DE112008002308B4 - Device and method for mapping sources for the local change in the earth's magnetic field - Google Patents

Abstract

Device for 3D magnetic field mapping of sources for changing the earth’s magnetic field, comprising sensors (1) for measuring the gradient of the earth’s magnetic field, a position measuring unit (3) for determining the location of the sensors (1) above the measuring field to be mapped, consisting of a movable and a stationary component, a data processing and storage unit (5) and a non-magnetic locomotion and carrier unit (6) which holds the sensors (1), the movable component of the position measuring unit (3) and the data processing and storage unit (5), wherein the signals of all sensors (1) and the position measuring unit (3) are recorded simultaneously by means of a data processing and storage unit (5), characterized in that the sensors (1) are SQUID gradiometers (11) with the same orientation, which are controlled by a cryostat ( 2) are surrounded, at least two of these SQUID gradiometers (11) oriented in the same direction and at a distance vertically al are arranged one above the other, the distance between the SQUID gradiometers (11) and the base length of these SQUID gradiometers (11) being of the same order of magnitude, with a linear dependence of the output signal of the SQUID by means of the data processing and storage unit (5) Gradiometer (11) is generated by the magnetic field gradient to be measured and the different characteristics of the signals measured by the SQUID gradiometers (11) in terms of size and lateral extent of the measured magnetic field gradient are used to determine the lateral and vertical distribution of the magnetic field sources, the distance between the SQUID gradiometers (11) is 10 cm.

Description

The invention relates to a device and a method for mapping sources for the local change in the earth's magnetic field.

Magnetometers or gradiometers have been known so far as magnetic field sensors that can be used, for example, to measure local changes in the earth's magnetic field.

DE 195 18 973 A1 discloses a device for 3D magnetic field mapping of sources for changing the earth's magnetic field and a method for finding ferromagnetic and / or other metallic objects, in particular underground objects, in which measurement signals are recorded along a measurement track by means of a metal detector, with at least two metal detector probes arranged at different heights , e.g. in the form of two fluxgate magnetometers that record measurement signals for direct depth determination of the object and the combined evaluation as a gradiometer 2 . Order for depth determination takes place and the signal of the first gradiometer is separately pre-evaluated for a location determination.

In addition, a device for carrying out the method is described in which the mounting of the metal search probes is designed with double gimbals and the height difference between the metal search probes is adjustable.

According to the technical teaching of the DE 195 18 973 A1 Due to the construction, it is not possible to arrange the two fluxgate magnetometers one above the other, which leads to some disadvantages.

A disadvantage of this technical solution of the two gradiometrically measuring fluxgate magnetometers is, for example, that the measurement is only possible if both individual gradiometers are in exactly the same place during the measurements. This is a very great disadvantage in the practical use of the device, especially when it is mounted on a movable tow truck.

Further disadvantages of the device are that the carrier unit has a complicated structure, that the two fluxgate magnetometers have to be transported and operated offset in the X direction with alternating height offset and that as a result, the measured values of the second fluxgate magnetometer, which follows in the direction of movement, are transmitted to exactly the must be included in the same place. The mechanics of the carrier unit and the time sequence of the measurements represent a cause of measurement inaccuracies that can hardly be avoided, which make precise mapping by means of this device difficult.

DE 195 04 841 A1 discloses a display device for a magnetometer with which a metallic disruptive body located in the ground can be located by measuring the interference field strength. The respective measured field strength is shown analogously on the display instrument of the magnetometer and further information, such as the respective sensitivity range or the computationally determined object depth, can be displayed in a digital display field. A simple keyboard enables a variety of different sensitivity levels to be selected and the measured values to be entered to determine the depth of an object in the ground.

The disadvantage of these two technical solutions is that they do not allow gradient measurements and no precise determination of the size and depth of objects to be detected.

2D magnetic field mapping with the LTS-SQUID system (gradiometer) are from the publication V. Schultze, A. Chwala, R. Stolz, M. Schulz, S. Linzen, H.-G. Meyer, and T. Schüler; A SQUID system for geomagnetic archaeometry, 6th Int. Conf. on Archaeological Prospection (Archeo2005), 14-17 Sept. 2005 , Proc. pp. 245-248 known. With this method, a SQUID measuring system is used in such a way that two LTS-SQUID magnetometers are carried or driven through the area to be examined.

These SQUIDs enable extremely precise 2D magnetic field mapping in a relatively shorter time than conventional systems and are at the same time robust enough to be used outdoors, e.g. for assessing excavation areas.

