MODULAR APPARATUS FOR THE ELECTRIC PROSPECTION OF A MEDIUM
DESCRIPTION
Background art
Various apparatuses for electric prospection of a medium are known which are used in geo-electric field for analyzing in a non-invasive manner the variation of resistivity, or more generally the impedance, in a medium, such as in the subsoil, typically associated to the variation of the composition, lithology, water content or the presence of cavity or buried object.
For executing a geo-electric measurement, it is assumed that the current conductivity in the subsoil follows the Ohm law, and therefore it is possible measure the resistivity of the soil starting from current and voltage measurement. For removing the influence of the contact resistance of the electrodes, the resistivity measuring is carried out by means of the "four points" called technique, wherein two electrodes inject current and other two are used for reading the voltage difference.
For executing a single measure of the resistivity of the examined medium, a prospection apparatus should comprise essentially a current or voltage generator, a current meter, a voltage meter, four electrodes, four cables for connecting the current and voltage meters to the electrodes.
However, such classical apparatuses have not shown adequacy for more complex inspection, such as hydrogeologic, engineering, archaeological inspection and those on cultural assets. In these cases the heterogeneity of the inspected medium requires the reconstruction of the impedance either on vertical sections of the examined medium (2D-thomografy) or on the whole examined volume (3D-thomografy).
Therefore, sequences of measures should be executed varying both the middle point between the electrodes and the position of the quadrupoles on the acquisition surface, or possibly in measurement wells.
For this reasons new apparatuses have been developed, named geo-resistivity meter, which apparatuses allow to automatically execute the quadrupole measure sequences without moving every time the electrodes and the connections with the various parts of the apparatus. In practice, similar new apparatuses provide the connection of the electrodes to the generator and to the current and voltage meters by means of a multipole cable. Therefore, by means of suitable switches it's possible to manage in a programmed and automatic manner the connection of the instrument provided with the electrodes. In particular it is possible to electrically connect in a selective manner the cables coming from the current generator among two current electrodes, while the voltage gauge is connected to a couple of cables which serves a couple of voltage electrodes.
For each connection, the apparatus realizes a simultaneous measurement of the current and voltage difference, for calculating the resistence.
In this field it's possible to define two main categories of geo-resitivity meters, operating according two different principles, as the switches for selecting the electrodes for which executing the measures are centralized or distributed along the multipole cable. In the first condition the multipole cable connecting the electrodes to the examined medium and to the instrumentations comprises a conductor for each electrode. Then a series of switches connect the module for current generation to a couple of any conductors of the multipole cable.
Instead a different series of switches connect the module for the voltage measurement to one other couple of electrodes. Generally in this type of instruments the current generation module, the current measurement module, the voltage measurement module and the switches are contained in a single box-like body, in such a manner to simplify the use and the transport of the instrumentations. However, these solutions have different drawbacks, a first of which is due to the difficulty of manage high intensity current. In this case it is necessary to increase the section of all the conductive channels of the— - multipole cable.
It is clear that similar cables are particularly heavy and cumbersone, besides to be particularly expensive because a great number of conductors corresponds to a great number of plugs, so limiting the number of electrodes which could be aligned. Moreover, also the labour cost for assembling the cable is high because it is necessary to provide the connections for the electrodes along the cable at regular distances, managing a multipole cable having dozens of conductors, as shown for example in GB2253912 and US2005/078011.
By contrast US6404203 and WO9005923 use multiple cable according different ways which cables having a couple of conductors for injecting current, a couple of conductors for voltage measuring, various conductors for communicating with the central instrument, wherein the current generator and the gauges are provided.
The switches are placed on the cable, inside a waterproof housing, close to the connection with the electrodes. By this way, the cable is lighter than the instruments with centralized switches, because it is made by a lower number of wires. Moreover, the design of cables which could sustain high currents (higher than 2,5 Ampere) is easier and so managing a high number of electrodes aligned on the ground.
However, also these solutions are not without drawbacks, due first of all to the fact that it is not possible simultaneously acquiring in a multi-channel fashion various measurements because the electrodes are connected in turn to the available conductors, while the measurement is executed only by the main unit.
As a consequence, unless increasing the number of the conductors in the cable and the complexity of the system, the measures are markedly slower.
Moreover it is necessary housing the switches along the cable inside low cumbersome, waterproof container resistant to the mechanical strains proper of the unwinding and winding of the cable on a roll, which are necessary each time that one is on a field for using the instrument, with a consequent high increasing of the cost of the cables, because both for materials and their assembly. Moreover, the housings of the switches are generally cumbersome and represent an eventual fragile area for the cable.
