INDIRECT CALIBRATION METHOD FOR AN ELECTROMAGNETIC INDUCTION METHOD AND MEASURING ARRANGEMENT FOR PERFORMING THE METHOD

Description

[0001] The invention relates to an indirect calibration method for an electromagnetic induction method for determining physical parameters of an electrically conductive medium from the ratio of a secondary magnetic field received in a receiving coil to a primary magnetic field emitted by a transmitting coil electrically connected to a transmitting signal generator, wherein a received voltage at an output of a receiving amplifier electrically connected to the receiving coil is measured as a measure of the ratio, which is calibrated by means of a series circuit consisting of at least one calibration coil and a variable calibration resistor as well as a calibration factor adjustable by the latter, and to a measuring arrangement for carrying out the method with at least one transmitting coil and a receiving coil as well as with a series circuit consisting of at least one calibration coil and a calibration resistor.

[0002] The electromagnetic induction method utilizes the effect that strong alternating magnetic fields induce eddy currents in a conductive medium. These eddy currents are the cause of secondary magnetic fields, which can be measured in terms of magnitude and phase. In this way, the average conductivity of the medium can be approximated as a physical parameter. The measurement can be performed either in the time domain (amplitude decay time of the secondary field) or in the frequency domain (amplitude and phase shift of the secondary field).

[0003] In the measurement setup for carrying out the method, a strong alternating magnetic field is generated by a transmitting coil. A receiving coil is positioned at a distance of approximately one meter from the transmitting coil. This receiving coil detects the secondary field emitted by the medium at the substrate and converts it into a voltage. This voltage is amplified and measured in a data acquisition system with respect to its magnitude and phase (complex quantity). At the location of the receiving coil, in addition to the secondary field, a very strong primary field is also present, which is generated by the transmitting coil. The secondary field is evaluated as the ratio between the secondary and primary fields at the location of the receiving coil. Since the primary field is several orders of magnitude stronger than the secondary field, the ratio of the fields is usually expressed in PPM (parts per million). Because the secondary field is very weak, the voltage induced in the receiving coil must be amplified.To prevent the strong primary field from driving the required amplifier into saturation, a compensation coil is typically installed in the measuring setup. This coil is positioned close to the transmitting coil and oriented to compensate for the voltage induced in the receiving coil by the primary field. To determine which measured voltage corresponds to which ratio of secondary field to primary field, the measuring setup must be calibrated. This calibration is performed using a calibration coil that generates a known ratio of a calibration field to the primary field.

[0004] The following section explains in more detail the functions and arrangements of the various coils in a measuring setup for carrying out the electromagnetic induction method: The transmitting coil generates the transmitting field (alternating magnetic field) through the transmitting current, which is generated by a signal generator. A receiver coil is arranged near the transmitting coil to measure the strength of the transmitting field. A compensation coil is also arranged near the transmitting coil and compensates for the voltage induced in the receiving coil by the primary field (alternating magnetic field) (increasing the sensitivity of the secondary field). The receiving coil is arranged a few meters away from the transmitting coil and receives the secondary field. The calibration coil generates a calibration field as a secondary field (also called a calibration signal or calibration pulse) with a precisely known amplitude and phase at the time of calibration.

[0005] In principle, direct calibration of the received voltage by directly generating a calibration pulse with known amplitude and phase can be distinguished from indirect calibration, in which a calibration coil is used to generate the calibration pulse. In the series connection of a calibration coil and a calibration resistor, a complex resistance results that influences the phase and amplitude of the variable calibration pulse. The ratio of the calibration field to the primary field is called the "calibration factor," which, in the prior art, has so far been determined purely mathematically from coil geometries. During calibration, the calibration field acts as a secondary field, and the calibration factor effectively represents the target value that the measuring arrangement should measure when the calibration pulse occurs in the calibration coil.The invention relates to the indirect calibration method in application to the electromagnetic induction method, specifically to the determination of the calibration factor. Stand der Technik

[0006] The indirect calibration method is explained in principle in DE 40 00 018 C2. For system calibration, a separate coil is installed. This calibration coil generates a known ratio of secondary field to primary field at the location of the receiving coil, both in terms of magnitude and phase, and enables the compensation of all gains and phase shifts in the measuring arrangement (sensor system and downstream electronics). The calibration coil is typically installed coplanarly between the transmitting and receiving coils. During the calibration process, it is ensured that no secondary field can be measured through the conductive medium under investigation. By bridging the terminals of the calibration coil with a known, fixed, complex resistance, a current is induced in the coil by the primary field, which in turn results in a secondary calibration field at the receiving coil.By calculating the magnetic moment of the calibration coil, the ratio of the secondary calibration field to the primary field can be determined for calibrating the measuring setup. To calculate the magnetic moment, the distances between the transmitting, calibration, and receiving coils must be determined very precisely. These distances are cubed in the calculation of the moment, making this measurement a significant source of error. Furthermore, the coils must be a considerable distance apart for the magnetic moment approximation to be applicable. However, due to the existing distance between the calibration and receiving coils, their inductive coupling is very weak and not directly measurable.

