Buried object magnetic exploration system

Abstract

To provide a buried object magnetic survey system capable of improving the quality of measurement data by suppressing false recognition of small detection as noise and discriminating clear noise.SOLUTION: The buried object magnetic search system includes a sensor parallel arrangement frame 10 suspended and towed from a search barge. The sensor parallel arrangement frame 10 has a ladder shape. A plurality of both coil type magnetic gradiometers 11 and a plurality of cesium magnetometers 12 are alternately provided corresponding to each step of a ladder. Further, the sensor parallel arrangement frame 10 is provided with a motion sensor 13.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a magnetic exploration system for detecting the presence or absence of buried objects on the seabed from the sea. [Background technology]

[0002] There are various buried objects on the seabed and riverbed. For example, during construction of bridge piers, if the buried objects are dangerous objects such as unexploded ordnance, there is a risk of a serious accident. Therefore, it is necessary to investigate the presence of buried objects on the seabed when planning construction work.

[0003] The Earth's magnetic field affects buried objects on the seafloor, and magnetic surveys can be used to determine whether or not they are buried. The instrument is towed at a specified distance from the seafloor and scans the area to be surveyed. If the magnetic survey indicates the presence of buried objects, divers then conduct a visual survey to confirm the presence of buried objects.

[0004] Instruments used in magnetic exploration (buried iron exploration) include a double-coil magnetic gradiometer (see, for example, Patent Document 1) and a cesium magnetometer. Note that a fluxgate magnetometer (single-axis differential type) may also be used instead of a double-coil magnetic gradiometer. For simplicity of explanation, in this application, a fluxgate magnetometer and a double-coil magnetic gradiometer are considered to be equivalent. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Utility Model Registration No. 3183187 Summary of the Invention [Problem to be solved by the invention]

[0006] Both coil-type magnetic gradiometers measure the magnetic gradient of buried objects. They are highly sensitive in that they are not affected by long-wavelength anomalies such as the Earth's magnetic field, and can detect only short-wavelength anomalies in buried iron. However, they are subject to vibration noise, which is particularly unavoidable during towing. Furthermore, it is empirically known that they are suitable for detecting relatively shallow buried objects.

[0007] Cesium magnetometers measure total magnetic field, so they are less affected by fluctuation noise. Furthermore, it is empirically known that they are suitable for detecting relatively deep buried objects. However, they are contaminated with long-wavelength noise. While long-wavelength noise can be removed by high-pass filtering, the measurement data can also be distorted depending on the extent of the high-pass filtering.

[0008] Furthermore, when considering magnetic anomalies due to magnetic poles, the amplitude of the magnetic field decays inversely proportional to the square of the distance, while the amplitude of the magnetic field gradient decays inversely proportional to the cube of the distance. This also suggests that cesium magnetometers are capable of detecting buried objects at relatively deep depths compared to both coil-type magnetic gradiometers.

[0009] As such, both coil-type magnetic gradiometers and cesium magnetometers have their advantages and disadvantages.

[0010] However, if there is a possibility that the buried object may be a dangerous object such as an unexploded bomb, and it is impossible to determine whether it is noise or not, a safe erroneous judgment is made. In other words, although negligence related to overestimation is permissible, negligence related to overlooking (for example, mistaking a small detection for noise) is not permissible.

[0011] On the other hand, magnetic surveys alone cannot determine whether the object is a dangerous object such as an unexploded bomb, so a visual survey by divers is also required to confirm. Taking an overly cautious approach would place an excessive burden on the diving survey team, making the survey difficult in practice.

[0012] Therefore, it is important to distinguish obvious noise from the measurement data.

[0013] The present invention aims to solve the above-mentioned problems and to provide a buried object magnetic exploration system that can prevent small detections from being mistaken for noise, distinguish obvious noise, and improve the quality of measurement data. [Means for solving the problem]

[0014] In order to achieve the above-mentioned objective, the buried object magnetic exploration system of the present invention comprises a sensor parallel arrangement frame that is suspended and towed from an exploration barge, the sensor parallel arrangement frame being ladder-shaped, with a double-coil type magnetic gradiometer and a cesium magnetometer provided corresponding to each rung of the ladder.

