Methods for exploring underground structures

Abstract

The present invention relates to a method and apparatus for exploring subsurface structures. The method comprises transmitting electromagnetic waves into the subsurface structure, receiving echoes of the electromagnetic waves from the subsurface structure, and processing the echoes to derive internal features of the subsurface structure. The transmitting electromagnetic waves into the subsurface structure comprises subsequently transmitting into the subsurface structure a plurality of electromagnetic probe signals having different frequency spectra, each probe signal comprising at least two non-zero spectral components. The receiving echoes comprises receiving an echo signal for each probe signal. The processing echoes comprises determining at least one of an amplitude and a phase for each echo signal.

Description

[Technical field]

[0001] The present invention relates to a method and apparatus for probing underground structures using electromagnetic waves. [Background technology]

[0002] Electromagnetic waves are routinely used for the exploration of man-made structures and underground structures, e.g., underground. Ground-penetrating radar (GPR) is a technique often applied in geophysical surveys or non-destructive testing, e.g., of concrete structures, using electromagnetic waves typically in the range of 10 MHz to 2.6 GHz.

[0003] A known method for GPR data acquisition is stepped frequency continuous wave (SFCW). Instead of transmitting a single broadband pulse, SFCW uses a constant frequency continuous wave probe signal that varies over time or between subsequent probe signals. Examples of such methods and suitable devices are described in WO 2018 / 161183.

[0004] The SFCW method is advantageous in terms of data quality and thus of the resulting images of the subsurface structure, in particular in terms of signal-to-noise ratio (SNR) and resolution. On the other hand, the acquisition speed of such a method is limited, which means that GPR measurements with a defined resolution of e.g. 2 cm can only be acquired if the GPR device does not move faster relative to the subsurface than its maximum acquisition speed, which is conventionally in the order of e.g. 10 km / h to 20 km / h. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2018 / 161183 Summary of the Invention

[0006] It is therefore a general object of the present invention to provide a method and an apparatus for exploring subsurface structures that allows for faster data acquisition, and in particular for an apparatus for exploring subsurface structures, while maintaining a high resolution of the acquired data.

[0007] Features described below with respect to the method also relate to the device, and vice versa. Furthermore, the described features are intended to be disclosed both independently of one another and, where reasonable, in combination.

[0008] Method for exploring subsurface structures In order to carry out the above and further objects of the present invention, which will become more readily apparent in the following description, a method for exploring a subterranean structure, e.g. an artificial structure or a subsurface, comprises transmitting electromagnetic waves into the subterranean structure, the electromagnetic waves being polarized, e.g. linearly polarized, the transmitting of the electromagnetic waves into the subterranean structure being in particular carried out by an antenna. The method also comprises the step of receiving echoes of the electromagnetic waves from the underground structure. Advantageously, a portion of the electromagnetic waves transmitted into the underground structure is reflected by an internal feature of the underground structure, the internal feature being characterized by a change in an electrical property, for example a change in the dielectric constant. The reflected portion of the electromagnetic waves may be received as an echo from the underground structure, i.e. as a reflected electromagnetic wave. The method also comprises processing the echoes to derive internal features of the subsurface structure, which may be, for example, the position and / or gradient of internal boundaries within the subsurface structure, characterization of scatterers or reflectors, variation in an electrical property, e.g., dielectric constant, at a particular location within the subsurface structure, etc.

[0009] The step of transmitting electromagnetic waves into the underground structure comprises the subsequent transmission into the underground structure of a plurality of electromagnetic probe signals having different frequency spectra. In particular, the frequency spectrum of the probe signal is given by its Fourier transform. Each probe signal comprises at least two non-zero spectral components. In particular, this is a difference with conventional SFCW, which means that each probe signal has only one spectral component and only one frequency is transmitted at a time.

[0010] In one embodiment, each probe signal comprises more than two non-zero spectral components, in particular between 3 and 20, in particular between 5 and 15, in particular 10 non-zero spectral components. In the extreme case, each probe signal comprises noise, e.g. white noise, over a defined frequency spectrum, which may advantageously have a width of at least 1000 MHz.

[0011] The step of receiving echoes comprises receiving an echo signal for each probe signal, in particular, these echo signals being received in succession.

[0012] The step of processing the echoes comprises determining at least one amplitude and phase for each echo signal, advantageously one echo and phase for a plurality of spectral components, in particular one for each non-zero spectral component.

[0013] The method of exploring an underground structure as described has the advantage that a plurality of spectral components, in particular at least two spectral components, are transmitted into the underground structure contemporaneously, i.e. at the same time. Compared to conventional SFCW, the method can result in a speedup of data acquisition by a factor equal to the number of spectral components in each probe signal. This means that the respective device for exploring an underground structure can move faster, in particular faster by a factor equal to the number of spectral components in each probe signal, while maintaining the same resolution. This makes it easier to mount the device, for example, on a car or an aerial drone, and perform data acquisition over larger areas and / or inaccessible locations.

[0014] However, the method as described has challenges and further advantages, which will be explained in the light of the following advantageous embodiments.

[0015] Orthogonal Spectral Components In an advantageous embodiment, the spectral components are simultaneous subcarrier signals with different subcarrier frequencies. In particular, the spectral components are located in non-overlapping frequency bands. In the field of telecommunications, a similar method is known in a completely different context as Frequency Division Multiplexing (FDM). The non-overlapping frequency bands allow low-power emitters to cover a wide spectral range.

[0016] In a further advantageous embodiment, at least two spectral components are orthogonal to each other. "Orthogonal" means in particular that they satisfy an orthogonality condition, which is generally defined as their inner product, or that the integral of their product over a time interval is zero. In particular, the first subcarrier signal f(t) and the second subcarrier signal g(t) are such that the time integral of their product is zero.

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number

[0017] In particular, adjacent subcarrier frequencies differ from each other by a regular subcarrier spacing, i.e. a subcarrier spacing which may be the same for all pairs of adjacent subcarrier frequencies. Advantageously, the subcarrier spacing satisfies an orthogonality condition, for example in that the subcarrier frequencies are integer multiples of the subcarrier spacing. In particular, the subcarrier spacing may be between 1 MHz and 10 MHz, in particular about 4 MHz.