[0007] WO 2012 1 146930 A2 discloses an indirect calibration method in the field of materials testing, in which one or more calibration coils are temporarily brought near a magnetometer during calibration to simulate the effect of a metal object near the magnetometer. The calibration current in the calibration coils is generated by the magnetic field of the magnetometer, and the calibration coils are intended to increase the magnetometer's sensitivity to this field. No information is provided regarding the calibration process itself, and in particular, how the calibration factor is determined. EP 2 790 030 A1 discloses a direct calibration method in which an active pulse of known magnitude is passed through the calibration coil. The aim is to measure the absolute field strength and to correct the hysteresis in the sensor.German patent DE 10 2009 026 403 A1 discloses a system for a geophysical measurement method in which the path between the transmitting and receiving coils is monitored by measuring the inductive coupling between them. For diagnostic purposes, it suffices to measure a first electrical signal in the primary circuit with a closed short-circuit path, through which a defined inductive coupling occurs between the transmitting and receiving coils, although the definition of this coupling remains unclear. The impedance of the two coils is measured, with the primary and secondary coils connected via a short-circuit path, and the voltages or currents across these coils are measured. EP 2 657 762 A2 deals with the measurement of subsurface conductivities in geophysics. However, no calibration is performed; instead, fluctuations in the primary field are compensated.German patent application DE 10 2018 220 532 A1 describes a direct calibration method that uses a known pulse applied to the receiving coil. Calibration is performed by applying a pulsed DC voltage to the transmitting or receiving coil via the calibration device. A separate calibration coil is not required. German patent application DE 20 2005 020 193 U1 also discloses a direct calibration method in which defined magnetic fields are generated to calibrate magnetic field probes. The aim is to compensate for the influence of these fields on the generated field.

[0008] From US patent 2015 / 0268369A1, a direct calibration method is known in which the calibration pulse has a known voltage and shape. The voltage across the calibration coil, which must have few turns to avoid high inductance and is only temporarily placed close to the receiving coil, is then measured absolutely. From this measurement, the current through the calibration coil in series with the calibration resistor is determined. This known current is then used for the direct calibration of the receiving coil. The sensitivity of the measuring setup is calculated from the geometry of the receiving coil. The calibration factor must be calculated beforehand at a low frequency and serves as a reference point for relative amplitude and phase measurements at higher frequencies. The purpose of the known measuring setup is to correct fluctuations in the strength of the transmitted signal.

[0009] From the Veröffentlichung Wang, Haowen [et al.]: A time-domain feedback calibration method for air-coil magnetic sensor, in: Measurement, Vol. 135, 2029, pp. 61-70. - ISSN 0263-2241. It is known to attempt to reconstruct the calibration signal, which is fed into the calibration coil, at the output of the receiving amplifier as accurately as possible using an iterative algorithm. The transfer function of the receiving system is adjusted until the error between the calibration signal and the received signal is sufficiently small. For this, the shape of the calibration pulse must be determined as precisely as possible. In this direct calibration method, the receiving system is also calibrated using a known calibration signal; thus, the calibration factor is again known. A decaying field, generated by interrupting a DC voltage through the calibration coil, is used as the calibration signal. The voltage across the calibration coil is not measured.The algorithm makes the determination of the transfer function independent of the mutual inductance between the calibration coil and the receiving coil. For a secondary field to be present, the calibration must be performed near ground level, with the transmitting coil switched off throughout the entire calibration process.

[0010] From the Veröffentlichung Haas, Christian [et al.]: Airborne electromagnetic measurements of sea ice thickness: methods and application. Luxembourg: Arctic sea ice thickness: past, present and future. Office for Official Publications of the European Communities, 2006, pp. 136-148 - ISBN 92-79-02803-0. It is known to use an indirect calibration method for an electromagnetic induction method to determine the physical parameters of an electrically conductive medium. The physical parameter to be determined is the conductivity of the conductive medium, in this case seawater, from which the thickness of floating sea ice can then be derived.

