A method for removing abnormal undulations of drag-type shallow profiling based on wave cycle

Abstract

The present invention discloses a method for removing abnormal fluctuations of dragged shallow-profile strata based on the wave cycle. The method comprises picking up the seabed time of shallow-profile data, and correcting the picked up seabed time to the real seabed time to eliminate the abnormal fluctuation phenomenon of the strata. The method approximates the real seabed time at a calm sea surface by calculating the average value of the abnormal seabed fluctuation amount within a complete wave cycle, and correcting the picked up seabed time of shallow-profile data to the real seabed time to eliminate the abnormal fluctuation phenomenon of the strata. The scheme of the present invention is highly reliable, convenient and efficient, has a wide range of applications, and has higher practical application value.

Description

[0001] Original application number: 2021104168251

[0002] Original application date: April 19, 2021

[0003] Original invention title: A method for removing abnormal undulations in drag-type shallow profile formations Technical Field

[0004] The present invention relates to the field of marine high-resolution shallow-profile seismic data processing, and in particular to a method for removing abnormal fluctuations of dragged shallow-profile strata based on ocean wave cycles. Background Art

[0005] A sub-bottom profiler uses sound waves to detect the cross-sectional structure of shallow strata. Improved on the ultra-wideband seafloor profiler, it displays the profiles of the strata at the bottom of oceans, rivers, and lakes. Combined with geological interpretation, it can detect subsurface geological structures. It is widely used in marine geological surveys, geophysical exploration, marine engineering, ocean observation, seabed resource exploration and development, waterway and port engineering, and submarine pipeline laying.

[0006] There are many types of shallow profiling in common use. The present invention is mainly aimed at the towed shallow profiling system, which is a shallow profiling system in which the source and receiver are independent of each other and are towed behind the hull during the acquisition process. Its equipment manufacturers include AAE of the United Kingdom, GEO of the Netherlands, and SIG of France. During the acquisition process, if the sea conditions are good, the shallow profiling equipment of these companies will generally obtain better seismic profiles. However, when the sea conditions are poor, the seismic profile will be greatly affected, especially the problem of abnormal stratum fluctuations in the seismic profile. Figure 1 As shown, it can be clearly seen that there are obvious abnormal strata fluctuations on the seabed and the strata below. This is not an actual geological phenomenon, but is caused by poor sea conditions. Therefore, how to eliminate this "geological illusion" during processing is extremely important for later geological interpretation.

[0007] It is generally believed that under poor sea conditions, the distance between the source and the geophone changes, i.e., the offset changes, which in turn affects the seabed reflection time. Therefore, it is believed that the fluctuation of the formation is caused by the change in offset, and eliminating the influence of the offset can solve the problem of formation fluctuation. However, in practice, eliminating the influence of offset only eliminates the influence of lateral space. In poor sea conditions, in addition to the spatial change between the source and the geophone, there is also the fluctuation of the sea waves, which is the main cause of formation fluctuation. Existing methods can only "mitigate" the formation fluctuation problem, and it is difficult to completely solve it. Summary of the Invention

[0008] In response to the defects in the existing technology, the present invention proposes a method for removing abnormal fluctuations in the dragged shallow profile of the stratum based on the wave cycle, which mainly removes the abnormal fluctuations in the stratum by eliminating the influence of the up and down fluctuations of the waves on the seismic profile.

[0009] The present invention is implemented by adopting the following technical solution: a method for removing abnormal fluctuations of a drag-type shallow profile stratum based on the wave cycle, comprising the following steps:

[0010] Step A: If there is echosounder data, the water depth data measured by the echosounder is integrated with the shallow profile data, and then step B is performed; if there is no echosounder data, step B is directly performed;

[0011] Step B, picking up the seabed time of shallow profile data;

[0012] Step C, determining the true seabed time;

[0013] Step D: Correct the seabed time of the shallow profile data picked up in step B to the real seabed time, so as to eliminate the abnormal undulation of the formation.

[0014] Furthermore, in step A, a spatial position matching method is used to perform data fusion, specifically including:

[0015] Step A1, quality control of echo sounder data: eliminate zero-value interference and calibrate the position of the echo sounder;

[0016] Step A2: relocate the shallow section data spatial position to obtain the corrected shallow section position coordinates;

[0017] Step A3, least squares spatial distance fitting: match the spatial positions of all channels of the shallow profile data with the spatial positions of the echo sounder water depth data to achieve matching between the echo sounder water depth data and the shallow profile data.