[0018] According to an embodiment, the frequency spectrum of the probe signal is located between 10 MHz and 8000 MHz, in particular between 40 MHz and 3440 MHz. Advantageously, the total frequency spectrum of the multiple subsequently transmitted probe signals has a width of at least 500 MHz, in particular at least 1000 MHz, more particularly at least 2000 MHz. With such a total frequency spectrum, the method can reach a high resolution, for example below 2 cm, and a large penetration depth, for example above 5 m, depending on the electrical characteristics of the underground structure.

[0019] Non-zero initial phase shift Transmitting a probe signal comprising multiple orthogonal subcarrier signals without further measures may lead to a high peak transmission power. This is due to the fact that at certain times, peaks of orthogonal functions with zero initial phase shift are constructively added, thus generating a high cumulative signal. The "initial phase shift" can be defined as the phase difference between the zero phase of the lower frequency function and the nearest zero phase of the higher frequency function. In an apparatus for probing underground structures, the front end, i.e. the antenna and analog components for processing the received signal, as well as the probe signal generator for generating the probe signal and the echo signal processor for processing the echo signal, must be adapted to handle the peak transmission power. Therefore, for reasons of signal-to-noise ratio and low intermodulation, it is not desirable to have a high peak transmission power that is much larger than the average transmission power.

[0020] Thus, in an advantageous embodiment, at least two of the at least two spectral components have a non-zero initial phase shift. In particular, the initial phase shift of the at least two spectral components is such that the maximum amplitude of the probe signal is smaller, in particular at least 10%, at least 25% or at least 50% smaller, than the maximum amplitude of a virtual probe signal with the same spectral components but with an initial phase shift of the spectral components of zero. This allows the peak transmission power to be kept low, in particular slightly higher than the average transmission power, for example by a factor of two or three. Thus, a good signal-to-noise ratio and low intermodulation can be achieved.

[0021] In practice, the initial phase shifts satisfying the above conditions for a given number of subcarrier signals can be obtained by empirical methods, for example numerical simulations, which subcarrier signals are summed with random initial phase shifts, thus generating the test probe signal. The maximum ratio of the test probe signal to the average of the test probe signals is then derived. These two steps are repeated with different random initial phase shifts, thus forming a set of test probe signals. For practical applications in the exploration of underground structures, the test probe signal with the minimum ratio is selected. In this approach, the initial phase shifts may be obtained from a random number generator, but are not changed in this application. In particular, the initial phase shifts of the at least two spectral components may be constant, i.e. identical, for each probe signal.

[0022] Probe signal generation In an advantageous embodiment, the method further comprises a step of generating the probe signals by frequency conversion of at least two modulated signals of different modulation frequencies on at least one carrier signal. In particular, the step of generating the probe signals comprises changing the carrier frequency of the carrier signal between subsequent probe signals. In this way, the modulation frequency can be the same for all probe signals.

[0023] Advantageously, the step of generating the probe signal and / or the step of processing the echo signal are performed partly in the digital domain and partly in the analogue domain, thereby taking advantage of the benefits of both domains, e.g. the precise control over signals with existing digital data processing routines, and the simplicity and speed of analogue components. In one embodiment, the step of generating the probe signal comprises generating at least two digitally modulated signals. The step of generating the probe signal may also comprise generating at least one analogue carrier signal. According to the above description, at least two of the at least two digitally modulated signals advantageously have a non-zero initial phase shift.

[0024] A typical modulation frequency is below 100 MHz, in particular in the range of 40 MHz to 80 MHz. Advantageously, the modulation frequency varies with the subcarrier spacing. Furthermore, the carrier frequency of the carrier signal may be at least 100 MHz.

[0025] In one embodiment, the step of generating the probe signal comprises: obtaining at least two analog modulated signals by digital-to-analog converting at least two digitally modulated signals; and mixing the at least two analog modulated signals with at least one mixed signal from at least one analog oscillator by at least one analog mixer. Further, the step of generating the probe signal may comprise mixing the at least two analog modulated signals with the mixed signals from the multiple analog oscillators in sequence by multiple analog mixers.

[0026] Advantageously, the step of generating the probe signal comprises applying an up-conversion to the analog modulated signal, in particular an up-conversion and then a down-conversion. Direct up-conversion typically requires an image-rejecting mixer, which brings about several limitations such as limited image suppression, local oscillator (LO) leakage and complex filter stages. By first up-converting the signal, the image and LO leakage can be attenuated by a filter. Since the LO of the down-conversion stage is always higher than the desired output signal, the LO leakage can be neglected. In this way, complex filter stages can be avoided.

[0027] In a further embodiment, the step of generating a digital modulation signal comprises switching off the defined modulation signal. This can still be performed in the digital domain, for example by applying a switching stage. Switching off the defined modulation signal changes the number of modulation frequencies of the corresponding probe signal. This causes the peak voltage to be modified. A dynamic correction can then be applied to the modulation signal in order to keep the peak voltage constant at a specifically intended level.

[0028] Furthermore, it may be useful for the first probe signal to have a different number of non-zero spectral components than the second probe signal. For example, the higher frequency probe signal may have a smaller number, for example half as many, of spectral components than the lower frequency probe signal. In this case, the subcarrier spacing of the higher frequency probe signal may be larger, for example twice as large, than the subcarrier spacing of the lower frequency probe signal. This is particularly reasonable in order to minimize the data that needs to be processed and ultimately stored and transmitted. By expanding the subcarrier spacing in the higher frequency probe signal, not much is lost in terms of penetration depth, while a smaller subcarrier spacing in the lower frequency probe signal may be required for a high penetration depth. Again, deleting certain subcarrier signals can be performed for each modulated signal that is still in the digital domain.