[0011] All known calibration methods can therefore be classified as either direct or indirect calibration, with all known methods being fundamentally single-stage. Direct calibration involves the direct application of a known voltage or current to the calibration coil. In indirect calibration using a calibration coil, the calibration factor is either calculated from the coil geometries and the measurement of the resistance of the calibration coil, or it is assumed to be known. The indirect calibration method most closely related to the invention is known from WO 2012 / 146930 A1 in the context of DE 40 00 018 C2. Aufgabenstellung

[0012] Based on this, the Aufgabe The present invention is understood to provide a calibration method for an electromagnetic induction method in which the calibration factor does not need to be calculated in a complex and potentially error-prone manner. Furthermore, the invention is intended to enable a particularly compact measuring arrangement. The invention Lösung The purpose of this task is described in the main claim. Advantageous embodiments of the method according to the invention, as well as a preferred application of the method, are described in the dependent claims. A preferred measuring arrangement according to the product claim, with advantageous modifications shown in the associated dependent claims, also contributes to the solution.

[0013] In the calibration method according to the invention, which is an indirect calibration method using a calibration coil, the calibration factor is initially determined purely metrologically in the first stage by measuring the mutual inductance between the receiving coil and the inductively coupled calibration coil caused by an electrically generated calibration pulse. The inductive coupling between the calibration coil and the receiving coil is strong enough to be measurable. The invention does not involve a purely computational analysis of the coil geometries or a measurement of the resistance of the calibration coil, thus significantly reducing the error in determining the calibration factor.In the second stage of the method according to the invention, the actual calibration of the measuring arrangement is carried out using the calibration factor determined metrologically in the first stage, whereby the calibration pulse is inductively generated by the primary field. While in the first stage of the method the received voltage is measured at the terminals of the receiving coil (i.e., at the input of the receiving amplifier), in the second stage the received voltage is measured downstream of the receiving amplifier (i.e., at the output of the receiving amplifier). This "unknown" received voltage is then related to the ratio of secondary to primary field to be measured using the known calibration factor. The value of the calibration resistor set in the first stage, which is short-circuited via the calibration coil in the second stage, remains unchanged.The calibration factor thus indicates the ratio of secondary field to primary field present at the receiving coil at the moment of calibration, which allows the measured voltage value to be related to it.

[0014] The difference between the method according to the invention and known indirect calibration methods lies in the way in which the calibration pulse is generated in the calibration coil and in the purely metrological determination of the size of the calibration field or the calibration factor. The combination of arranging the calibration and receiving coils at the smallest possible distance, and the resulting direct measurement of the inductive coupling using simple means, as well as the subsequent calibration via the present primary field, is characteristic of the invention.

[0015] Furthermore, the invention eliminates the significant source of error introduced by the coil spacing in the calculation of the calibration factor, resulting in considerably more accurate calibration. In the first stage, the transmitting coil is inactive (no current flows through it) or shielded, preventing any signal from reaching the receiving coil. Instead, a calibration signal generator is electrically connected to the calibration coil, outputting a variable calibration current. This current can be a sinusoidal signal, but other pulsating alternating signals for generating induced currents are also suitable. The calibration coil is electrically connected in series with the calibration signal generator and the calibration resistor, which can be variable and purely resistive or more complex. The resulting complex calibration resistance is formed by the calibration coil.The pulsating calibration current generates a calibration pulse in the calibration coil (an induced current in the coil resulting in a secondary magnetic field). This pulse, in turn, generates a magnetic flux in the receiving coil via inductive coupling (hence the terms "inductive coupling" and "magnetic coupling" are equivalent). This magnetic flux then produces a received voltage at the terminals of the receiving coil (at the input of the receiving amplifier). The received voltage and the calibration voltage are then measured simultaneously at the terminals of the signal generator, i.e., via the series connection of the calibration coil and the calibration resistor (the complex calibration resistance), and before the receiving amplifier.The mutual inductance between the receiving and calibration coils is calculated using these voltages. From this, the calibration factor is determined as the ratio of the secondary field induced in the calibration coil by the primary field to the primary field at the receiving coil. Since a voltage ratio is used to determine the calibration factor, the amplitude and phase of the calibration signal fed into the calibration coil are irrelevant to the invention.

[0016] The secondary field generated by the environment is very small compared to the calibration field of the calibration coil and can be considered negligible in its influence on the receiving coil. Therefore, it can be present during calibration without causing a significant error. However, to avoid even this small error, according to a first modification of the invention, it is advantageous and preferred if the secondary field is shielded, at least in the second stage. This can be achieved, for example, by a magnetic field-shielding metallic substrate or by a very large spatial separation. For measuring the calibration factor in the first stage, the entire measuring arrangement can also be placed in an environment where no secondary field from a conductive medium can be measured. Alternatively, the conductive medium can also be shielded in the first stage.