[0018] Furthermore, in step A3, when performing least squares spatial distance fitting, the following method is specifically adopted:

[0019] (1) Based on the shallow profile data, calculate the distance between each point of the shallow profile data and each point of the depth sounder to obtain the minimum distance point. After all shallow profile data points are calculated, arrange all the minimum distance points in ascending order and calculate the average value of the middle 50% of the distance points;

[0020] (2) Set the percentage cutoff value according to the sample size. After the percentage cutoff value is determined, the minimum distance point corresponding to it is the threshold. Compare the threshold value with the average value to adjust the cutoff value;

[0021] (3) All minimum distance points within the threshold range are retained and set as the best matching points; the minimum distance points exceeding the threshold range are considered to be mismatched between the echo sounder and the shallow profile data, and are set as empty channels. The empty channels are interpolated to complete the matching of the spatial positions of all channels of the shallow profile data with the spatial positions of the echo sounder water depth data, and then the echo sounder water depth data is imported into the shallow profile data.

[0022] Furthermore, in step A3, considering that the distance between the shallow profile and the positioning device is difficult to measure accurately, resulting in a distance difference between the imported echo sounder water depth data and the shallow profile seabed in the horizontal direction, the distance between the shallow profile and the positioning device is re-determined by measuring the distance difference between the echo sounder water depth data and the shallow profile seabed in the horizontal direction, and then the least squares spatial distance fitting is re-performed to achieve a more accurate match.

[0023] Furthermore, the step B specifically includes the following steps:

[0024] Step B1, eliminating spherical diffusion effect: using seawater velocity of 1500 m / s to compensate for spherical diffusion;

[0025] Step B2: Determine the simple seabed surface:

[0026] (1) If there is depth data from an echosounder, smooth the depth data and use it as a simple seabed surface;

[0027] (2) If there is no depth data from the echo sounder, outline the seabed according to the seabed morphology, keep the distance between the picked points and the seabed the same when picking, and then interpolate the unpicked points to determine the simple seabed surface;

[0028] Step B3: using the simple seabed surface as a reference, and determining the automatically picked seabed time according to the amplitude energy within the fixed time window;

[0029] Step B4: Correct the abnormal points picked up in step B3, and then determine the seabed time of the shallow profile data.

[0030] Furthermore, in step C, when determining the real seabed time:

[0031] If there is depth data from a depth sounder, use the depth data from the depth sounder to determine the true seabed time;

[0032] If there is no depth data from the echo sounder, the true seabed time is simulated based on the actual seismic data: the period of abnormal fluctuations in the shallow profile data, that is, the number of sample points in the horizontal direction, is measured, and then the average seabed time within this period is calculated to obtain the true seabed time.

[0033] Compared with the prior art, the advantages and positive effects of the present invention are:

[0034] The solution of the present invention proposes two different strategies depending on whether there is echosounder water depth data. When echosounder water depth data exists, the seabed time at calm sea level is defined by the echosounder water depth data measured synchronously with the ship; if there is no echosounder water depth data, the seabed time at calm sea level is approximated by calculating the average value of abnormal seabed fluctuation within a complete cycle. By correcting the seabed time of the picked-up shallow profile data to the real seabed time, the abnormal fluctuation phenomenon of the formation is eliminated. This solution is convenient and reliable, has high processing efficiency, and has a wide range of applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a schematic diagram of abnormal stratum fluctuations caused by poor sea conditions in shallow section surveys;

[0036] Figure 2 This is a schematic diagram of the principle of abnormal fluctuation of shallow strata;

[0037] Figure 3 A schematic diagram of an obvious wave cycle according to an embodiment of the present invention;

[0038] Figure 4 This is a schematic diagram of the principle of the method according to an embodiment of the present invention;

[0039] Figure 5 Schematic diagram of processing shallow profile data points exceeding a threshold range according to an embodiment of the present invention;

[0040] Figure 6 This is a schematic diagram showing a lateral difference between the shallow seabed and the depth of the depth sounder when the distance difference D is inaccurate according to an embodiment of the present invention;

[0041] Figure 7 This is a schematic diagram of an example of the fusion of depth data and shallow profile data from a depth sounder according to an embodiment of the present invention;

[0042] Figure 8 This is a schematic diagram comparing shallow section data before and after the abnormal stratum fluctuation phenomenon is eliminated according to an embodiment of the present invention. DETAILED DESCRIPTION