[0029] In one embodiment, the total duration of each probe signal does not exceed 2 μs. Such a short duration of each probe signal prevents excessive electromagnetic energy from being transmitted into the environment, potentially degrading electrical communication signals or affecting living organisms.

[0030] Furthermore, the envelope for each probe signal may have a rise time and a fall time, both of non-zero duration, in particular greater than 0.5 μs and / or greater than 10% of the duration of each probe signal. In comparison with a "hard window", in particular consisting of two step functions, such a "smooth" probe signal has the advantage of avoiding resonance effects in the step of processing the echoes. Therefore, the envelope may in particular be formed as a Gaussian function or other known smooth window functions.

[0031] Echo signal processing Like the step of generating the probe signal, the step of advantageously processing the echo signal is also performed partly in the analogue domain and partly in the digital domain, thus combining the advantages of both, as mentioned above.

[0032] In one embodiment, the step of processing the echo signal comprises obtaining a mixed echo signal by mixing the echo signal with at least one mixed signal from at least one analog oscillator by at least one analog mixer. In particular, the step of processing the echo signal may comprise successively mixing the echo signal with a plurality of mixed signals from a plurality of analog oscillators by a plurality of analog mixers. It may be useful for the step of processing the echo signal to comprise applying a down-conversion to the echo signal, in particular applying an up-conversion followed by a down-conversion.

[0033] Additionally, the step of processing the echo signal may comprise analog-to-digital conversion of the mixed echo signal to a digital echo signal.

[0034] Analogously to the above, the step of processing the echo signals advantageously comprises correcting the digital echo signals for an initial phase shift of at least two spectral components, in particular an initial phase shift being known for each probe signal and thus for each corresponding echo signal and therefore capable of being corrected.

[0035] It may also be desirable for the step of processing the echo signal to comprise serializing at least two non-zero spectral components, in particular with a digital echo signal, to obtain a serialized echo signal. In particular, "serializing" means that the non-zero spectral components in the echo signal, e.g. a modulated signal extracted from the echo signal, are stringed together one after the other in time. In this way, the serialized echo signal resembles a signal obtained by a conventional SFCW method. This allows the use of known data processing methods for further processing, e.g. for assembling and processing A-scans or for applying a moving algorithm.

[0036] Finally, the step of processing the echoes may comprise deriving at least one characteristic of the subterranean structure from an amplitude difference and / or a phase difference between the echo signal and the probe signal, the amplitude difference and / or the phase difference being directly related to material properties and in particular caused by the material properties, in particular the electrical properties, and the material distribution encountered by the electromagnetic wave on its travel path from the antenna into the structure and back.

[0037] Equipment for exploring underground structures A second aspect of the invention relates to an apparatus for exploring subsurface structures. The device is configured to implement the above-mentioned method, comprising: an antenna, which in one embodiment can use the same antenna for transmitting the probe signal as well as for receiving the echo signal, and in another embodiment the antenna can comprise a first antenna configured to transmit the probe signal to the underground structure and a second antenna configured to receive the echo signal received from the underground structure, and in both embodiments the antenna, or respectively the first and second antenna, comprises a lower frequency sub-antenna, for example with a centre frequency of 350 MHz, and a higher frequency sub-antenna, for example with a centre frequency of 1500 MHz, such sub-antenna having the advantage of more effectively transmitting and receiving the probe signal and, respectively, the echo signal, especially at higher and lower frequencies around its centre frequency; a probe signal generator for generating an electromagnetic probe signal to be transmitted by an antenna into an underground structure, the probe signal generator comprising, in an advantageous embodiment, a digital signal generator configured to generate at least two digitally modulated signals, a digital-to-analog converter configured to convert the digitally modulated signals into an analog modulated signal, at least one analog signal generator configured to generate at least one carrier signal, and at least one analog mixer configured to generate the probe signal by mixing the at least two analog modulated signals with the at least one carrier signal; an echo signal processing device for processing echo signals received from an underground structure by an antenna, in an advantageous embodiment the echo signal processing device comprising at least one further analog mixer configured to generate a mixed echo signal by mixing the echo signal with at least one carrier signal from at least one analog signal generator, an analog-to-digital converter configured to convert the mixed echo signal into a digital echo signal, and a digital processing device configured to derive at least two non-zero spectral components from the digital echo signal; a control unit for controlling the probe signal generator and the echo signal processor, in particular the control unit being configured to perform the steps of the method of the present technique; Optionally, a communications module, such as a Wi-Fi module, configured to transmit the processed echo signals to a remote computing unit or display.

[0038] The invention will be better understood, and further objects thereof will become apparent, when a consideration is given to the following detailed description, such description making reference to the accompanying drawings, in which: [Brief description of the drawings]

[0039] [Figure 1] FIG. 1 shows a schematic frequency spectrum with the spectral components of a subsequently transmitted probe signal according to a prior art method. [Diagram 2] FIG. 2 shows an exemplary frequency spectrum with spectral components of subsequently transmitted probe signals in a method according to an embodiment of the invention. [Diagram 3] FIG. 3 is a block circuit diagram of an apparatus for exploring underground structures according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] Examples of prior art In conventional SFCW GPR, several probe signals are transmitted one after the other into the subsurface structure. Each probe signal is conventionally composed of only one spectral component, which means that each probe signal has one single frequency, for example a single sinusoidal signal of fixed duration. This is illustrated in Fig. 1, which shows a schematic full frequency spectrum of all probe signals. In a first step, a first lower frequency probe signal, for example 30 MHz, is transmitted into the subsurface structure. Then, in a second step, a second probe signal, with a slightly higher frequency, for example 34 MHz, is transmitted. These steps of transmitting probe signals with increasing frequencies are repeated until the last step, in which a higher frequency probe signal, for example 4030 MHz, is transmitted. All probe signals form a sweep that spans the entire bandwidth, for example 30 MHz to 4030 MHz in this example. By employing lower and higher frequencies, the resulting image of the subsurface structure is improved in terms of penetration depth, i.e. the depth to which the subsurface structure can be explored, and resolution, i.e. the size of the smallest feature that can be resolved.