[0017] According to a further modification of the calibration method according to the invention, the first stage can preferably and advantageously be carried out independently of the second stage. If the calibration factor is determined in the first stage on a purely metrological basis, independent of the calibration of the measuring arrangement in the second stage, it can, for example, be measured only once in a laboratory and then applied in the measurement operation in the second stage. However, the calibration factor can also be measured in the field immediately (and thus once, several times, or even as often as desired) before the actual calibration in the second stage. This closely temporally correlated determination of the calibration factor according to the first stage with the calibration according to the second stage results in a more precise calibration according to the invention, which also flexibly takes into account time-dependent effects such as ambient temperature and material aging of the coils.Further details on the determination of the calibration factor on a metrological basis, taking into account simplifying boundary conditions, can be found in the exemplary embodiments.

[0018] Furthermore, the invention claims a measuring arrangement for carrying out the previously described two-stage calibration method with at least one transmitting coil and one receiving coil, as well as with a series connection of a calibration coil and a variable calibration resistor. According to the invention, this measuring arrangement features the receiving coil and the calibration coil being arranged adjacent to each other, generating a permanent inductive coupling, and a calibration signal generator that can be electrically connected to the series connection. By eliminating the spatial distance between the receiving coil and the calibration coil, the measuring arrangement can be designed to be significantly more compact than before. This results in a space saving of up to 35% in the length of the measuring arrangement. The design of the calibration and receiving coils can be carried out together, thus making it particularly efficient and cost-effective.

[0019] Furthermore, in a modification of the measuring arrangement, it is preferably and advantageously shown that a first switch is provided, via which the calibration coil can be electrically connected to the calibration signal generator through the calibration resistor. After closing this switch and switching on the calibration signal generator, the calibration factor is determined metrologically in the first stage. In a further modification, it is preferably and advantageously provided that another switch is provided, via which the calibration coil can only be electrically connected to the calibration resistor. When the second switch is closed, the calibration signal generator for the calibration signal is disconnected from the series circuit. After closing the second switch and switching on the signal generator for the transmitted signal, the calibration can be carried out according to the second stage of the calibration procedure using the previously measured calibration factor.In connection with the method according to the invention, it has already been explained that the calibration factor in the first stage can be determined metrologically independently of the calibration in the second stage, either temporally and spatially (laboratory measurement) or in conjunction with it (field measurement). In a laboratory measurement, a simple measuring setup has only the first switch. The actual measuring setup in the field then has only the second switch. When the two stages of calibration are combined, the measuring setup then has both switches accordingly. This makes the calibration flexible, and environmental influences and the aging of the individual components can be taken into account.

[0020] In a further modification of the measuring arrangement claimed by the invention, it is preferably and advantageously provided that the receiving coil and the calibration coil are arranged on a common core and have an area ratio of 1. This design facilitates the metrological determination of the calibration factor in the first stage and further simplifies the measuring arrangement. The receiving coil and the calibration coil can, for example, have a turns ratio of 200 to 1 or 200 to 2. Both coils can also preferably and advantageously have a turns ratio of 1, in which case they have the same number of turns. This boundary condition also simplifies the metrological determination of the calibration factor in the first stage. However, it is more common for the receiving coil to have a higher number of turns than the calibration coil, so that the turns ratio is not equal to one and is taken into account in the determination of the calibration factor.Further explanations of the present invention in its embodiment as a method and as an arrangement and its respective modifications can be found in the illustrated exemplary embodiment.

[0021] Beforehand, a particularly preferred application of the method and measuring arrangement claimed by the invention will be described, in which the thickness of sea ice is determined as a physical parameter. This can be carried out using an airborne application of the magnetic induction method, as is used for various applications in geophysics, for example, for geological mapping or groundwater exploration. Magnetometers are used in each case, the operation and design of which can be significantly improved by the present invention. In the application for determining the thickness of sea ice, a strong alternating magnetic field is emitted by a towed probe. In In a conductive medium, such as saline seawater, eddy currents are induced, which in turn generate secondary fields. These secondary fields are received by the towed probe and evaluated for amplitude and phase, including calibration. From the amplitude and phase information, either the apparent conductivity of the medium or the distance to the surface of a conductive medium can be calculated, which corresponds to the distance to the base of the ice. A further distance measurement, for example with a laser distance meter, can determine the distance to the top of the ice. The difference between these two distances is the ice thickness. The conductive medium can also be a conductive substrate in a natural environment. Ausführungsbeispiele

[0022] The indirect calibration method claimed in the present invention, as well as a measuring arrangement implementing this method, are explained in more detail below with reference to the schematic figures for a better understanding of the invention. The following shows the Fig. 1 a simplified measurement setup for the metrological determination of the calibration factor (first stage) S1), Fig. 2 the simplified measurement setup according to Fig. 1 during the calibration process (second stage) S2), Fig. 3 the simplified measurement setup according to Fig. 1 , 2 in detail (levels) S1+S2) Fig. 4 the simplified measurement setup according to Fig. 1 , 2 , 3 in measurement operation and Fig. 5 a complete measurement setup according to the Fig. 1 , 2 , 3 , 4 during measurement operation.