[0043] In order to more clearly understand the above-mentioned objects, features and advantages of the present invention, the present invention is further described below with reference to the accompanying drawings and embodiments. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can also be implemented in other ways than those described herein. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0044] In order to understand this scheme more clearly, the principle of abnormal stratum fluctuation is first explained: Figure 2As shown in the figure, when the sea conditions are poor, the ups and downs of the waves cause the earthquake source and the detection point to deviate from the calm sea surface. There are two extreme cases: one is that the earthquake source and the detection point are both located at the crest of the wave. At this time, the seismic wave is excited from the earthquake source, reflected from the seabed and received by the detection point, and its travel time is significantly increased, forming a trough of abnormal undulating strata on the seismic profile; the other is that the earthquake source and the detection point are both located at the trough of the wave. At this time, the seismic wave is excited from the earthquake source, reflected from the seabed and received by the detection point, and its travel time is significantly reduced, forming a crest of abnormal undulating strata on the seismic profile.

[0045] When the wave height is 3 meters, Figure 2 The increase in relative calm sea surface when traveling in state A is approximately (1.5m+1.5m) / 1.5m / ms=2ms. Figure 2 The reduction in the average B-state relative to calm seas is approximately (1.5m + 1.5m) / 1.5m / ms = 2ms. Therefore, in poor sea conditions, the abnormal stratum fluctuation on the seismic profile can reach 4ms, far exceeding the error tolerance of high-resolution shallow profiles. Therefore, it is necessary to eliminate the influence of the ups and downs of the waves on the seismic profile.

[0046] According to the above analysis, when the earthquake source and the detection point are respectively corrected to the calm sea surface, the travel time tends to be consistent and the stratum fluctuation phenomenon can be eliminated. Therefore, the key to the problem is how to find the best seabed time when the sea surface is calm.

[0047] In the solution of the present invention, when there is echo sounder water depth data, the seabed time at calm sea level is defined by the echo sounder water depth data measured synchronously with the ship. Because the echo sounder is generally installed at the rear of the hull where the center of gravity is most stable, its amplitude of fluctuation with the sea surface is small, and the measured water depth data is more accurate. If there is no echo sounder water depth data, the seabed time at calm sea level can be approximated by calculating the average value of abnormal seabed fluctuation within a complete wave cycle. Because the source shot time is a fixed value and the fluctuation of the waves is also periodic, this makes the stratum fluctuation have a certain regularity, that is, the stratum fluctuation often fluctuates around the seabed time at calm sea level and also has a certain periodicity ( Figure 3 ), so this method has a wider range of applicability.

[0048] like Figure 4 As shown, this embodiment proposes a method for removing abnormal undulations of the towed shallow profile. When there is no echosounder data, the abnormal undulations of the formation are removed based on the wave cycle. Specifically, the method includes the following steps:

[0049] Step A: If there is depth data from the echo sounder, the depth data measured by the echo sounder is fused with the shallow profile data:

[0050] The purpose of fusion is to import the echosounder water depth data into the shallow profile data. Regarding data fusion, traditional methods generally use time matching. Echosounder water depth data is generally recorded once every 1 second, while shallow profile data is generally recorded every 4-5 seconds. Therefore, in theory, the water depth data recorded by the echosounder must include the water depth data at all shallow profile locations. However, in actual application, it was found that this method did not match the time because the echosounder system and the shallow profile system belong to two separate systems and lack a synchronization controller between the two. In addition, the time scales used for echosounder and shallow profile data recording may be different, that is, there is a conversion between Beijing time and Greenwich time. If the time spans across days, the time will be confused. In addition, when there are recording gaps between the two, it is even more difficult to handle. Therefore, it is relatively troublesome to unify the two.

[0051] This embodiment proposes a spatial position matching method for data fusion, that is, a judgment is made based on the principle of the shortest distance between the echo sounder position and the shallow profile data. This method is relatively simpler and more convenient. The specific steps are as follows:

[0052] Step A1: Quality control of depth data from echo sounder:

[0053] In complex sea conditions, the depth sounder may sometimes have difficulty receiving reflected signals due to the large shaking of the hull, causing the depth data of the depth sounder to appear zero. Therefore, the depth data of the depth sounder must first eliminate zero-value interference, that is, use the surrounding normal values ​​to interpolate the zero-value data.

[0054] In addition, under normal circumstances, there is a certain distance between the echo sounder and the positioning device. Therefore, there is a certain deviation between the GPS information recorded by the positioning device and the actual echo sounder position. It is necessary to measure the relative position between the two according to the actual situation and perform position correction. To facilitate position calculation, it is necessary to convert spherical coordinates into plane coordinates. Projections generally use Mercator projection or UTM projection. The actual echo sounder position coordinates need to be calculated based on the ship's sailing direction, as shown in Formula 1:

[0055]

[0056] Where X1 is the horizontal coordinate of the echo sounder before position correction, Y1 is the vertical coordinate of the echo sounder before position correction, X is the horizontal coordinate of the echo sounder after position correction, Y is the vertical coordinate of the echo sounder after position correction, S is the distance between the echo sounder and the positioning device, and α is the heading angle.