Moreover, both the instrumentations with centralized and distributed switches are limited by a too high length of the line for transmitting the analogical signal from the voltage electrodes to the voltage measurements module, which could bring to a distance between the measurement points and the instrument up to hundred of meters. This causes that the instruments are sensible to noise or electromagnetic interference, most of all those irradiated from the 50 or 60 Hz electric lines. Moreover the high entering impedance of these instrumentations make them vulnerable to the capacitive coupling of the conductors with the multiple cable.
Scope of the invention
A main scope of the present invention is overcoming the above drawbacks providing a modular apparatus for the electric prospection of a medium that is particularly economic and effective.
A particular scope is realizing a similar apparatus which allows the multichannel acquisition of data of electric current and of the voltage referred to several electrodes at a time.
One other scope is to realize a similar modular apparatus which allows to highly increase the prospection speed through the simultaneous acquisition on all the measurement electrodes.
A further scope of the invention is realizing a similar modular apparatus having high flexibility and easily transportable.
Another scope is realizing a similar modular apparatus which is substantially invariable with respect of the electromagnetic interference provided by external elements and by the capacitive coupling of the cables.
This scopes, and others that become clearer later, are obtained with a modular apparatus for the electric prospection of a medium, particularly of a ground or body, according claim 1.
The voltage measurement means comprise a plurality of advanced measurement modules, independent with each other and electrically connected to a generator and to a respective electrode.
Each measurement module could comprise corresponding devices for making by itself whole voltage and/or current measures, intended as assembly for conditioning, digitally converting and locally storing the measures, and for their possible transmission and a switch susceptible of connecting the respective electrode of said plurality to first cables and/or at least one second cable for its selective enabling/disabling.
Thanks to the use of advanced measurement modules, autonomous and able to executing and storing by themselves the local measures, the simultaneous measuring of the current in correspondence of at least one first electrode and the measure of the voltage induced between two electrodes other than the first is allowed.
In particular, it will be possible to measure simultaneously a number of voltage equal to that of the electrodes, through voltage measuring modules. This cause the maximum reduction of the acquiring time for the measures. Moreover the simultaneous measuring on all the electrodes allows the analysis of the spontaneous voltages in the examined medium, because only a simultaneous measurement make sure the reliability of the analysis of said spontaneous voltages because these are variable during time. Opportunely for measuring the spontaneous voltages not-polarizable electrodes will be used. Advantageously, the measuring modules may be designed for managing the several instruments of the apparatus, such as the generator, the current measuring device and the voltage measuring means, besides the electrodes in such a manner to automatically execute on the examined medium any simultaneous sequence of voltage and current measures. These simultaneous measures could be executed by selecting one or more electrodes as reference electrode for voltage measures, one or more couples of further electrodes as current electrodes and one or all the remaining electrodes as voltage measuring electrodes, whose measuring will be conduced relative to the reference electrodes. It could be observed that the voltage measuring may be executed by the modules in a distributed fashion and in parallel. Analogously, the injection of the current may be executed simultaneously through one or more couples of electrodes, allowing faster measures.
These advantages are also obtainable with the association of the connectors to the modules in a distributed fashion along the connection cables, with a switch for each single measuring module, making the measuring modules operatively independent with each other.
Advantageously, the voltage measuring module may be connected to the cables through a bus-like connection, for operating with a high and variable number of aligned electrodes even with a low number of cables.
Brief description of the drawings
Further features and advantages of the invention will be more evident with a detailed description of a preferred but not limiting embodiment of the prospection apparatus according to the invention, shown as a non limiting example with the aid of the alleged drawings tables, wherein:
Figure 1 is an installation scheme of an apparatus of the invention;
Figure 2 schematically shows a particular of the apparatus of Figure 1 which represents the structure of the advanced measuring and switching modules used in the instrumentations.
Disclosure of a preferred embodiment
As it could be observed in Figure 1, a prospection apparatus according the invention comprises a plurality of electrodes 1 locatable in contact relationship with a medium 2 for injecting current thereinside and/or measuring the induced voltage in the medium 2.
Particularly, the electrodes 1 may be selected among those commonly available in the market and could be placed lying on the medium 2 or inserted, even only partially, thereinto. Thus, the apparatus comprises one or more current generators 4, which can work in constant current and/or alternate current and/or at constant voltage and/or alternate voltage, for injecting the current in the medium 2.
A first couple of first powering electric cables 7 is also provided for connecting the generator 4 with the electrodes 1 and a second electric cable 7 linked to the electrodes 1 for distributing the voltages and transmitting the analogical signal from a reference electrode to the others electrodes for measuring the voltages.
The apparatus also comprises one or more devices 5 for measuring the current connected to the generator 4 and means for measuring the induced voltage between one or more couples of electrodes 1.