The disadvantage of this 2D magnetic field mapping is that the depth of the structures found in the subsurface cannot be clearly determined. In addition to their position, the only features obtained are their dimensions and magnetic field strength. In this case, however, a measurement signal of different strengths for different structures can be caused not only by different strengths of the magnetic field source, but also by different depths. Likewise, a different lateral extent of the measured structures can be caused not only by actually different lateral extent of the magnetic field source, but also by different depths.

Stolz, R. et. al: "Magnetic full-tensor SQUID gradiometer system for geophysical applications" in "The leading edge, society of exploration geophysicists, TULSA, OK, US, LNKD-DOI: 10.1190 / 1.2172308, vol. 25, no.2,1, 01 February 2006 , Pages 178-180 discloses 3D magnetic field mapping of Sources for changing the geomagnetic field comprising sensors for measuring the gradient of the geomagnetic field, a position measuring unit for determining the location of the sensors above the measuring field to be mapped, a data processing and storage unit and a non-magnetic locomotion and carrier unit which holds the data processing and storage unit, wherein the sensors are at least two gradiometers which are oriented in different directions and the signals of all sensors and the position measuring unit are recorded simultaneously by means of a data processing and storage unit.

The disadvantage of this technical solution is that good depth discrimination of the source to be detected is not possible at greater depths.

the end U.S. 3,829,768 A an arrangement for measuring the first and second derivative of the earth's magnetic field is known, in which two superconducting gradiometers in the form of two spaced superconducting coils are used to improve the localization of objects to be detected.

The disadvantage of this technical solution is that the basic length of the gradiometer in the form of superconducting coils is relatively large (and therefore the distance between the two superconducting coils must be very large for practical measurements in order to allow depth discrimination at all) and, at the same time, the sensitivity is very small is, so that in the near-surface area the depth of the objects cannot be determined and at great depths only objects with a very strong magnetic signature can be determined, with only homogeneous gradients being detectable.

the U.S. 3,916,248 A discloses a magnetometric system for determining the distance to a source of a magnetic field comprising the combination of two gradiometers of superconducting material oriented in the same direction, superconducting means for detecting and generating first and second output signals, means for adding the first and second signal and scaling the sum thereof to produce a sum signal, means for producing a difference signal proportional to the difference between the first and second signals and scaled relative to the distance between the magnetometers, and means for providing a system output signal which is proportional to the ratio of the sum and difference signals, the output signal being proportional to the range.

The disadvantage of this technical solution is that the detection of sources that change the earth's field locally is basically possible at greater depths, but this detection is limited to a special case for point-like magnetic sources (dipole sources), so that this device is only estimations for the distances to the sources, since with the teaching of the D1 only the measurement of the sum or difference signals of both gradiometers is possible.

the U.S. 5,629,626 A discloses a mobile system for detecting buried ferromagnetic objects that includes an all-terrain vehicle and a towed sensor platform with a plurality of magnetometers, a global positioning navigation system, and a data acquisition hardware and software system.

the U.S. 3,617,865 A discloses an apparatus for locating a buried metallic line having means for inductively coupling an alternating current to the line to generate a magnetic field around the line, wherein magnetic field gradient amplitudes can be measured.

the AT 404 883 B discloses a method for the highly precise location of underground cables and pipes based on the principle of alternating current measurement in connection with a correlation analysis.

the US 6,841,994 B1 discloses a magnetic anomaly detection system comprising a plurality of triaxial magnetometer (TM) sensors, each of the TM sensors having X, Y, Z magnetic detection axes, with the plurality of TM sensors in a three dimensional array having respective one of these is arranged, the X, Y, Z magnetic measuring axes being parallel to each other in the three-dimensional arrangement. The three-dimensional arrangement has a defined geometry that (i) collinearly aligns at least three of the plurality of TM sensors in a spaced-apart manner along an axis, around a plurality of single-axis gradiometers along the X, Y, Z axes magnetic detection axes; and (ii) positioning at least four of the plurality of TM sensors in a spaced apart manner in a planar arrangement perpendicular to the axis and processing means to provide data that can be used to determine the position of a magnetic target relative to the three-dimensional array.

the DE 42 27 876 A1 discloses a circuit arrangement for operating a SQUID, in particular a DC SQUID with a coupling coil in a damped measuring circuit, at least one resistor, connecting lines and Contact pads for connecting the SQUID, a gradiometer and control electronics, whereby the circuit arrangement enables a reaction-free integrated control loop circuit and special functions and at the same time significantly simplifies their production, through a first and a second connecting line that runs between the measuring circuit and a first and second contact pad for connection at least of the output signal of the control electronics and for galvanic coupling of the integrated control current provided by this are provided, the SQUID signal being available as an input signal of the control electronics via a fifth and sixth contact pad.