[0015] This allows the measurement data from the two-coil magnetic gradiometer and the measurement data from the cesium magnetometer to complement each other, thereby preventing small detections from being mistaken for noise.

[0016] In the above invention, preferably, a plurality of double-coil type magnetic gradiometers and a plurality of cesium magnetometers are alternately provided in the sensor parallel arrangement frame corresponding to each stage.

[0017] This ensures that the measurement data corresponding to the two coil-type magnetic gradiometers and the measurement data corresponding to the cesium magnetometer mutually support each other.

[0018] In the above invention, preferably, the interval length between the steps is 0.5 to 2.0 times the maximum length of the target buried object.

[0019] This allows the target buried object to be properly explored.

[0020] In the above invention, preferably, there is provided output means for displaying, in parallel, the measurement data of the two coil-type magnetic gradiometers and the data obtained by high-pass filtering the measurement data of the cesium magnetometer.

[0021] The parallel display allows easy comparison of measurement data from both coil-type magnetic gradiometers and cesium magnetometers, and the two data complement each other.

[0022] In the above invention, preferably, the sensor parallel arrangement frame is provided with a motion sensor.

[0023] This allows the movement of the sensor parallel arrangement frame (tilt of the frame) to be monitored.

[0024] In the above invention, preferably, the device further comprises output means for displaying in parallel the measurement data of the two coil-type magnetic gradiometers, the data obtained by high-pass filtering the measurement data of the cesium magnetometer, and the measurement data of the motion sensor.

[0025] The parallel display makes it possible to distinguish vibration noise in the sensor parallel arrangement frame. [Effects of the Invention]

[0026] The buried object magnetic exploration system of the present invention can prevent small detections from being mistaken for noise, and can distinguish between obvious noise, thereby improving the quality of measurement data.

[0027] The increased reliability of the measurement data enables highly effective underwater surveys. If the buried object is a dangerous object such as an unexploded ordnance, the dangerous object can be detected with certainty. [Brief explanation of the drawings]

[0028] [Figure 1] Overall system configuration diagram [Figure 2] Parallel sensor layout frame overview [Figure 3] Measurement data (before high-pass filter processing) [Figure 4] Measurement data (after high-pass filter processing) [Figure 5] Example of a malfunction related to a double-coil magnetic gradiometer [Figure 6] Measurement data excerpt [Figure 7] Measurement data examples [Figure 8] Measurement data examples [Figure 9] Measurement data examples [Figure 10] Measurement data excerpt [Figure 11] Example of a malfunction related to a double-coil magnetic gradiometer [Figure 12] Validation data (after high-pass filtering) [Figure 13] Example of validation data [Figure 14] Measurement data examples [Figure 15] Measurement data excerpt DETAILED DESCRIPTION OF THE INVENTION

[0029] First Embodiment Figure 1 shows the overall system configuration. The system is composed of a tugboat, a survey barge, and a sensor parallel arrangement frame 10.

[0030] The tugboat tows the survey barge. The sensor parallel arrangement frame 10 is suspended from the survey barge and towed by the tugboat.

[0031] The sensor parallel arrangement frame 10 is maintained at a constant distance (for example, 1 m) from the seabed by winch operation from the survey barge.

[0032] Figure 2 is a schematic plan view of the sensor parallel arrangement frame 10. The towing direction is added for reference. The sensor parallel arrangement frame 10 is ladder-shaped so that multiple sensors can be arranged in parallel and to reduce towing resistance. In other words, there are openings between the ladder rungs. Sensors 11 and 12 are arranged in parallel on each rung of the ladder. In the example shown, they are arranged at 1m intervals.

[0033] In general, the sensor spacing should be correlated with the expected size of the buried object. Let's assume that the expected maximum length of an unexploded ordnance, etc. is 1-2 m. The sensor spacing should preferably be 0.5-2.0 times the maximum length of the buried object. In other words, the sensor spacing should be approximately 0.5-4 m. In practice, the sensor spacing should preferably be approximately 0.5-2.5 m.