[0023] In the Fig. 1 is a simplified measurement setup 01 shown, in which a transmitting coil is on the left 02 shown is the one with a transmitting signal generator. 03 for generating a variable (in the exemplary embodiment in the frequency domain; in the time domain it would be a pulsating signal) transmitted signal (in the exemplary embodiment sinusoidal), a transmitting amplifier 04 and a resonant capacitor 05 is connected. Since in the Fig. 1 the metrological determination of the calibration factor H Cx / H P in the first stage S1 What needs to be explained is the transmit signal generator. 03 switched off, the transmitting coil 02 Therefore, it does not transmit a transmission field. H Tx from, which via a primary path 06 to a receiving coil 08 could get there. Also, in the illustrated embodiment, there is no conductive medium. 09 in influencing proximity, through which a secondary field H s via a secondary path 07 in the receiving coil 08 could be generated. Reference symbols mentioned but not shown can be inferred from the other figures.

[0024] In the Fig. 1 The receiving coil is on the right. 08 shown, to which a calibration coil is attached at the smallest possible distance. 10 is arranged so that the two coils 08,10 are inductively coupled to each other with particular strength, which can be further enhanced by the use of iron cores. The receiving coil 08 is connected to a receiver amplifier 11 connected, at whose input a receiving coil 08 induced voltage U Rxin is located at the output of the receiver amplifier. 11 A voltage will be created U Rx generated. The calibration coil 10 is connected to a calibration signal generator 12 to generate a calibration pulse in the calibration coil 10and an adjustable calibration resistor 13, about the amplitude and phase of the calibration pulse from the calibration coil 10 generated calibration field H Cx (electrical generation in the first stage) S1 through the active calibration signal generator 12 The series circuit can be connected and adjusted. It can be opened and closed via a first switch. 14, the determination of the calibration factor H Cx / H P serves (first stage) S1 In the series circuit, current flows when the first switch is closed. 14 and with the calibration signal generator switched on 12 a current I cal , a tension U cal This is due to the series connection of the calibration coil. 10 and calibration resistor 13 (these two together generate a complex calibration resistance) which is used to determine the strong inductive coupling between the calibration coil10 and receiving coil 08 is used. The measure of this is the mutual inductance. M RC . In the Fig. 1 is another switch 15 shown, which is shown during the actual calibration in stage S2 is used, compare Fig. 2 .

[0025] To determine the calibration factor H Cx / H P is the transmit signal generator 03 switched off to generate the transmission signal, the conductive medium 09 In the selected embodiment, the receiving coil is either absent or shielded. 08 and the calibration coil 10 are strongly inductively coupled to each other. The first switch 14 is closed, the other switch 15 is open. The calibration signal generator 12 It is switched on and generates a pulsed current. I Cal in the calibration coil 10, which in turn creates a magnetic flux through the receiving coil 08which in turn generates a voltage U Rxin at the terminals of the receiving coil 08 induced. About the voltages U Cal (Voltage across the series circuit from the calibration coil) 10 and the calibration resistor 13, (serves as a reference) and U R-Xin will the mutual inductance M RC between the receiving coil 08 and the calibration coil 10 determined. For this purpose, the tensions are measured. U cal and U RXin measured, whereby the voltage U RXin according to amount and phase in relation to U cal is measured.

[0026] For a transmit signal in the exemplary embodiment that is sinusoidal in the time domain (with exactly one frequency in the frequency domain) and correspondingly for a sinusoidal calibration signal of the same frequency as well as for the steady state, it can be shown that the mutual inductance M RC which can be calculated as follows: M _ RC = U _ Rxin ⋅ Z _ Cx jω ⋅ U _ cal

[0027] All underlined quantities are complex. U Rxin and U cal are the voltages at the receiving coil 08 and via the series circuit of calibration resistor 13 and calibration coil 10. Z Cx is the complex resistance (impedance) of the series circuit consisting of a calibration coil 10 and calibration resistor 13, ω is the angular frequency and j the imaginary unit. Furthermore, it can be shown that the calibration factor is a ratio of H Cx and H P which can be calculated as follows: H _ Cx H _ P = jω ⋅ M _ RC ⋅ A Cx ⋅ N Cx A Rx ⋅ N Rx ⋅ Z _ Cx