[0041] For correct data acquisition and correct imaging, it is necessary that the entire sweep is transmitted into the subsurface structure and its echoes are received while the antenna is not moving further (faster) than the resolution. Obviously, this condition places an upper limit on the acquisition speed, i.e., the speed at which the antenna can move relative to the subsurface structure.

[0042] In the above example, to cover the full 4000 MHz bandwidth of the sweep with frequency steps of 4 MHz, several thousand steps are required. If we assume that each probe signal has a duration, also called dwell time, of 2 μs, with interruptions of duration, also called lock time, of 5 μs, the total time required for the entire sweep is 7000 μs. With a desired resolution of, say, 20 mm, the antenna may not move faster than about 10 km per hour, especially not faster than walking speed.

[0043] On the other hand, acquisition can conveniently be performed today with an antenna mounted on a car or a drone. In that case, acquisition speeds of the order of 100 km per hour may be desirable. Such speeds are ten times higher than can be reached with prior art methods. In other words, the total time of one sweep would have to be ten times lower, while keeping other parameters fixed, in particular the resolution, penetration depth, dwell time and lock time.

[0044] Exemplary Methods The problem of limited acquisition speed is solved by a method according to an embodiment of the present invention, which is illustrated in the schematic frequency spectrum of FIG. 2. Here again one probe signal is transmitted step by step, from the first to the last step. However, now each probe signal comprises a number of, for example at least 10, spectral components. The frequency step between adjacent spectral components is again fixed, for example at 4 MHz, in order to maintain the same penetration depth and resolution as above. By transmitting 10 spectral components at once, i.e. within the same probe signal, the total time of the sweep is reduced by a factor of 10. Thus, the maximum acquisition speed for a given resolution is increased by a factor of 10. In the above example, the maximum acquisition speed is now about 100 km per hour, which is suitable for large-scale surveys, for example with cars or drones.

[0045] In general, the simultaneous transmission of multiple spectral components, also called subcarrier signals, in each probe signal increases the maximum acquisition speed by a factor equal to the number of spectral components. In order for the simultaneous transmission of such subcarrier signals to ultimately lead to data or images with a structure comparable to that of conventional SFCW, e.g., in terms of signal-to-noise ratio and low intermodulation, the subcarrier signals must satisfy certain conditions. These conditions are described above in the sections "Orthogonal Spectral Components" and "Non-Zero Initial Phase Shift".

[0046] Exemplary Apparatus 3 shows a block circuit diagram of an exemplary apparatus for exploring subsurface structures, the apparatus being configured to perform a method according to an embodiment of the present invention. Accordingly, in the following, aspects relating to the apparatus and the method are described.

[0047] The device comprises a digital processing device 1 and an analog processing device 2. In the digital processing device 1, a plurality of modulated signals, for example ten modulated signals, are generated and processed by digital components 11-15, which will be described in detail below. In a digital-to-analog converter (DAC) 20 with a clock source 21, the digital modulated signals are converted into the analog domain and then further processed by analog components 22-28. In particular, a probe signal comprising a plurality of, for example ten, subcarrier signals is generated by mixing, i.e. frequency converting, different carrier and modulated signals from analog oscillators 23 and 26. The probe signal is then transmitted by a transmitting antenna 29 into the underground structure. The above components 11-28 may be incorporated as a probe signal generator.

[0048] The echo signals return from the underground structure and are received by the receiving antenna 30. In general, the receiving antenna 30 and the transmitting antenna 29 can coincide, for example in a monostatic radar. The echo signals are processed in analog components 32-36. In particular, the echo signals are mixed again with different carrier signals from the analog oscillators 23 and 26, or in other words frequency converted. The frequency converted echo signals are converted to the digital domain by an analog-to-digital converter (ADC) 37. The echo signals are then further processed by the analog components 40-47. The components 32-47 can be incorporated as an echo signal processing device.

[0049] The device further comprises a control unit 48 configured to control the probe signal generator and the echo signal processing device. Advantageously, the device also comprises a communication unit 49, e.g. a Wi-Fi® module. The communication unit 49 is configured to transmit the processed echo signals to a remote computing device or a display device. The transmission of the processed echo signals can be performed at the output signal of any of the components 43-47, in particular at component 44, meaning that further processing steps are performed by the remote computing device. Furthermore, the communication unit 49 can be configured to receive control commands from the remote computing device, e.g. for starting or ending the data acquisition or for setting parameter values ​​such as lower limits, upper limits or frequency steps in the overall frequency range of the subsequently transmitted probe signal or other parameters as will become clear below.

[0050] Probe Signal Generation 3, the modulation signals are generated by a number of Numerically Controlled Oscillators (NCOs) in oscillator section 11. Alternatively, the NCOs may be replaced by Inverse Discrete Fourier Transforms (IDFTs), replacing components 11, 12 and 13.

[0051] As input, the NCO receives a frequency ω n , frequency offset θ n , and the initial phase shift α n where n=1, 2, ..., N, and N is the number of modulating signals, and therefore the number of subcarrier signals in each probe signal, for example 10. The modulation frequency ω n can be in the range of 40 MHz to 80 MHz with a frequency step of 4 MHz between adjacent modulation frequencies.

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[0052] Advantageously, the initial phase shift α n is non-zero and is different for at least some, and in particular all, different n. This has the advantage that the ratio of maximum probe signal amplitude to average probe signal amplitude can be kept low over time, and thus the transmit power of the device can be kept low while maintaining a high signal-to-noise ratio and low intermodulation.

[0053] The first phase shift α n The feasible choices of are in the interval 0 to 2, α n It is known that the function can be obtained by randomly generating a random number α n , and the low maximum amplitude criterion above can be checked in advance. Then, α n The values ​​of can be stored as a table in the device's internal memory and used to generate the probe signal.

[0054] Similarly, the modulation frequency ω n is the frequency offset θ n can be offset by different θ n at different frequencies ω n However, by applying the frequency offset θ n should not be too large, for example bounded by the interval (-0.5, +0.5) x Δω, to avoid overlapping with adjacent modulation frequencies. In particular, the frequency offset can be 400 kHz to maintain the orthogonality criterion defined above. In particular, all frequency offsets θ n can be the same, i.e., θ n =θ.