Advantageously, a control module 6 may be provided which is used for controlling the apparatus and collecting and managing the measures and the acquired data, with possibility of interface with data net or external computers. Advantageously, the measuring means comprise a plurality of measuring modules 3 mutually independent, each able to execute, storing and possibly transmitting the measure, distributed along the cables 7 and electrically connected both to the generator 4 and to a respective electrode 1.
Advantageously, each measuring module 3 may be connected to the respective electrode 1 through suitable electric connecting means which may be designed in such a manner to selectively enable the electrode 1 and/or connect it with at least one of the cables 7, for increasing function and working autonomy of the modules 3. By this way the minimum embodiment physically possible for this type of measure could be realized, obtaining high immunity against interference and external noise. As matter of fact the measure and the conversion of the voltage is executed on the voltage measuring module 3, which are distributed along the cables 7 and placed preferably close to the corresponding electrodes 1.
The electric connection between the electrode 1 corresponding to the measuring module 3 and the electrode 1 used as reference will be realized by means one of the cables 7 for distributing the reference voltages, and one could obtain, according the disposition of the cables 7 on the examined medium 2, the shorter transmission line physically possible, that is the one wherein the voltage cables 7 run along a straight line from the electrode 1 used as reference to the respective measuring module 3.
Each measuring module 3 comprises a switch susceptible of connecting the respective electrode 1 to first and/or to the second cable 7 for its selective enabling/disabling.
Therefore the selection of each electrode 1 as current or voltage electrode or its disabling could be possible, in a totally autonomous manner with respect of the function of the other electrodes 1, allowing the simultaneous measure of the current in correspondence of at least one first electrode and the measure of the induced voltage between to or more different electrodes other than the first.
Preferably only one current generator 4 could de provided which is connected only to two first supplying cables 7. Moreover, a single electrode 1 may be used as reference for the voltage measures, which electrodes being anyone of the provided electrodes 1. Also the current measuring device 5 may be a single and centralized device, but more than one of similar devices may be provided, with maximum one for each electrode 1.
For powering the modules 3 one of the first supplying cables 7 and possibly an auxiliary cable could be used, so that only four cables may be used, minimizing their number and lighting the whole apparatus. In the case that powering accumulators are used for powering measuring module 3, it could be also possible not providing the auxiliary cable which could be however useful for recharging the modules 3 during the measuring operations, if the internal accumulators should discharge during the acquisition of the measures. The auxiliary cable, or the auxiliary cables, may also be used for transmitting the data acquired from the measuring modules 3, as well as for their calibration. Opportunely, cables having adequate section for high current flow may be used, for mere example cable of the AWG 18 type for managing 10 Ampere current. The electric connection means comprise a plurality of connecting plugs 8 distributed on the first cables and/or on the second electric cable 7 which allows the connection of all the cables 7 to the modules 3 with a configuration according a bus topology.
Preferably, the plugs 8 will be higher or equal in number with respect of the measuring modules 3. By this way it could be possible adding channels to the apparatus simply adding voltage measuring modules, without changing the cables.
Moreover, it makes simple to overcome obstacles interposed along the path on which the cables extend. As matter of fact it is possible using a portion of the length of the same only for overcoming the obstacles, without taking care that some of the plugs 8 are disconnected. The measuring and switch capability of the apparatus does not depend from the number of the output of the cables 7, but from the number of the voltage measuring module 3.
Moreover, it is possible, with only one set of connection cables, using several distances between consecutive electrodes 1. For example it may be possible, for using 48 voltage measuring modules 3, to have a cable with 101 plugs, with the first 48 spaced of lm each other, the following 24 spaced of 2m each other and the last 29 spaced of 5m.
By this way it could be possible to place the 48 electrodes on the examined medium spaced according variable schemes, for examples with distances of lm, 2m or 5m, in function of the type of analysis which is in execution and/or of the properties of the examined medium.
In place of the cables 7 a single multipole cable may be used, which cable is provided with a predetermined number of conductors or a plurality of multiple cables with several conductors, which total number of conductors will be equal to the number of cables that would be used alternatively.
The multiconductor cable may be a plate cable, for allowing that the plugs 8 be connected thereto by direct crimping, simplifying significantly the assembling of the multiconductr cable, allowing the repairing the cable on the filed if any plugs fault and reducing the whole costs.
In the shown embodiment, the generator 4, the current measuring device 5 and the control module 6 are contained in a unitary box-like body 9, for simplifying the layout of the instrumentations and the transport. However it is also possible to realize an apparatus formed only by the voltage measuring modules 3 and the cables 7 and wherein the measuring modules 3 enclose a control module, a current measuring device and a current generator. Of course each intermediate embodiment between these may be possible.
As shown in Figure 2, in a preferred embodiment, the measuring modules 3 comprise a digital control circuit 21 implemented by means of a microcontroller or programmable logic.