The object of the present invention is to provide a device and a method for mapping sources for the local change in the earth's magnetic field, which avoids the aforementioned disadvantages of the prior art, in particular compared to the previously known 3D magnetic field mapping or arrangements with superconducting coils Determine the specific depth and size of the objects to be detected.

The object is achieved by the features of the first and sixth patent claims. Advantageous configurations are covered by the subordinate claims.

1. 1. hochempfindliche Sensoren (Gradiometer) zur Magnetfeldgradientenmessung, besonders vorteilhaft SQUID, zur Messung eines Gradienten des Erdmagnetfeldes, die es gestatten, bei kleiner Basislänge einen lokalen Gradienten hoch aufgelöst zu messen, (Unter Gradient versteht man dabei den Unterschied des Magnetfeldes an zwei verschiedenen Orten und unter Basislänge versteht man den Abstand der Zentren der beiden Orte, zwischen denen der Unterschied des Magnetfeldes gemessen wird.)
2. 2. im Falle der Verwendung von SQUID's einen Kryostat zur Aufrechterhaltung der tiefen Temperatur (realisiert bspw. durch verflüssigtes Helium oder Stickstoff) für den Betrieb der SQUID's,
3. 3. eine Elektronik zur Erzeugung einer linearen Abhängigkeit des Ausgangssignals der Sensoren vom zu messenden Magnetfeldgradienten,
4. 4. eine Messeinheit zur simultanen Bestimmung des Ortes der Gradiometer über dem zu kartierenden Messfeld (Unter Gradiometer versteht man dabei einen Sensor zur Messung des lokalen Gradienten), bestehend aus einer differentiellen GPS-Einheit (dGPS) mit einer GPS-Antenne zur Aufnahme der Position des beweglichen Messwagens mit den verwendeten Sensoren, einer zweiten fest stehenden Referenz-GPS-Antenne und einer Radioübermittlung der Referenzstationsdaten zum bewegten Messwagen, zur Erhöhung der Genauigkeit der lokalen Ortszuweisung von mehreren Metern (einfaches GPS) zu wenigen Zentimetern (dGPS) sowie aus einer Inertialeinheit, welche die Lage der Messwagens bestimmt [Lage: ausgedrückt über die drei Winkel Nicken, Rollen, Gieren] und es so gestattet, bei beliebiger Lage des Messwagens aus der über das differentielle GPS bestimmten Position der GPS-Antenne die benötigte Position der Gradiometer zu berechnen,
5. 5. eine Aufzeichnungseinheit für die während der Messung aufgenommen Daten (lokale magnetische Gradienten, mit dGPS aufgenommener Ort, Winkel der Inertialeinheit) und
6. 6. eine nichtmagnetische, weitestgehend metallfreie Trägereinheit für das Ensemble der Sensoren und Datenverarbeitungs- und Speicherungseinheiten (vorzugsweise in Form einer Fortbewegungseinheit, bspw. einem Karren auf Rädern, der von einem Zugfahrzeug, z.B. einem Geländewagen, gezogen werden kann, möglicherweise auch einem Schlitten o.ä.).

The essence of the invention is that a device for mapping sources for the local change in the earth's magnetic field is provided, which comprises the following components:

1. 1.Highly sensitive sensors (gradiometers) for measuring magnetic field gradients, especially SQUID, for measuring a gradient of the earth's magnetic field, which allow a local gradient to be measured with high resolution with a small base length (gradient is the difference in the magnetic field at two different locations and the base length is the distance between the centers of the two locations between which the difference in the magnetic field is measured.)
2. 2. in the case of using SQUIDs, a cryostat to maintain the low temperature (realized for example by liquefied helium or nitrogen) for the operation of the SQUIDs,
3. 3. Electronics for generating a linear dependence of the output signal from the sensors on the magnetic field gradient to be measured,
4. 4. A measuring unit for the simultaneous determination of the location of the gradiometer over the measuring field to be mapped (a gradiometer is a sensor for measuring the local gradient), consisting of a differential GPS unit (dGPS) with a GPS antenna for recording the position of the moving measuring vehicle with the sensors used, a second fixed reference GPS antenna and a radio transmission of the reference station data to the moving measuring vehicle, to increase the accuracy of the local location assignment from several meters (simple GPS) to a few centimeters (dGPS) as well as from an inertial unit , which determines the position of the measuring car [position: expressed via the three angles of pitch, roll, yaw] and thus allows the required position of the gradiometer to be calculated from the position of the GPS antenna determined by the differential GPS for any position of the measuring car ,
5. 5. a recording unit for the data recorded during the measurement (local magnetic gradients, location recorded with dGPS, angle of the inertial unit) and
6. 6. a non-magnetic, largely metal-free carrier unit for the ensemble of sensors and data processing and storage units (preferably in the form of a locomotion unit, e.g. a cart on wheels that can be pulled by a towing vehicle, e.g. an off-road vehicle, possibly also a sledge or the like .ä.).

• • dass die Größe des gemessenen Magnetfeldgradienten mit der vierten Potenz des Abstandes zwischen dem Sensor und der Quelle abfällt (wohingegen das Magnetfeld selbst nur mit der dritten Potenz schwächer wird, wodurch über die Verwendung von Gradiometern die Tiefendiskriminierung deutlich schärfer als mit Magnetometern möglich ist),
• • dass die lokale Ausdehnung des gemessenen Feldes mit dem Abstand zwischen der Quelle und dem Sensor zunimmt,
• • dass insbesondere der Unterschied zwischen der Größe der von zwei übereinander angeordneten Gradiometern gemessenen Signale einer Magnetfeldquelle mit zunehmendem Abstand zwischen der Quelle und dem Ensemble aus übereinander angeordneten Sensoren kleiner wird,

wobei die Gesamtheit der Abhängigkeiten der Größe und lokalen Ausdehnung der von den verschiedenen, insbesondere aber den übereinander angeordneten Gradiometern aufgenommenen Signale von der Tiefe der Quellen für die lokalen Unterschiede des gemessenen Erdmagnetfeldes verwendet wird, um diese Tiefe zu bestimmen (zur Veranschaulichung siehe 2, welche die Signalgröße und den Signalverlauf von zwei Gradiometern, die im Abstand von 10 cm übereinander angeordnet sind, beim Überfahren eines darunter befindlichen magnetischen Dipols mit dem Verfahrweg darstellt, wobei in den drei Grafiken - obere, mittlere und untere - das Gradiometer einen Abstand zum Dipol von 50, 100 und 150 cm aufweist.)It is essential to the invention that at least two sensors for measuring magnetic field gradients (particularly advantageously SQUID gradiometers) are provided for measuring a local magnetic field gradient, which have identical detection directions and are arranged at a distance from one another, the signals from all sensors (but in particular the two vertically arranged one above the other Gradiometers) are recorded simultaneously and the different characteristics of these at least two gradiometers arranged one above the other are used to determine the depth of the sources for the measured local changes in the earth's magnetic field by making use of the following features:

• • that the magnitude of the measured magnetic field gradient decreases with the fourth power of the distance between the sensor and the source (whereas the magnetic field itself only becomes weaker with the third power, which means that the use of gradiometers enables depth discrimination to be much sharper than with magnetometers),
• • that the local expansion of the measured field increases with the distance between the source and the sensor,
• • that in particular the difference between the size of the signals measured by two gradiometers arranged one above the other a magnetic field source becomes smaller with increasing distance between the source and the ensemble of sensors arranged one above the other,

The totality of the dependencies of the size and local extent of the signals recorded by the various, but especially the one above the other arranged gradiometers on the depth of the sources is used for the local differences in the measured geomagnetic field in order to determine this depth (for illustration see 2 , which shows the signal size and the signal course of two gradiometers, which are arranged 10 cm apart, when the travel path traverses a magnetic dipole below, whereby in the three graphics - upper, middle and lower - the gradiometer shows a distance to Has a dipole of 50, 100 and 150 cm.)

In the general case, the distance between the gradiometers and the base length of these gradiometers should be of the same order of magnitude. A good depth discrimination of the sources to be detected is then possible up to a depth that is approximately ten to one hundred times this gradiometer distance.