[0055] Advantageously, the frequency offset θ n is the same for all modulated signals within one probe signal. Furthermore, advantageously, the frequency offset θ nis the same for all probe signals in one cycle. To reduce the total cycle time, n can be applied to various cycles.

[0056] In general, the modulation frequency ω n , frequency offset θ n , and the initial phase shift α n The input values ​​for probe signal generation for φ i need to be at least temporarily stored or known for subsequent processing of the echo signals, particularly in the digital mixer 41 .

[0057] In a next step, the switch matrix 12 is applied to the digitally modulated signal. By applying the switch matrix 12, it is possible to switch off the modulated signal at a certain modulation frequency. This is useful to reduce the number of subcarrier signals in certain probe signals of a sweep, for example from N=10 to 5, or only one subcarrier signal per probe signal, while maintaining the number of subcarrier signals in other probe signals of the same sweep at N. In this way, the peak power can be lowered. Also, less processing power is required and less data needs to be transmitted at the end, for example by the communication unit 49, for example the Wi-Fi unit after the serializer 42. For example, it may be useful to use a frequency step of Δω=4 MHz for lower frequency probe signals, for example between 40 MHz and 1500 MHz, and a frequency step of Δω=8 MHz for higher frequency probe signals, for example above 1500 MHz. In this case, the switch matrix 12 zeros out each second modulated signal when providing a modulated signal for a probe signal above 1500 MHz.

[0058] A band combiner 13 then performs the summation of all the digitally modulated signals. Subsequently, a dynamic amplitude correction 14 can be performed on the combined signal as described above.

[0059] Furthermore, a gate 15 is applied to the combined signal, still in the digital domain. The gate advantageously applies a smooth time window function to the combined signal. The time window can be, for example, a Gaussian function, which reduces artifacts that typically result from applying a hard window, such as an abrupt switch-on or switching of the signal.

[0060] Here, the combined modulated signal is converted to the analog domain by a DAC 20, which receives a clock signal having a clock frequency from a clock oscillator 21. In the exemplary apparatus of Figure 3, the clock frequency is 200 MHz and is led to an analog signal which is then band-pass filtered by a filter 22 in the frequency range from 120 MHz to 160 MHz.

[0061] This signal is then mixed with a first carrier signal at a first carrier frequency generated by an analog oscillator 23. This mixing, in this case up-conversion, is performed by an analog mixer 24. This mixing process is also called "heterodyning", in particular only the "up-side heterodyne", i.e. the frequency equal to the sum of the first carrier frequency and the modulation frequency, is used, whereas the "low-side heterodyne", i.e. the frequency equal to the difference between the first carrier frequency and the modulation frequency, is truncated, for example filtered out. In general, mixing of signals towards higher frequencies is called "up-conversion" and mixing of signals towards lower frequencies is called "down-conversion".

[0062] In this example device, the first carrier frequency is 3600 MHz and is introduced into a first mixed signal, which is then band-pass filtered by a filter 25 in the frequency range from 3720 MHz to 3760 MHz. The first mixed signal is then down-converted by an analog mixer 27. For that purpose, the analog mixer 27 receives a second carrier signal from an analog oscillator 26. The analog oscillator 26 is a variable frequency oscillator arranged to generate a second carrier signal having a second carrier frequency from 3800 MHz to 7200 MHz. This analog oscillator 26 is advantageously controlled by a control unit 48. In particular, the second carrier frequency is changed between subsequent probe signals in steps of 40 MHz, i.e. in the bandwidth of one probe signal, generally from 3800 MHz to 7200 MHz. This results in the generation of a second mixed signal. This second mixed signal has in particular a bandwidth of 40 MHz and a subcarrier frequency of Δω=4 MHz. The second mixed signal after sweeping is low-pass filtered by a filter 28, which advantageously has a cut-off frequency of 3440 MHz. The filtered second mixed signal, which constitutes the actual probe signal, has a frequency in the frequency range from 40 MHz to 3440 MHz. The probe signal is then transmitted by a transmit antenna 29.

[0063] As an alternative to the example device, the mixing, i.e. modulation, of the analog modulation signal to the probe signal can be performed by a single analog mixer. Up-conversion in one step requires more filtering, since harmonics must be removed from the signal. In contrast, in the example device of Figure 3, for example the "mirror frequency" of the upper heterodyne is automatically truncated by applying several mixing steps one after the other.

[0064] Echo Signal Processing Echoes from the underground structure are received by the receiving antenna 30 as analog echo signals and filtered by a low pass filter 32, particularly in the frequency range of the probe signal, particularly at a cut-off frequency of 3440 MHz.

[0065] Further processing of the echo signal in the analog processing device 2 is mirrored from the probe signal generation as described above. Typically, the probe signal is down-converted to the frequency range of the modulated signal, in particular to the range of 40 MHz to 80 MHz. This can be done in one step or, as in the exemplary device of FIG. 3, in several steps.

[0066] In this example device, the echo signal is mixed, in particular frequency converted, and mixed with a second carrier signal from the second analog oscillator 26 by a mixer 33. This means an up-conversion of the echo signal to a first mixed echo signal, which is then filtered by a band-pass filter 34 in the frequency range between 3720 MHz and 3760 MHz. This first mixed echo signal is then down-converted by mixing with the first carrier signal from the first oscillator 23 by a mixer 35. The resulting second mixed echo signal is filtered by a band-pass filter 36 in the frequency range between 120 MHz and 160 MHz. The second mixed echo signal is then converted into the digital domain by an ADC 37, which receives a clock signal of the clock frequency from the clock oscillator 21.