[0034] If the sensor spacing is too wide, not only will insufficient resolution be obtained, but the frame will become too large and the towing resistance will increase. If the sensor spacing is too narrow, the resolution will be excessive. If the frame is too small, the scanning distance in the survey area will be too long, which will cause excessive strain.

[0035] The short side length of the sensor parallel arrangement frame 10 is slightly longer than the sensor length. In general, the double coil type magnetic gradiometer 11 is longer than the cesium magnetometer 12, about 1-2 m.

[0036] In the illustrated example, five double-coil type magnetic gradiometers (e.g., model number 56S-W) 11 and four cesium magnetometers (e.g., model number G-882) 12 are arranged alternately. For ease of explanation, the double-coil type magnetic gradiometers 11 are numbered No. 1-5 from the right side, and the cesium magnetometers 12 are numbered No. 1-4 from the left side.

[0037] The double-coil magnetic gradiometer 11 is a sensor that observes changes in the magnetic field. It is characterized by having two coils. Changes in the magnetic field generate an electromotive force in the coils. The two coils are arranged so that the output polarity of the electromotive force from each coil is opposite. By detecting the difference between the two outputs, the fluctuation noise is canceled out and the magnetic field generated by buried objects can be detected. However, if the fluctuation is large, fluctuation noise will occur.

[0038] The cesium magnetometer 12 is an optically pumped magnetometer. It measures the total magnetic field by using optical pumping. While it is less susceptible to fluctuation noise than the two-coil magnetic gradiometer, it does generate long-wavelength noise.

[0039] The signals from both the coil-type magnetic gradiometer 11 and the cesium magnetometer 12 are transmitted via cables to a recording device on the survey barge and recorded as data. The processed data is then displayed on a display device or printed out from an output device.

[0040] Figure 3 shows an example of measurement data (before high-pass filtering). From top to bottom, the measurement data is displayed for sensor 11 (No. 1), sensor 12 (No. 4), sensor 11 (No. 2), sensor 12 (No. 3), sensor 11 (No. 3), sensor 12 (No. 2), sensor 11 (No. 4), sensor 12 (No. 1), and sensor 11 (No. 5).

[0041] The horizontal axis is the towing distance (m). The maximum towing distance is 500m. The vertical axis corresponding to sensor 11 is the electromotive force (V), and the vertical axis corresponding to sensor 12 is the total magnetic force (nT). Note that these are the differences from the median values.

[0042] The data in question was taken at a location 40 m away from the revetment, and the data corresponding to sensor 12 shows long wavelength noise that is presumed to be noise caused by the revetment structure.

[0043] Figure 4 shows the measurement data shown in Figure 3 after appropriate high-pass filtering of the data corresponding to sensor 12. High-pass filtering is performed appropriately and optimally depending on the measurement data. Long wavelength noise is removed and the short wavelength side is displayed. In Figure 4, the characteristic measurement results of this invention are enclosed in a dashed rectangle.

[0044] Figure 5 is an illustration of an example of a malfunction involving a double-coil magnetic gradiometer. If the buried object is an unexploded ordnance, it may have a long shape. Generally, long objects have positive and negative magnetism at both ends of their long axis.

[0045] When the towing direction and the longitudinal direction of the buried object are aligned, as in Case A shown in the figure, the magnetic field is parallel to the measurement line direction, and there is a high possibility that areas with strong magnetic force will be detected. On the other hand, when the towing direction and the longitudinal direction of the buried object are perpendicular, as in Case B shown in the figure, the magnetic field is perpendicular to the measurement line direction, particularly in the central area, and areas with weak magnetic force will be detected, which may not be distinguished from noise. Similarly, when the buried object is located relatively deep, there is a risk that a weak magnetic force will be detected and not distinguished from noise.

[0046] Figure 6 shows an excerpt of the measurement data shown in Figure 4, and corresponds to the dashed square in Figure 4. This is measurement data from Sensor 12 (No. 2), Sensor 11 (No. 4), and Sensor 12 (No. 1) at towing distances of approximately 150-250 m.

