ÁDAS can be replaced by another one at any time while the first one is brought back to the laboratory for downloading.
Used in a network. This solution requires a special interface that allows the use of up to 64 different ÁDAS using only one computer. The ÁDAS (slaves) are connected to an interface which is defined by an address used by the computer (master) to establish the connection. In this case, a three-wire cable can make a connection over a distance of about 1 kilometre.
The instruments require different electronic interfaces depending on the type of sensors which are described in the table 2 (capacitive dC or resistive dR using bridge or single structure).
Standardised interfaces for EDAS consist of six electronic configurations which use a single DC grounded supply with 11 to 18 V output. Generaly the ÁDAS recorder is connected in parallel with the four transducers and the complete system consumes less than 50 mA which needs a 30 cm x 30 cm solar panel and a 10 A.h battery.
The type of signals selected are frequency modulated, which are well adapted to be transferred at long distances and through opto-electronics if DC-DC converters are used to isolate completely, local ground of the instrument with the main supply. Generally this grounding system justifies a careful approach mainly when using solar panels.
Passive lightning protections consist in input circuits that can be replaced easily.
Described below are the six EDAS interfaces that have standardised supplies and output signals.1. EDAS Oscillator
The first interface is a relaxation oscillator that uses a voltage comparator as it's central component. Its purpose is to convert a dC or a dR reading into a square wave signal whose frequency variations are proportional to the dC or the dR input signal. The lowest recommended value for C being 10 pF. The amplitude of the oscillations is set so that it is ranging in the limits of the active zone of the components. The oscillator, by its design, is able to work at frequencies higher than 100 kHz but this value is generally taken as the upper limit fixed for the DAS. The oscillator is also able to deal with pulse or FM inputs like an interface with an eventual adjustable hysteresis that suppresses false counting induced by electrical noise.2. EDAS Voltage-To-Frequency converter
Another important interface is the Voltage-To-Frequency converter (V-F) which admits analogue signal inputs (coming from a resistor bridge for instance). The main element of the V-F interface is an instrumentation amplifier provided with an adjustable gain ranging from 1 to 1000. The two inputs of the amplifier are referred to an active ground set at 4 volts and are equipped with tension dividers so that the input voltages can be adjusted around this reference value. The upper and lower limits around the reference voltage are +2 volts and -2 volts. The V-F converter works at frequencies ranging from 0 to 80 kHz but is generally used between 25 and 55 kHz for stability purposes. It can be stated that from an input tension of (4 ▒ 1 x U) V, the output frequency will be (40 ▒ 10 x U) kHz.
Any kind of low frequency analogical signal is interfacable with this circuit to the ÁDAS without need of common grounding.3. EDAS Floating Bridge
Based on the V-F converter geometry, a series of other interfaces have been developed.
The Floating Bridge Interface (FlBr) has been designed in order to treat the signal coming from a capacitor or resistor bridge whose central element is floating. The power supply of the interface must be floating and can vary between 12 and 20 volts since it is then stabilised at 8 volts by a regulator. A oscillator drives the divider so that we have a 8-volts square wave signal at our disposal with a frequency of 9 kHz which could be decreased for special purposes (thermistors, ...).
This signal is then inverted twice in order to feed the Capacitor Bridge and a phase detector by two signals that are in phase opposition. At the output of the bridge, the signal is sent to an operational amplifier.We select the inputs (+) for resistive sensor or (-) for capacitive one.The amplifier is feedback with resistors and capacitors depending on the bridge compounds. After filtering and amplification, the signal is sent to the phase detector that is constituted by two pairs of electronic switches placed in parallel. An amplifier subtracts rectified signals sent in low-pass filters.
This VRL circuit does not need a transformer and has a very wide dynamic range. Working frequency is relatively low (less than 9 kHz) and the circuit doesn't require special radioelectrical protection and will not perturb the environment.
The circuit can be used with capacities ranging from 10 to 100 pF. If the distance z between the plates of the bridge electrodes is 1 mm, the dynamic response of the circuit allows measuring displacements dz of 1 nanometer. When used in a narrow bandwidth, it is even possible to measure displacements of 0.1 nanometer. The physical dimensions of the circuit make it integrable inside the instruments themselves.
The use of resin moulding significantly reduces the noise induced by electrostatic discharges and thermal shock.4. EDAS Grounded Bridge
This circuit was developed for measurements using capacitor bridges with the central element connected to the ground (i.e. : recording the displacement of the mass of a seismometer moving between two floating plates). This circuit is called Grounded Bridge (GrBr).