[0067] In the digital processing device 1, a weighting in time 40 is applied to the echo signal. In particular, this is used to reduce the spectral leakage of the subsequent Discrete Fourier Transform (DFT). The echo signal is then input to a digital mixer 41, which advantageously transforms the echo signal into the frequency domain, for example by applying a DFT. The different spectral components of the echo signal corresponding to the frequencies of the modulating signal, for example 10 in the case of N=10, are separated and a first phase shift α n is corrected for.

[0068] Furthermore, the offset or average in each spectral component can be removed. The averaging after the mixing stage reduces the signal to one complex coefficient for each band. It also defines the equivalent noise bandwidth (ENBW) of the system given by the duration of the received signal and the shape and weighting 40 of the gates 15.

[0069] In order to obtain a processed echo signal that is comparable to the echo signal of the conventional, i.e., non-multiplexed, SFCW method, the different spectral components need to be “serialized”, i.e., sequenced in time. This can be done by serializing 42, for example by adding a time shift Δt for each of the N spectral components. n As a result of the serialization 42, the echo signal has the form of a sweep of successive signals, each subsequent signal consisting of only one spectral component and differing in frequency from the previous signal by a frequency step Δω.

[0070] Further processing of the echo signal advantageously comprises correcting the echo signal for the device response, i.e. correcting amplitude and / or phase changes due to device components. For this purpose, a calibration signal 31 may be measured between the transmit antenna 29 and the receive antenna 30, for example before each sweep of the probe signal. The calibration signal 31 may be used to correct the echo signal in a calibration 43. In particular, signal drift or device response over time may be corrected by using the current calibration signal 31 as well as historical calibration signals, which may be recorded, for example, by the device manufacturer.

[0071] Furthermore, spatial averaging 44 can be applied to the echo signals, which increases the signal to noise ratio and reduces the amount of data. Since the echo signals are to be stored or transmitted, for example by a communication unit 49, spatial averaging 44 is advantageously applied before the transmission of the data.

[0072] It is common practice to apply further signal processing steps to the echo signals, such as at least one of weighting 45, zero padding 46 and inverse Fourier transformation 47. These steps are advantageously performed on a remote computing device. As a result of the inverse Fourier transformation 47, the echo signals are converted into time domain waveforms, also called A-scans in the field of GPR.

[0073] In general, it is advantageous to use dedicated hardware for the analog processing unit 2 as well as for the digital processing unit 1. The digital processing unit 1 is advantageously one or more Field Programmable Gate Arrays (FPGAs) configured to perform one or more of the described processing steps. The use of an FPGA facilitates efficient probe signal generation and / or echo signal processing, especially real-time processing.