• • Zur Realisierung des Grundprinzips der Tiefenbestimmung von lokalen Quellen für Unterschiede im Erdmagnetfeldgradienten ist die Anordnung zweier gleich orientierter Gradiometer übereinander notwendig (im Folgenden auch als Sensorebenen bezeichnet).
• • Zur schnellen Erzielung einer kompletten Charakterisierung des auszumessenden Areals werden zumindest in einer Sensorebene, erfindungsgemäß in der unteren, wo aufgrund des geringeren Abstandes der Sensoren zur Quelle die Signale größer und die laterale Auflösung schärfer ist, zwei Gradiometer verwendet, die in zwei senkrecht aufeinander stehende laterale Richtungen schauen. So ergibt sich stets eine ausreichende Sensitivität gegenüber beliebig lateral orientierten Mustern von lokalen Quellen von Magnetfeldgradienten.
• • Zur weiteren Verbesserung der Qualität der Erkundung der lateralen und vertikalen Verteilung von Quellen für lokale Unterschiede im Magnetfeldgradienten kann auch die obere Sensorebene mit zwei senkrecht auseinander stehenden Gradiometern ausgerüstet werden.

The following expansion variants for the arrangement of several gradiometers are within the scope of the invention:

• • To implement the basic principle of determining the depth of local sources for differences in the geomagnetic field gradient, it is necessary to arrange two gradiometers with the same orientation one above the other (hereinafter also referred to as sensor levels).
• • To quickly achieve a complete characterization of the area to be measured, two gradiometers are used at least in one sensor plane, according to the invention in the lower one, where the signals are larger and the lateral resolution is sharper due to the smaller distance between the sensors and the source look at lateral directions. This always results in sufficient sensitivity to any laterally oriented patterns from local sources of magnetic field gradients.
• • To further improve the quality of the investigation of the lateral and vertical distribution of sources for local differences in the magnetic field gradient, the upper sensor level can also be equipped with two vertically spaced gradiometers.

Instead of two sensor levels arranged one above the other, three or more can be used. It is also possible to vary the vertical distance between the sensor levels. Both of these can be used to adjust the accuracy of the resolution of different depths of local sources of magnetic field gradient differences to the mean depth of most interest of these sources.

The device according to the invention and the method according to the invention are distinguished from previous technical solutions by the fact that 3D magnetic field mapping is possible in that the specific depth and size of the objects to be detected can be determined.

• 1: eine schematische Darstellung einer Ausführungsform der erfindungsgemäßen Vorrichtung und
• 2: eine graphische Darstellung der Signalgröße und des Signalverlaufs von zwei Gradiometern, die in einem Abstand von 10 cm übereinander angeordnet sind, beim Überfahren eines darunter befindlichen magnetischen Dipols.

The invention is explained in more detail below with reference to the drawings and the exemplary embodiment. Show it:

• 1 : a schematic representation of an embodiment of the device according to the invention and
• 2 : a graphical representation of the signal size and the signal course of two gradiometers, which are arranged at a distance of 10 cm on top of each other, when passing over a magnetic dipole located below.

In the 1 The device shown comprises several SQUID gradiometers 1 that are in a cryostat 2 are located, a unit of measurement 3 for the exact determination of the location of the SQUID gradiometer during the measurement, consisting of a differential GPS unit, which is driven by a GPS antenna that moves with the SQUID gradiometers 31 and a stationary second GPS antenna 33 as well as the radio antennas 32 and 34 which receives the signal from the fixed GPS antenna 33 to the one with the SQUID gradiometers 1 moving GPS antenna 31 transmits, as well as an inertial unit 35 to measure the position of the SQUID gradiometer, there is the data lines 4th , via which all recorded data is sent to the data acquisition and storage unit 5 as well as a non-magnetic, largely metal-free locomotion unit 6th , on which the moving parts of the measuring unit are firmly mounted, with at least two gradiometers 11th oriented in the same direction and arranged at a distance one above the other.

The signals of all SQUID gradiometers 1 (Sensors), in particular of the two SQUID gradiometers arranged vertically one above the other 11th , can be recorded at the same time.

The different characteristics of these at least two SQUID gradiometers arranged one above the other 11th are used to determine the depth of the sources for the measured local changes in the earth's magnetic field, whereby it should be noted that the size of the measured magnetic field gradient is to the fourth power of the distance between the SQUID gradiometer 1 (Sensor) and the source drops (whereas the magnetic field itself only becomes weaker with the third power, which is via the use of SQUID gradiometers 1 the depth discrimination is clearly sharper than with magnetometers) and that the local expansion of the measured field with the distance between the source and the SQUID gradiometer 1 (Sensor) increases, in particular the difference between the size of two SQUID gradiometers arranged one above the other 11th measured signals of a magnetic field source with increasing distance between the source and the ensemble of SQUID gradiometers arranged one above the other 11th (Sensors) gets smaller.