[0028] By introducing the mutual inductance M RC The impedance is eliminated Z Cx and the calibration factor H Cx / H P simplifies to H _ Cx H _ P = U _ Rxin U _ cal ⋅ A Cx ⋅ N Cx A Rx ⋅ N Rx

[0029] This includes A Cx and A Rx the effective areas and N Cx and N Rx the number of turns of the calibration coil 10 or the receiving coil 08. H Cx Is that in the calibration coil?10 generated secondary field (electrical generation in the first stage) S1 through the active calibration signal generator 12, inductive generation in the second stage S2 through the active transmit signal generator 03 ) and H P the primary field occurring at the location of the receiving coil 08 (with and without active transmitting coil) 02 The underlined field sizes H Cx and H P This again indicates that these are treated as complex quantities and therefore have a magnitude and a phase. It has proven particularly advantageous that the windings of the receiving coil 08 and the calibration coil 10 They can be wound onto the same frame. With sufficiently thin wires, the surfaces can thus be covered. A Cx and A Rx They are assumed to be identical, and their ratio becomes one. The same applies if both coils have the same number of turns. 08, 10,whose ratio then also becomes one, whereby it has already been explained above that, as a rule, the number of turns of the receiving coil 08 is orders of magnitude larger than the number of turns of the calibration coil 10. The calibration factor H Cx / H P Therefore, by simply measuring the voltage U cal standardized voltage U Rxin at the input of the receiver amplifier 11 measured and on to the actual calibration in the second stage S2 be applied.

[0030] In the Fig. 2 is a simplified measurement setup 01 demonstrated, whereby the setup of the simplified measuring arrangement 01 according to Fig. 1 corresponds to the metrological determination of the calibration factor. H Cx / H P serves (first stage) S1 The simplified measurement setup 01 according to Fig. 02 However, it serves the actual calibration (second stage). S2It should be noted that in the illustrated embodiment, the measuring arrangement 01 both the first switch 14 as well as the other switch 15 in a combination. Such a combination enables a timely determination of the calibration factor. H Cx / H P also in field measurement operation and a compact measurement setup 01. The two calibration levels S1 and S2 However, they can also be carried out completely separately in terms of time and location. In the second stage, calibration takes place in the field during measurement operations. S2 the calibration factor H Cx / H P used, which is unique in the first stage S1 for example, determined metrologically in a laboratory. Time-dependent changes in the measurement setup 01 However, environmental influences cannot then be taken into account.

[0031] To carry out the actual calibration (second stage) S2) the measuring setup 01 In the illustrated embodiment, an environment has been created in which no secondary field exists. H S from a conductive medium 09 This can be measured, for example, by flying an airplane to a high altitude above the ground. Then the next switch is activated. 15 closed (in a combined arrangement, the first switch) 14 (opened). The connections of the calibration coil 10 are about the variable, but in the second stage S2 constant to the value from the first stage S1 set calibration resistance 13 connected and thus short-circuited. The transmitting signal generator 03 is switched on and is acting on the transmitting coil 02 with a time-varying current I Tx This generates the corresponding transmission field. H Tx , which is via the primary path 06 to the calibration coil 10reached, there the primary field H P (now under the influence of the transmission field) H Tx ) generated and the calibration current I Cx induced by the calibration resistor 13 its phase and amplitude can be influenced. The current I Cx This in turn generates a magnetic flux through the surface of the receiving coil. 08, which is synonymous with the magnetic calibration field H Cx at the location of the receiving coil 08, the one with the calibration coil 10 is strongly inductively coupled. The generated calibration field H Cx at the time of calibration in the second stage S2 therefore a known secondary field field H S (in relation to the unknown, emerging primary field) H P The tension U Rx at the output of the receiver amplifier 11 Measurements are taken before and during the calibration field H Cx is present. Through the one from the first stage S1known calibration factor ( H Cx / H P ) the measuring setup 01 then calibrated.

[0032] The Fig. 3 shows the measuring setup 01 with a combination from the first stage S1 and the second stage S2 according to Fig. 1 , 2 in detail for a specific measurement setup (the transmitting coil) 02 The receiving coil is on the right. 08 (shown on the left). With the measuring device 18 will the tension U cal (at the output of the calibration signal generator) 12 for measuring mutual inductance M RC ), with the measuring device 19 the tension U Rxin (due to strong inductive coupling of the calibration coil) 10 with the receiving coil 08 ) and with the measuring device 20 the tension U Rx The actual measurement signal is measured. Reference symbols not mentioned can be found in the other figures.