[0074] While presently preferred embodiments of the invention have been shown and described, it is to be expressly understood that the invention is not limited thereto and may be otherwise embodied and practiced within the scope of the following claims. The present disclosure also includes the following inventions. The first aspect is 1. A method for exploring a subsurface structure, comprising: The method comprises: transmitting electromagnetic waves into the underground structure; receiving echoes of said electromagnetic waves from said underground structure; and processing the echoes to derive internal features of the subsurface structure. transmitting the electromagnetic waves into the subterranean structure comprises subsequently transmitting into the subterranean structure a plurality of electromagnetic probe signals with different frequency spectrums; each probe signal having at least two non-zero spectral components; the step of receiving echoes comprises the step of receiving an echo signal for each probe signal; The method, wherein processing the echoes comprises determining at least one of an amplitude and a phase for each echo signal. The second aspect is The method of the first aspect, wherein the at least two non-zero spectral components are simultaneous subcarrier signals having different subcarrier frequencies. The third aspect is The method of the first or second aspect 2, wherein the spectral components are located in non-overlapping frequency bands. The fourth aspect is A method in any one of the first to third aspects, wherein the at least two spectral components are orthogonal to each other, and in particular adjacent subcarrier frequencies differ from each other by a subcarrier spacing, in particular the subcarrier spacing satisfies an orthogonality condition, and / or in particular the subcarrier spacing is 1 MHz to 20 MHz, in particular 4 MHz. The fifth aspect is The method according to any one of the first to fourth aspects, wherein the frequency spectrum of the probe signal is located between 10 MHz and 8000 MHz, in particular between 40 Hz and 3440 MHz. The sixth aspect is The method according to any one of the first to fifth aspects, wherein the total frequency spectrum of the multiple subsequently transmitted probe signals has a width of at least 500 MHz, particularly at least 1000 MHz, more particularly at least 2000 MHz. The seventh aspect is The method according to any one of the first to sixth aspects, wherein at least two of the at least two non-zero spectral components have a non-zero initial phase shift. The eighth aspect is A method according to a seventh aspect, wherein the initial phase shifts of the at least two spectral components are such that the maximum amplitude of the probe signal is smaller, in particular at least 10%, at least 25%, or at least 50% smaller, than the maximum amplitude of a virtual probe signal having the same spectral components but with an initial phase shift of the spectral components being zero. The ninth aspect is The method according to any one of the first to eighth aspects, wherein each probe signal comprises between 3 and 20, particularly between 5 and 15, particularly 10 non-zero spectral components. A tenth aspect is The method further comprises generating the probe signal by frequency converting at least two modulated signals of different modulation frequencies on at least one carrier signal; In particular, the step of generating the probe signals comprises changing the carrier frequency of a carrier signal between subsequent probe signals; and / or In particular the method according to any one of the first to ninth aspects, wherein said modulation frequency is the same for all probe signals. An eleventh aspect is the step of generating a probe signal comprises generating at least two digitally modulated signals; In particular, the method of a tenth aspect, wherein the step of generating a probe signal comprises generating at least one analog carrier signal. A twelfth aspect is 11. The method of claim 10, wherein the at least two of the at least two digitally modulated signals have a non-zero initial phase shift. A thirteenth aspect is A method according to any one of the tenth to twelfth aspects, wherein the modulation frequency is less than 100 MHz, in particular, the modulation frequency is in the range of 40 MHz to 80 MHz, and / or, in particular, the modulation frequency varies depending on the subcarrier spacing. A fourteenth aspect is The method according to any one of the tenth to thirteenth aspects, wherein the carrier signal has a carrier frequency of at least 100 MHz. The fifteenth aspect is the step of generating the probe signal comprises: obtaining at least two analog modulated signals by digital-to-analog converting the at least two digital modulated signals; and mixing the at least two analog modulated signals with at least one mixing signal from at least one analog oscillator (23, 26) by at least one analog mixer (24, 27); In particular, the step of generating the probe signal comprises successively mixing the at least two analog modulated signals with mixing signals from a plurality of analog oscillators (23, 26) by a plurality of analog mixers (24, 27); and / or In particular, the method according to any one of the eleventh to fourteenth aspects, wherein the step of generating the probe signal comprises applying up-conversion to the analog modulated signal, in particular applying up-conversion and subsequent down-conversion. A sixteenth aspect is The method according to any one of the eleventh to fifteenth aspects, wherein the step of generating a digital modulated signal comprises switching off a prescribed modulated signal. A seventeenth aspect is A method according to any one of the first to sixteenth aspects, wherein a first of the probe signals comprises a different number of non-zero spectral components than a second of the probe signals. The eighteenth aspect is The method according to any one of the first to seventeenth aspects, wherein the total duration of each probe signal does not exceed 2 μs. A nineteenth aspect is A method according to any one of the first to eighteenth aspects, wherein an envelope for each probe signal has rise and fall times of non-zero duration, and in particular, the envelope is embodied as a Gaussian function. The twentieth aspect is A method according to any one of the first to nineteenth aspects, wherein the step of transmitting the electromagnetic waves to the underground structure is performed by an antenna, in particular by a higher frequency sub-antenna and a lower frequency sub-antenna. A twenty-first aspect is The step of processing the echo signal comprises obtaining a mixed echo signal by mixing the echo signal with the at least one mixed signal from the at least one analog oscillator (23, 26) by at least one analog mixer (33, 35); In particular, the step of processing the echo signal comprises successively mixing the echo signal with the mixed signals from the analog oscillators (23, 26) by a plurality of analog mixers (33, 35); and / or In particular, the method according to any one of the fifteenth to twentieth aspects, wherein the step of processing the echo signal comprises applying down-conversion to the echo signal, in particular applying up-conversion and then down-conversion. A twenty-second aspect is A method according to a twenty-first aspect, wherein the step of processing the echo signal comprises analog-to-digital conversion of the mixed echo signal to a digital echo signal. A twenty-third aspect is The method of any one of the seventh and twenty-second aspects, wherein the step of processing the echo signal comprises compensating the digital echo signal for the initial phase shift of the at least two spectral components. A twenty-fourth aspect is A method in any one of the first to twenty-third aspects, wherein the step of processing the echo signal comprises obtaining a serialized echo signal by serializing the at least two non-zero spectral components, particularly using the digital echo signal. The twenty-fifth aspect is A method in any one of the first to twenty-fourth aspects, wherein the step of processing the echoes comprises deriving at least one characteristic of the underground structure from an amplitude difference and / or a phase difference between the echo signal and the probe signal. A twenty-sixth aspect is 1. An apparatus for exploring an underground structure, comprising: The apparatus comprises: Antenna (29); a probe signal generator for generating an electromagnetic probe signal transmitted by said antenna into said underground structure; an echo signal processor for processing echo signals received from the underground structure by the antenna; a control unit (48) for controlling the probe signal generator and the echo signal processing device; The apparatus is an apparatus for exploring an underground structure, configured to execute the method according to any one of the first to twenty-fifth aspects. A twenty-seventh aspect is The apparatus comprises: a first antenna (29) configured to transmit said electromagnetic probe signal into said underground structure; a second antenna (30) configured to receive the echo signals received from the underground structure; In particular, the apparatus according to a twenty-sixth aspect, wherein both the first antenna and the second antenna comprise a lower frequency sub-antenna and a higher frequency sub-antenna. The twenty-eighth aspect is The probe signal generator includes: a digital signal generator (1) configured to generate at least two digitally modulated signals; a digital-to-analog converter (20) configured to convert the at least two digitally modulated signals into an analog modulated signal; at least one analog signal generator (23, 26) configured to generate at least one carrier signal; and at least one analog mixer (24, 28) configured to generate the electromagnetic probe signal by mixing the at least two analog modulated signals with the at least one carrier signal. A twenty-ninth aspect is The echo signal processing device includes: at least one further analog mixer (33, 35) configured to generate a mixed echo signal by mixing the echo signal with the at least one carrier signal from the at least one analog signal generator (23, 26); an analog-to-digital converter (37) configured to convert the mixed echo signal into a digital echo signal; and a digital processing device (1) configured to derive at least two non-zero spectral components from the digital echo signal.

Claims

1. 1. A method for exploring a subsurface structure, comprising: The method comprises: transmitting electromagnetic waves into the underground structure; receiving echoes of said electromagnetic waves from said underground structure; and processing the echoes to derive internal features of the subsurface structure. transmitting the electromagnetic waves into the subterranean structure comprises subsequently transmitting into the subterranean structure a plurality of electromagnetic probe signals with different frequency spectrums; each probe signal having at least two non-zero spectral components; the step of receiving echoes comprises the step of receiving an echo signal for each probe signal; the step of processing the echoes comprises determining at least one of an amplitude and a phase for each echo signal; The method comprises: generating said probe signal by frequency converting at least two modulated signals of different modulation frequencies on at least one carrier signal; the step of generating a probe signal comprises generating at least two digitally modulated signals; 11. The method of claim 1, wherein the step of generating a probe signal comprises: obtaining at least two analog modulated signals by digital-to-analog converting the at least two digitally modulated signals; and mixing the at least two analog modulated signals with at least one mixing signal from at least one analog oscillator (23, 26) by at least one analog mixer (24, 27).

2. The method of claim 1 , wherein the at least two non-zero spectral components are simultaneous subcarrier signals having different subcarrier frequencies.