Each module 3 may also comprise thereinside a measuring system 22, 23, adapted to measure the voltage difference between the respective electrode 1 and one of the reference junctions 20, a data storing unit 24, a remote communication device 25 fore receiving commands or transmitting the executed and/or stored measures toward a remote unit, and a programmable switching device 26.
This latter allows to selectively connect the respective module 3 with the current generator 4 through its poles 27, as well to the measuring device 22 and/or to the reference junction 20. These components may be contained together in a programmable logic control unit for controlling and storing the registered measures. Such a unit will be connected to a centralized control module 6 connected in turn also to the generator 4 for controlling the same and that will be provided with a storing unit for acquiring data.
The connection of each measuring module 3 to the control module 6 may be realized through a data transmission circuit adapted to send the registered measures from the memory unit. The transmission of data may also be performed in a wireless way.
The measuring system may comprise an inlet buffer amplifier 28, a programmable gain differential amplifier 22 and an analogical/digital converter 23 with resolution higher or equal to 16 bit, a non volatile memory for storing the acquired data 24, a radiofrequency communication interface 25, and the switch device of the signals 26 made by relay or solid state switches. The switching system allows to connect the local electrode 1 to one of the two poles 27 of the current generator 4 if one wants to inject current from the electrode 1, or to none of the two if one wants to read its voltage. This latter, adequately pre-amplified, can be used as reference voltage for the measure executed by the other modules, connecting it to the reference junction 20. The inlet buffer amplifier 28 allows the pre-amplification of the inlet signal coming from a voltage reference electrode and its transmission on the corresponding second cable 7 with low impedance, increasing the immunity of the signal against electromagnetic noise and the capacitive coupling of the cables.
For knowing at least the correct order of connection of the modules along the cables, because each voltage measuring module 3 may be connected in any point of the connection cable 7 and for not measuring each time in a single manner the position of the all measuring modules 3, the same will be also provided with respective automatic localization unit 29, designed for determining the relative positions and/or the mutual distances of the measuring modules 3 along the cables 7. Then, this unit 29 allows, knowing the disposition of the cable 7 and the distance between the plugs 8, to obtain the absolute position.
In a first embodiment, the automatic localization unit 29 will be designed for measuring the strength of the length of the first cables 7 comprised between two consecutive modules 3, or between the generator 4 and one of the modules 3. In particular, the localization unit 29 acts measuring the electric resistance of a length of one or more of the first connection cables 7, with the aid of the control module 6 and of the current generator 4.
As matter of fact, the electric resistance, measurable as the associated voltage drop, is proportional to the length of the cable 7 and thus provides the information about the sequence of the connection of the voltage measuring modules 3, as well of the relative distances therebetween, as the resistance per length unit is known.
Opportunely, at least one or more of the measuring modules 3, typically one each ten, will comprise a detector of the absolute position, such as a GPS or the like, for the geographical reference of the position of the modules.
The measuring modules 3 may also comprise a luminous and/or acoustic signalling device adapted to communicate the operation state and the measuring phase of the same module. For example, the device will signal if the respective module is on or off, if it's connected or disconnected or in which measuring phase it is and whether the corresponding electrode 1 are injecting current, increasing the safety of the measures.
The modules 3 may also comprise, integrated in the control logic unit, a control device for the integrity of the cables designed for valuating the integrity of the cables 7 by means of the resistance thereof and also allowing to detect the position of the damages.
Preferably, the injection step of the electric current, through the current generator 4 may be realized by flowing the current between the electrodes 1 first in a sense and then un the other, for eliminating the spontaneous voltage, if it works at constant current or voltage.
The voltage measuring module 3 will measure the voltages and the current measuring devices 5 will measure the current also during the transitory period preceding the stabilization of the current, for reconstructing the charge distribution of the medium 2, together with the resistivity distribution and define measures of the polarization induced in the time field, as the current generator 4 works at constant current or voltage.
If it works at alternated current or voltage the measures may be executed at several frequencies for the current signal, typically between 10"3 and 103 Hz, for reconstructing the resistivity and charge distribution of the examined medium. Opportunely, the voltage measuring modules 3 and the current measuring devices 5 contain an inner memory, for saving the acquired data before their transmission. However it is also possible that the data saving may be executed only in the control modules, and that the data are transferred in real time through a suitable communication interface 10 during the acquisition from the measuring modules 3 and from the current meter 5 to the control modules 6. Each intermediate embodiment between the two listed will be finally possible. Preferably, in the apparatus of the invention is implemented an automatic calibration process for the voltage and current measuring on all the voltage measuring modules 3 and the current meters 5.
From above, it is apparent that the invention fulfils the intended objects.