[0033] In the Fig. 4 The measurement operation is carried out in a simplified measurement setup 01 shown. The transmission current I Tx through the transmitting coil 02 The transmission field is generated H Tx This spreads in various directions, with a fundamental distinction between the primary pathway. 06 and the secondary path 07 a distinction is made. Along the primary path 06 The signal propagation occurs directly from the transmitting coil. 02 to the receiving coil 08. The transmission field H Tx generated at the location of the receiving coil 08 the primary field H P Along the secondary path 07 The propagation occurs through the conductive medium 09 generated signals. The transmission field H Tx induced in the conductive medium 09 (for example, a geological, conductive subsurface) eddy currents (representation in Fig. 4 , 5(through circles), which in turn are the source of further alternating magnetic fields of the same frequency. These eddy currents generate eddy currents at the location of the receiving coil. 08 the secondary field H S . From the relationship H S / H P can the conductivity of the medium 09 can be calculated. Alternatively, for the application of measuring ice thickness, the ratio can be used. H S / H P the distance from the surface to a conductive medium 09 - here, seawater - can be calculated. All alternating magnetic fields induce in the receiving coil. 08 a tension U Rxin , which are from the receiver amplifier 11 amplified and then as tension U Rx is measured and output. In order to be able to make a statement about the voltage. U Rx what ratio H S / H P The calibration coil corresponds to 10 used, which is the known ratio of H Cx / H P (= calibration factor) is generated.

[0034] The representation of the measuring setup 01 in Fig. 4 It is not complete with regard to optimal functionality. The elements required only for the actual measurement are missing. Fig. 5 shows the complete setup of the measuring arrangement 01. There is also a pickup coil here. 16 and a compensation coil 17 shown. Since the receiving coil 08 only the secondary field H s It is supposed to receive, but the primary field H P the receiver amplifier 11 would drive it into saturation, the influence of the primary field will H P through the compensation coil 17 minimized. The pickup coil 16 This in turn measures the transmission field. H Tx . With the help of the pickup coil 16 Fluctuations in the transmission current can occur. I Tx and thus in the emitted magnetic field H Tx They will be compensated. The tensions are still shown. U B at the connections of the compensation coil 17, U RxL at the terminals of the receiving coil 08 and U PU at the terminals of the pickup coil 16. In its entirety Fig. 5 The tension results U Rxin at the input of the receiver amplifier 11 through series connection U B and U RxL , thereby reducing the influence of the primary field H P at the location of the receiving coil 08 on the receiving voltage U Rxin This minimizes fluctuations in the transmission current. I Tx not mistakenly considered fluctuations in the secondary field H S The tension will be interpreted U PU the pickup coil 16 used to compensate for fluctuations in the transmission current I Tx to compensate, since the tension U PU directly proportional to this (transmitting and receiving coils are also inductively coupled). Formelzeichenliste

[0035] A Cx effective area of ​​the calibration coil 10 A Rx effective area of ​​the receiving coil 08 H Cx Calibration field, which is used during calibration (second stage) S2 ) as a secondary field H S with known strength (phase and amplitude, in relation to the occurring, unknown primary field) H P ) functions H P Primary field, alternating magnetic field at the location of the receiving coil 08, that via the primary path 06 from the transmitting coil 02 to the receiving coil 08 reached H S Secondary field, alternating magnetic field at the location of the receiving coil 08, via the secondary path 07 from the transmitting coil 02 to the receiving coil 08 reached H Tx Transmitting field, from the transmitting coil 02 emitted alternating magnetic field H Cx / H P Calibration factor I cal Current through the calibration coil10 during the determination of the calibration factor (first stage) S1 ) I Cx Current through the calibration coil 10 during calibration (second stage) S2 ) I Tx Current through the transmitting coil 02 during calibration (second stage) S2 ) and the actual measurements j imaginary unit M RC Mutual inductance as a measure of the strong inductive coupling between the calibration coil 10 and receiving coil 08 N Cx Number of turns of the calibration coil 10 N Rx Number of turns of the receiving coil 08 U B Voltage at the terminals of the compensation coil 17, serves to compensate for the contribution of the primary field H P on the receiving voltage U Rxin U cal Voltage at the terminals of the calibration signal generator 12, is used as a reference for determining the strong inductive coupling between the calibration coil10 and receiving coil 08 used U PU Voltage at the terminals of the pickup coil 16, It serves to compensate for fluctuations in the transmitted current. I Tx U Rx Voltage at the output of the receiver amplifier 11 (Measurement signal) U Rxin Voltage at the input of the receiver amplifier 11 as a sum (series connection) of U Bx and U RxL in a simplified measurement setup 01 according to Fig. 1 bis 4 U RxL Voltage at the terminals of the receiving coil 08 in the complete measurement setup 01 according to Fig. 5 to distinguish between the voltage at the receiving coil 08 and the tension U Rxin , which are about the influence of the primary field H P compensated Z Cx complex resistance (impedance) of the series circuit of calibration coil 10 and calibration resistor 13 (variable load resistance) to change the calibration field ω angular frequency Bezugszeichenliste