[0047] There is a small amplitude at a towing distance of approximately 180 m for both coil-type magnetic gradiometers 11 (No. 4), but it is difficult to determine whether this is noise or not. On the other hand, a clear peak with reversed polarity was detected at a towing distance of approximately 180 m for cesium magnetometers 12 (No. 2) and 12 (No. 1). This suggests that both coil-type magnetic gradiometers 11 (No. 4) may have detected a buried object. The estimated buried object is shown in Figure 6.

[0048] Figure 7 shows measurement data from another example. At a towing distance of approximately 380 m, sensors 12 (No. 3), 11 (No. 2), and 12 (No. 4) exhibit a phenomenon similar to that described in Figure 6. This suggests the possibility of detecting buried objects.

[0049] Figure 8 also shows measurement data from another example. At a towing distance of approximately 80 m for Sensor 12 (No. 2), Sensor 11 (No. 3), and Sensor 12 (No. 3), a phenomenon similar to that described in Figure 6 is observed. This suggests the possibility that buried objects may have been detected.

[0050] As described above, even when the magnetic force detected by the two-coil magnetic gradiometer 11 is weak, the use of data corresponding to the cesium magnetometer 12 as an auxiliary means can prevent the magnetic force from being mistaken for noise.

[0051] Figure 9 shows measurement data for another example. There is a small amplitude at around 450 m towing distance for sensor 11 (No. 4), sensor 12 (No. 1), and sensor 11 (No. 5), but it is difficult to determine whether this is noise or not.

[0052] Figure 10 shows an excerpt of the measurement data shown in Figure 9, and corresponds to the solid-line square in Figure 9. This is measurement data from sensors 11 (No. 4), 12 (No. 1), and 11 (No. 5) at a towing distance of approximately 450 m.

[0053] By applying appropriate high-pass filtering to the data from the cesium magnetometer 12, it may be possible to detect buried objects. However, there are various causes of long-wavelength noise, and appropriate high-pass filtering must be applied to each individual case. Therefore, it is largely up to the judgment of the person conducting the analysis.

[0054] At this time, by using the data corresponding to the two adjacent coil-type magnetic gradiometers 11 as supplementary data, it can be confirmed that the high-pass filter processing is appropriate.

[0055] The example in Figure 10 suggests that buried objects may be detected, which can prevent them from being mistaken for noise.

[0056] ~Second embodiment~ Figure 11 is an illustration of an example of a malfunction with a double-coil magnetic gradiometer. In magnetic exploration of buried objects on the seafloor, it is necessary to maintain the sensor position at a fixed distance from the seafloor. Specifically, the seafloor depth is confirmed using an echo sounder and then operated with a winch.

[0057] However, as mentioned above, both coil-type magnetic gradiometers are prone to picking up noise, so it is important to operate the winch carefully to avoid generating noise.

[0058] In the prior art, it was assumed that noise would not be generated if the winch was operated carefully so as not to generate noise, and no verification was made as to whether noise would actually be generated.

[0059] Therefore, in the present verification test, sudden up and down movements of about 1 m of the frame 10 were intentionally reproduced in the water depth range of 3-11 m to verify noise generation.

[0060] Figure 12 shows an example of verification data. The upper part of Figure 12 shows the measurement data of five sensors 11 and four sensors 12 displayed side by side, similar to Figure 4. However, the maximum towing distance is 700 m.

[0061] A motion sensor 13 is provided approximately in the center of the frame 10. The motion sensor 13 is generally capable of measuring three components of acceleration, three components of angular velocity, and three components of magnetic field. As a result, the attitude (inclination amount) of the frame 10, i.e., pitch, roll, and yaw, can be measured in real time. It is important that the frame 10 maintain a horizontal attitude during towing, and this is adjusted via the motion sensor 13 before exploration.

[0062] Starting from a depth of 3m, the depth of frame 10 is lowered by 1m for every 40m towed, until the deepest depth is 11m, and then the depth of frame 10 is raised by 1m for every 40m towed, until the depth is 3m.

[0063] The vertical axis in the lower part of Fig. 12 represents the depth of frame 10 and the corresponding attitude of frame 10. The horizontal axes in the upper and lower parts of Fig. 12 represent the towing distance (m), and correspond to each other.