[0057] Step A2: Repositioning of shallow data space position:

[0058] Similar to the principle of depth sounder position correction in step A1, the shallow depth source is typically located several dozen meters behind the survey vessel, leaving a distance difference from the positioning equipment, thus also requiring position correction. The distance D between the shallow depth source and the positioning equipment is typically between 40 and 80 meters, depending on the field acquisition conditions and determined based on actual circumstances. In this embodiment, the shallow depth coordinate repositioning is shown in Formula 2:

[0059]

[0060] Where X′1 is the horizontal coordinate of the shallow section before position correction, Y′1 is the vertical coordinate of the shallow section before position correction, X′ is the horizontal coordinate of the shallow section after position correction, Y′ is the vertical coordinate of the shallow section after position correction, D is the distance between the shallow section and the positioning device, and α is the heading angle. It should be noted that the D value is generally determined by measuring the release length of the shallow section source, which is not necessarily reliable and requires further matching with the shallow section seabed for correction.

[0061] Step A3, least squares spatial distance fitting:

[0062] (1) Based on the shallow profile data, calculate the distance between each point of the shallow profile data and all spatial positions of the depth sounder to obtain the minimum distance point

[0063]

[0064] Among them D m黨n is the minimum distance point between the shallow profile data point and all spatial positions of the echo sounder, X is the horizontal coordinate of the echo sounder position after correction, Y is the vertical coordinate of the echo sounder position after correction, X′ is the horizontal coordinate of the shallow profile position after correction, Y′ is the vertical coordinate of the shallow profile position after correction, after all shallow profile data points are calculated, all minimum distance points are arranged in ascending order, and the middle 50% of the distance points are selected to calculate the average value;

[0065] (2) Set the percentage cutoff value. This value is generally set according to the sample size. The larger the sample size, the larger the percentage cutoff value, and the smaller the sample size, the smaller the percentage cutoff value. It is generally more appropriate to set it to 90%-95%.

[0066] (3) After the percentage cutoff value is determined, the corresponding minimum distance point is the threshold. At this time, it is necessary to further compare the threshold and the average value. If the difference between the two is too large, it indicates that the percentage cutoff value is too large. The percentage cutoff value needs to be reduced until the deviation between the two is within a reasonable range.

[0067] (4) All the minimum distance points within the threshold range are retained and considered as the best matching points; the minimum distance points exceeding the threshold range are considered to be mismatched between the depth sounder and the shallow profile data and are therefore set as empty channels. Figure 5), interpolate the empty channels, complete the matching of the spatial positions of all channels in the shallow profile data with the spatial positions of the echo sounder water depth data, and then realize the import of the echo sounder water depth data into the shallow profile data.

[0068] In addition, considering that the distance D between the shallow profile and the positioning equipment in formula 2 is difficult to measure accurately, there may be a distance difference between the imported echosounder water depth data and the shallow profile seabed horizontally. By measuring the distance difference between the two, the accurate D value can be regained based on the distance difference between the two ( Figure 6 ), and then re-performing the least squares spatial distance fit, the echosounder water depth data and the shallow profile data can be matched and integrated. Once the D value is determined, the coordinates of the remaining survey lines in the project no longer need to be re-measured.

[0069] An example of fusion of bathymetry and shallow seabed data is Figure 7 As shown, the echosounder water depth data and the shallow profile data are well integrated. Clearly, the echosounder water depth data confirms the existence of the seabed scarp, while the other seabed topography is false, caused by swells. If there is no echosounder water depth data, skip this step and proceed directly to step B.

[0070] Step B: Pick up the seabed time of shallow profile data:

[0071] Since shallow profile data is generally large in volume, manual picking is too inefficient. In addition, due to the large differences in shallow profile acquisition environments and the many influencing factors such as environmental noise, fully automated picking is currently unavailable. The best seabed picking method at this stage is a combination of automated picking and manual correction. The specific steps are as follows:

[0072] Step B1, eliminating spherical diffusion effect: mainly to make the seabed energy consistent and avoid excessive energy difference between deep and shallow water areas, the seawater speed of 1500m / s is used to compensate for spherical diffusion.