[0036] 01 Measuring setup 02 Transmitting coil 03 Transmitting signal generator 04 Transmitter amplifier 05 Resonant capacitor 06 Primary pathway 07 Secondary path 08 Receiving coil 09 conductive medium 10 Calibration coil 11 Receiver amplifier 12 Calibration signal generator 13 variable calibration resistor 14 first switch for S1 15 additional switch for S2 16 Pickup coil 17 Compensation coil 18 Measuring device for U cal 19 Measuring device for U Rxin 20 Measuring device for U Rx S1 First stage: metrological determination of the calibration factor H Cx / H P S2 second stage, performing the calibration

Claims

1. Indirect calibration method for an electromagnetic induction method for the determination of physical parameters of an electrically conductive medium (09) from the ratio (HS / HP) of a secondary magnetic field (Hs), received in a receiving coil (08), to a primary magnetic field (HP) emitted by a transmitting coil (02) electrically connected to a transmission signal generator (03), wherein as a measure for the ratio (HS / HP) a received voltage (URx) is measured at an output of a receiving amplifier (11) electrically connected to the receiving coil (08), said received voltage being calibrated using a series circuit of at least one calibration coil (10) and a variable calibration resistor (13) and using a calibration factor (HCx / HP) which is adjustable thereby, wherein the receiving coil (08) is permanently inductively coupled to the calibration coil (10), generating a mutual inductance (MRC), wherein a calibration pulse generated in the calibration coil (10) induces a calibration field (HCx) and this induces a magnetic flux in the receiving coil (08) and wherein the areas and numbers of turns of the receiving coil (08) and calibration coil (10) are known, and wherein the calibration method has two stages and comprises a • first stage, in which the transmission coil (02) is inactive or shielded and a calibration signal generator (12) is connected to the series circuit of calibration resistor (13) and calibration coil (10), and in which the calibration pulse is generated electrically by the calibration signal generator (12) and by adjusting the calibration resistor (13), via which the amplitude and phase of the calibration field (HCx)can be adjusted, a calibration voltage (Ucal) at the series circuit of calibration coil (10) and calibration resistor (13) and the received voltage (URxin) at the receiving coil (08) at an input of the receiving amplifier (11) are simultaneously measured to determine the mutual inductance (MRC) generated between the calibration coil (10) and the receiving coil (08), from which the calibration factor (HCx / HP) is determined, taking into account the known areas and numbers of turns of the calibration coil (10) and receiving coil (08), and a • second stage, in which the transmitting coil (02) is active via the transmit signal generator (03) and the calibration coil (10) is short-circuited via the calibration resistor (13) that was adjusted to a constant value in the first stage, and in which the calibration pulse is inductively generated by the primary field (HP) generated by transmission signal generator (03) and transmitting coil (02), and in which the received voltage is measured at the output of the receiving amplifier (11) in order to calibrate said voltage as a measure of the ratio (HS / HP) by using the calibration factor (HCx / HP) metrologically determined in the first stage.

2. Indirect calibration method according to Claim 1, characterized in that at least in the second stage, the secondary field (Hs) is shielded.

3. Indirect calibration method according to Claim 1 or 2, characterized in that the first stage is carried out independently of the second stage.

4. Measuring assembly (01) for carrying out the indirect calibration method for an electromagnetic induction method having at least one transmitting coil (02) and a receiving coil (08) and having a series circuit comprising at least one calibration coil (10) and a calibration resistor (13), according to any one of Claims 1 to 3, characterized in that the receiving coil (08) and the calibration coil (10) are arranged adjacent to each other, generating a permanent inductive coupling, and that a calibration signal generator (12) is provided, which can be electrically connected to the series circuit.

5. Measuring assembly (01) according to Claim 4, characterized in that a first switch (14) is provided, by means of which the calibration coil (10) can be electrically connected to the calibration signal generator (12) via the calibration resistor (13).

6. Measuring assembly (01) according to Claim 4 or 5, characterized in that a further switch (15) is provided, by means of which the calibration coil (10) can be electrically connected only to the calibration resistor (13).

7. Measuring assembly (01) according to any one of the preceding Claims 4 to 6, characterized in that the receiving coil (08) and the calibration coil (10) are arranged on a common core and have an area ratio of 1.

8. Measuring assembly (01) according to any one of the preceding Claims 4 to 7, characterized in that the receiving coil (08) and the calibration coil (10) have a winding ratio of 1.