[0064] Although a long-period yaw occurs during the 700m tow, it is thought that this does not affect the measurements and can be ignored. There is also almost no roll. It can be seen that pitch occurs with the up and down movement of frame 10. Therefore, other types of sensors can be used for the motion sensor 13 as long as they can detect pitch movement.

[0065] Figure 13 shows an excerpt of the validation data, from the 50-150m towing distance section and the 350-350m towing distance section.

[0066] The data corresponding to sensor 11 (No. 4), sensor 12 (No. 1), and sensor 11 (No. 4), the pitch data of frame 10, and the depth data of frame 10 are displayed in parallel.

[0067] The pitch behavior is upwards at the front of the frame 10 when ascending, and downwards at the front when descending. Clear peaks are detected by all sensors 11, while the peaks are unclear at all sensors 12.

[0068] Furthermore, the positive and negative polarities are reversed (opposite polarities) when the frame 10 rises and falls. That is, the peaks from the sensor 11 and the peaks from the pitch data are linked.

[0069] Therefore, if all sensors 11 detect peaks of the same polarity, linked to peaks in the pitch data, and the peaks are unclear in all sensors 12, this suggests that the peaks are noise caused by the up and down movement of the frame 10.

[0070] In the prior art, it was assumed that careful winch operation would prevent noise, but the results of the present verification test confirmed noise caused by the up and down movement of the frame 10. The noise caused by the up and down movement of the frame 10 has a specific tendency and can be identified.

[0071] It should be noted that the above verification test intentionally reproduced a sudden vertical movement of the frame 10 for verification purposes, and it is difficult to imagine a sudden vertical movement of 1 meter in an actual magnetic survey. Therefore, this does not completely negate the conventional wisdom that assumes that noise will not occur if the winch is operated carefully. Below, this knowledge is used as a supplement to the measurement data.

[0072] 14 shows measurement data according to another example. There is a small amplitude in the vicinity of a towing distance of 200 m for the sensors 11 (Nos. 1 to 5), but it is difficult to determine whether this is noise or not.

[0073] Figure 15 shows an excerpt of the measurement data shown in Figure 14, and corresponds to the solid-line square in Figure 14. This is measurement data from sensors 11 (No. 4), 12 (No. 1), and 11 (No. 5) at a towing distance of approximately 200 m. The pitch data in box 10 is also displayed side by side.

[0074] In the example of FIG. 14, sensor 11 detects a peak of the same polarity, which is linked to the peak due to the pitch data, and furthermore, the peak is unclear in sensor 12, suggesting that it is noise due to the up and down movement of frame 10.

[0075] By identifying obvious noise, the burden on the next step, the underwater survey, can be reduced. [Explanation of symbols]

[0076] 10 Sensor parallel arrangement frame 11 Double-coil magnetic gradiometer 12 Cesium magnetometer 13 Motion Sensor

Claims

1. It is equipped with a parallel sensor arrangement frame that is suspended and towed from the exploration barge, The sensor parallel arrangement frame is in a ladder shape, and each step of the ladder is provided with: a double-coil magnetic gradiometer; Cesium magnetometer and It is established A buried object magnetic exploration system.

2. The sensor parallel arrangement frame has the following arrangements corresponding to each stage: a plurality of double-coil magnetic gradiometers; Multiple cesium magnetometers are arranged alternately 2. The buried object magnetic exploration system according to claim 1.

3. The interval length of the steps is 0.5-2.0 times the maximum length of the target buried object.

2. The buried object magnetic exploration system according to claim 1.

4. Measurement data from both of the coil-type magnetic gradiometers; data obtained by high-pass filtering the measurement data of the cesium magnetometer; An output means for displaying the 2. The buried object magnetic exploration system according to claim 1, further comprising:

5. The sensor parallel arrangement frame is provided with a motion sensor. It is established 2. The buried object magnetic exploration system according to claim 1.

6. Measurement data from both of the coil-type magnetic gradiometers; data obtained by high-pass filtering the measurement data of the cesium magnetometer; Measurement data of the motion sensor; An output means for displaying the 6. The buried object magnetic exploration system according to claim 5, further comprising:

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