[0073] Step B2: Determine the simple seabed surface:

[0074] If there is depth data from an echosounder, it can be smoothed to serve as a simple seabed surface.

[0075] If there is no depth data from the echo sounder, the seabed contour can be roughly outlined at a certain position above the seabed according to the seabed morphology. When picking, keep the distance between the picked point and the seabed roughly the same, and then interpolate the unpicked points to achieve a simple seabed surface.

[0076] Step B3: Using the simple seabed surface as a reference, determine the automatically picked seabed time based on the amplitude energy within a fixed time window:

[0077] The fixed time window is a time window with a fixed length starting from a simple seabed surface and including the seabed, which is generally 30-50ms long. Within this time window, the first point that reaches the set amplitude energy is searched. The point at this time is the automatically picked seabed time. The amplitude energy point generally needs to be set according to the actual shallow profile data. The commonly used amplitude energy is 2000.

[0078] Step B4: Correcting abnormal points: When the ambient noise is high, the data will often be picked up due to noise. In addition, when the seabed topography is large, the seabed energy may be too high or too low, resulting in inaccurate seabed geological data. Therefore, manual quality control is required after automatic data collection to manually correct the abnormal points.

[0079] Step C: Determine the real seabed time:

[0080] Since the water depth data of the echo sounder is much more accurate than that of the shallow profile, if the water depth data of the echo sounder is available, the water depth data measured by the echo sounder is the real seabed time;

[0081] If there is no depth data from the echo sounder, actual seismic data is needed to approximate the real seabed time: Since the fluctuation of waves is periodic, the abnormal fluctuation of the seabed stratum often also presents periodic characteristics, which represents the periodic change of the relative height difference between the earthquake source and the detection point. This change basically makes a periodic reciprocating motion around the calm sea surface. Therefore, the average seabed time of a cycle is generally the seabed time when the sea surface is calm. For this purpose, the period of abnormal fluctuation of the shallow profile data is measured, such as Figure 4 As shown in the figure, that is, the number of sample points in the horizontal direction, and then the average seabed time within this range is calculated to obtain the real seabed time.

[0082] Step D: Correct the picked-up shallow seabed time to the real seabed time to eliminate the abnormal stratum fluctuation phenomenon:

[0083] The correction amount calculation formula is:

[0084] Δt=T1-T2 (3)

[0085] Where Δt is the correction amount, T1 is the real seabed time, and T2 is the picked seabed time.

[0086] Figure 8 This shallow profile data was collected from the Bohai Bay area. Bathymetric data was not available for this acquisition, so the seafloor time was approximated by smoothing the seafloor time after seafloor picking. Due to the high quality of this acquisition, automatic seafloor picking allows for almost complete accuracy, significantly improving picking efficiency. With the elimination of abnormal stratum fluctuations, the seafloor becomes smoother, and the underlying stratum continuity improves. The overall profile signal-to-noise ratio is also improved, making stratum tracking and identification easier.

[0087] The above description is merely a preferred embodiment of the present invention and does not constitute any other form of limitation to the present invention. Any person skilled in the art may utilize the technical contents disclosed above to change or modify them into equivalent embodiments with equivalent changes for application in other fields. However, any simple modification, equivalent change, and modification of the above embodiments made in accordance with the technical essence of the present invention without departing from the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for removing abnormal undulations of a drag-type shallow profile based on the wave cycle, characterized in that: The following steps are involved: Step B, picking up the seabed time of shallow profile data; Step B1, eliminating spherical diffusion effect: using seawater velocity of 1500 m / s to compensate for spherical diffusion; Step B2: Determine the simple seabed surface: According to the seabed morphology, outline the seabed contour, keep the distance between the picked points and the seabed the same when picking, and then interpolate the unpicked points to determine the simple seabed surface; Step B3: using the simple seabed surface as a reference, and determining the automatically picked seabed time according to the amplitude energy within the fixed time window; Step B4, correcting the abnormal points picked up in step B3, and then determining the seabed time of the shallow profile data; Step C, determining the true seabed time: simulating the true seabed time based on actual seismic data: measuring the wave cycle of abnormal undulations in the shallow profile data, i.e., the number of sample points in the horizontal direction, and then calculating the average seabed time within this wave cycle range to obtain the true seabed time; Step D: Correcting the seabed time of the shallow profile data picked up in step B to the real seabed time to eliminate abnormal stratum fluctuations; The correction amount calculation formula is: Δt=T1-T2; Where Δt is the correction amount, T1 is the real seabed time, and T2 is the picked seabed time.

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