3. The method of claim 1 or 2, wherein the spectral components are located in non-overlapping frequency bands.

4. 4. The method according to claim 1, wherein the at least two spectral components are orthogonal to each other, in particular adjacent subcarrier frequencies differ from each other by a subcarrier spacing, in particular the subcarrier spacing satisfies an orthogonality condition and / or in particular the subcarrier spacing is between 1 MHz and 20 MHz, in particular 4 MHz.

5. The method according to any one of the preceding claims, wherein the frequency spectrum of the probe signal lies between 10 MHz and 8000 MHz, in particular between 40 Hz and 3440 MHz.

6. The method according to any one of the preceding claims, wherein the total frequency spectrum of the multiple subsequently transmitted probe signals has a width of at least 500 MHz, in particular at least 1000 MHz, more particularly at least 2000 MHz.

7. The method of any one of claims 1 to 6, wherein at least two of the at least two non-zero spectral components have a non-zero initial phase shift.

8. 8. The method according to claim 7, wherein the non-zero initial phase shifts of the at least two spectral components are such that the maximum amplitude of the probe signal is smaller, in particular at least 10%, at least 25% or at least 50% smaller, than the maximum amplitude of a hypothetical probe signal having the same spectral components but with an initial phase shift of the spectral components being zero.

9. A method according to any one of the preceding claims, wherein each probe signal comprises between 3 and 20, in particular between 5 and 15, in particular 10 non-zero spectral components.

10. The step of generating the probe signals comprises changing a carrier frequency of a carrier signal between subsequent probe signals; and / or The method according to any one of the preceding claims, wherein the modulation frequency is the same for all probe signals.

11. The method of claim 10 , wherein the step of generating a probe signal comprises generating at least one analog carrier signal.

12. The method of claim 11 , wherein the at least two of the at least two digitally modulated signals have a non-zero initial phase shift.

13. 5. The method according to claim 4, wherein the modulation frequency is less than 100 MHz, in particular the modulation frequency is in the range of 40 MHz to 80 MHz and / or in particular the modulation frequency varies with the subcarrier spacing.

14. A method according to any one of claims 10 to 13, wherein the carrier frequency of the carrier signal is at least 100 MHz.

15. and / or wherein the step of generating the probe signal comprises mixing the at least two analog modulated signals successively with mixing signals from a plurality of analog oscillators (23, 26) by a plurality of analog mixers (24, 27); The method according to any one of claims 11 to 14, wherein the step of generating a probe signal comprises applying an up-conversion to the analog modulated signal, in particular applying an up-conversion and a subsequent down-conversion.

16. A method according to any one of claims 11 to 15, wherein the step of generating a digitally modulated signal comprises switching off a defined modulated signal.

17. A method according to any preceding claim, wherein a first said probe signal comprises a different number of non-zero spectral components than a second said probe signal.

18. A method according to any preceding claim, wherein the total duration of each probe signal does not exceed 2 μs.

19. A method according to any one of the preceding claims, wherein the envelope for each probe signal has rise and fall times of non-zero duration, in particular said envelope being embodied as a Gaussian function.

20. The method according to any one of the preceding claims, wherein the step of transmitting electromagnetic waves into the underground structure is performed by an antenna, in particular by a higher frequency sub-antenna and a lower frequency sub-antenna.

21. The step of processing the echo signal comprises: obtaining a mixed echo signal by mixing the echo signal with the at least one mixed signal from the at least one analog oscillator (23, 26) by at least one analog mixer (33, 35); In particular, the step of processing the echo signals comprises successively mixing the echo signals with the mixed signals from the analog oscillators (23, 26) by a plurality of analog mixers (33, 35); and / or A method according to any one of claims 15 to 20, in particular wherein the step of processing the echo signals comprises applying a down-conversion to the echo signals, in particular applying an up-conversion and then a down-conversion.

22. 22. The method of claim 21, wherein the step of processing the echo signals comprises analog-to-digital conversion of the mixed echo signals to digital echo signals.

23. At least two of the at least two non-zero spectral components have a non-zero initial phase shift; 23. The method of claim 22, wherein processing the echo signal comprises correcting the digital echo signal for the non-zero initial phase shift of the at least two spectral components.

24. 24. The method according to claim 22 or 23, wherein the step of processing the echo signal comprises obtaining a serialized echo signal by serializing the at least two non-zero spectral components, in particular with the digital echo signal.

25. 25. A method according to any one of the preceding claims, wherein the step of processing the echoes comprises deriving at least one characteristic of the subsurface structure from amplitude and / or phase differences between the echo signals and the probe signal.

26. 1. An apparatus for exploring an underground structure, comprising: The apparatus comprises: An antenna (29); a probe signal generator for generating an electromagnetic probe signal transmitted by said antenna into said underground structure; an echo signal processor for processing echo signals received from the underground structure by the antenna; a control unit (48) for controlling the probe signal generator and the echo signal processing device; Apparatus for exploring subsurface structures, the apparatus being configured to carry out a method according to any one of claims 1 to 25.

27. The apparatus comprises: a first antenna (29) configured to transmit said electromagnetic probe signal into said underground structure; a second antenna (30) configured to receive the echo signals received from the underground structure; 27. The apparatus of claim 26, wherein in particular both the first antenna and the second antenna comprise a lower frequency sub-antenna and a higher frequency sub-antenna.

28. The probe signal generator includes: a digital signal generator (1) configured to generate at least two digital modulated signals; a digital-to-analog converter (20) configured to convert the at least two digitally modulated signals into an analog modulated signal; at least one analog signal generator (23, 26) configured to generate at least one carrier signal; and at least one analog mixer (24, 28) configured to generate the electromagnetic probe signal by mixing the at least two analog modulated signals with the at least one carrier signal.

29. The echo signal processing device includes: at least one further analog mixer (33, 35) configured to generate a mixed echo signal by mixing the echo signal with the at least one carrier signal from the at least one analog signal generator (23, 26); an analog-to-digital converter (37) configured to convert the mixed echo signal into a digital echo signal; 29. The apparatus according to claim 28, comprising a digital processing device (1) configured to derive at least two non-zero spectral components from the digital echo signal.

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