The GrBr is equipped with a quartz oscillator that provides a square wave at a fixed frequency. This signal is then duplicated and a 90░ - phase shift is introduced between the two signals. Such a change insures that the signals won't perturb each other by cross talk through the supply. One of the lines is directly connected to one of the two inputs of an exclusive NOR gate. The same signal is sent to the capacity of the sensor through a resistor constituting a RC cell. It produces a signal connected on the second input of the exclusive NOR gate that is deleted from the direct signal. The difference between the two signals will be proportional to the RC value and is the length of the pulse appearing on the output of the exclusive NOR. The output of the gates is a train of pulses since such a gate delivers a low output signal only when the two inputs are different. Using a low-pass integrator, we obtain a DC voltage that is proportional to the capacitance of the sensor.
The second line is equipped similarly and we obtain a second DC voltage proportional to the other capacitance of the bridge. These two signals will be combined and connected to a similar circuit as the V-F.5. EDAS 556 oscillator
The 556 oscillator consists of two classical 555 oscillators that are well compensated for supply variations and temperature changes, with a high current output usable over long electrical cables.
The first 555 is used in astable mode delivering a signal at a frequency given by the values of R and C components attached to it. This square wave signal is used to trigger the second 555 of the circuit that is used in monostable mode with a pulse length determined by the two components R' and C' attached to it. By filtering the output of the second 555 using a low-pass filter we obtain the mean voltage X which is:
Where t is the length of the astable pulse and t' is the length of the monostable pulse. Thus we can say that X at the output of the circuit will vary according to the equation
With K proportional to the value of the voltage supply.
Depending on the type of resistive or capacitive probes we are using, it will be connected to the first 555 astable or to the second monostable one, delivering a linear or inverse transfer function.
The mean voltage is then sent to an amplifier and a V-F converter in order to have a FM signal at our disposal for the data acquisition system.6. EDAS Maximum Voltage Retroaction
The MVR is a feedback system used for instruments equipped with a capacitive transducer bridge like a LaCoste-Romberg gravimeter. Its principle is based on the application of an electrostatic force on the plates of a capacitive sensor in order to keep a moving mass at a fixed position. In this case the force is proportional to the square of the modulated voltage difference.
The first task of the circuit is to determine during the charge of the capacitors which one of the two has the smallest value (the charging of the circuit with the smaller RC value will be quicker). It corresponds to the largest distance between the moving mass and the fixed plates of the capacitive transducer.
A high voltage will be applied on its terminal until the mass, drawn back by electrostatic forces, reaches and finally overshoots its equilibrium position. The system is then reversed and the full polarisation voltage is applied to the other capacitor to bring back the mass again. At each inversion of the voltage, the application of the full feedback force is governed by the choice of the smallest capacity. This system is auto-stabilised and can be connected with a direct or inverse polarity.
Referring to the problem of the monitoring of short-term signatures related to tectonic activities, we consider that only a multi-parameter approach minimises the risk of spurious conclusions.
Since geothermalism and fluidics are very complex phenomena that require a global approach and a rigorous methodology, we believe the EDAS concept is well adapted to the requirements concerning realistic interpretations of hydrological and thermal process.
Let's mention that a special effort is dedicated to the training of geophysicists who have to use the EDAS concept. The elementary conception of the EDAS didactic program makes it accessible to field operators with low technical background.
More than one hundred instruments are now installed in fifteen countries around the world and day after day new developments improve the system. With the help of various partners like The European Commission, we continue to collect questions and remarks of users for a maximum of efficiency in the solution of scientific questions.
The Comission of the European Communities, DG XII, Environment Programme, Climatology and Natural Hazards Units, in the framework of the contract EV5V-CT92-0189 and EV5V-CT93-0283 in part supported this research. The authors are appreciative of Pr. P. PÔquet, Director of the Royal Observatory of Belgium who provided the necessary support, especially through the efforts of Jean-Marc Delinte, Robert Laurent and Francis Renders. Many student projects from different institutes of education contribute to the EDAS establishment. The authors are grateful to all the persons, who are using EDAS instruments, and have participated to the improvement of the different parts of the system. In particular, authors are grateful to Pr. Ricardo Vieira (Lanzarote), Pr. Cai Weixin (BES), Ing. Jean Flick (ECGS) and Mr. Nicolas d'Oreye de Lantremange for giving opportunities to experiment EDAS. Fruitful contributions were given by Dr. Bernard Ducarme, Dr. Malte Westerhaus, Dr. Philippe Jousset, Ing. Gao Weimin, Miss Naphsica Grammatica, Mr. Frederic Hody and Mr. Philippe Faucon and Mr. Eamon Ryan. The final english vertion of this paper was prepared with the effective support of Eamon Ryan at the Royal Observatory of Belgium during a stage granted by the ERASMUS european mobility project.
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