The totality of the dependencies of the size and local extent of the different, but especially the one above the other arranged SQUID gradiometers 11th The recorded signals from the depth of the sources for the local differences in the measured earth's magnetic field are used to determine their depth.

Use is made of the various features of 2D magnetic field mapping, which are recorded by the gradiometers arranged one above the other. The difference in the signal strength of individual structures is smaller, the deeper the associated signal source is. Likewise, the lateral extent of these structures measured at different heights has fewer differences the deeper the associated signal source is.

All of the features presented in the description and the following claims can be essential to the invention both individually and in any combination with one another.

List of reference symbols

1

SQUID gradiometer (sensors for measuring magnetic field gradients)

11

SQUID gradiometers with the same orientation, spaced one above the other

2

Cryostat

3

Position measuring unit

31

moving GPS antenna

32

Moving radio antenna for receiving the data from the fixed GPS unit

33

fixed GPS antenna

34

Fixed radio antenna for sending the data from the fixed GPS unit

35

Inertial unit

4th

Data lines

5

Data processing and storage unit

6th

Carrier and locomotion unit

Claims (6)

Device for 3D magnetic field mapping of sources for changing the earth’s magnetic field, comprising sensors (1) for measuring the gradient of the earth’s magnetic field, a position measuring unit (3) for determining the location of the sensors (1) above the measuring field to be mapped, consisting of a movable and a stationary component, a data processing and storage unit (5) and a non-magnetic locomotion and carrier unit (6) which holds the sensors (1), the movable component of the position measuring unit (3) and the data processing and storage unit (5), wherein the signals of all sensors (1) and the position measuring unit (3) are recorded simultaneously by means of a data processing and storage unit (5), characterized in that the sensors (1) are SQUID gradiometers (11) with the same orientation, which are controlled by a cryostat ( 2) are surrounded, at least two of these SQUID gradiometers (11) oriented in the same direction and at a distance verti cal are arranged one above the other, the distance between the SQUID gradiometers (11) and the base length of these SQUID gradiometers (11) being of the same order of magnitude, whereby by means of the data processing and storage unit (5) a linear dependence of the output signal of the SQUID Gradiometer (11) is generated by the magnetic field gradient to be measured and the different characteristics of the signals measured by the SQUID gradiometers (11) with regard to the size and lateral extent of the measured magnetic field gradient are used to determine the lateral and vertical distribution of the magnetic field sources, the distance between the SQUID gradiometers (11) is 10 cm. Device for mapping sources for the local change in the earth's magnetic field according to Claim 1 , characterized in that the position measuring unit (3) for the simultaneous determination of the location of the SQUID gradiometer (11) over the measuring field to be mapped has a moving GPS unit with GPS antenna (31) and a moving radio antenna (32), a stationary one GPS antenna (33) with a stationary radio antenna (34) and an inertial unit (35). Device for mapping sources for the local change in the earth's magnetic field according to Claim 1 or 2 , characterized in that two SQUID gradiometers (11) are perpendicular to each other. Device for mapping sources for the local change in the earth's magnetic field according to Claim 1 , 2 or 3 , characterized in that the locomotion and carrier unit (6) is mobile. Device for mapping sources for the local change in the earth's magnetic field according to Claim 1 , 2 or 3 , characterized in that the locomotion and carrier unit (6) is portable. Method for mapping sources for the change in the earth's magnetic field using an apparatus according to one or more of the preceding Claims 1 until 5 , in which the measured values of these at least two superimposed SQUID gradiometers (11) are used to determine the depth of a magnetic field source for the measured changes in the earth's magnetic field, the device being moved, the size of the measured magnetic field gradient to the fourth power of the distance between the SQUID -Gradiometer (11) and the magnetic field source decreases, the expansion of the measured field increases with the distance between the magnetic field source and the SQUID gradiometer (11), the difference between the size of the signals measured by two superimposed SQUID gradiometers (11) Magnetic field source becomes smaller with increasing distance between the magnetic field source and the device with the SQUID gradiometers (11) arranged one above the other, the entirety of the dependencies of the size and local extent of the fields measured by the different, but especially the SQUID gradiometers (11) arranged one above the other from the low e of the magnetic field source is used for the local differences in the measured geomagnetic field in order to determine their depth.

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