CN119148201A - OBN or OBS exploration transverse wave data conversion method based on seismic interferometry - Google Patents

Abstract

The invention discloses an OBN or OBS exploration transverse wave data conversion method based on a seismic interferometry, which belongs to the field of geophysical exploration and seismology and comprises the steps of carrying out longitudinal wave field separation on collected original X-component and Z-component seismic data, separating the original seismic data into longitudinal waves (PP waves) and converted waves (PS waves), selecting sea surface shot points and a submarine detector, and carrying out cross-correlation operation on direct waves, the PP waves and the PS waves received by the submarine detector by using the seismic interferometry to obtain virtual source PP waves and PS wave records with shot points all located on the sea bottom. The method aims at eliminating the influence of a water layer to obtain a virtual source record of which the offset points are all positioned on the sea bottom, and based on the obtained virtual source record of which the offset points are positioned on the sea bottom, performing second interferometry processing on reflected waves, namely combining the PP wave and the PS wave to obtain a virtual source record of the SS wave, and finally obtaining the section of the SS wave of the virtual source by using a conventional longitudinal wave data processing method.

Description

OBN or OBS exploration transverse wave data conversion method based on seismic interferometry

Technical Field

The invention belongs to the field of geophysical exploration and seismology, and particularly relates to an OBN or OBS exploration transverse wave data conversion method based on a seismic interferometry.

Background

In marine oil and gas exploration, two exploration modes, OBN and OBS, are increasingly applied to actual acquisition. How to handle the data collected in two ways is always a problem of hot spots. In the conventional PS wave interferometry, longitudinal wave data is mainly used for processing. As the seismic waves propagate, the seismic waves are reflected by the interface and decomposed at the interface. Therefore, the longitudinal wave is also decomposed into transverse waves in the propagation process, and then continues to propagate in a transverse wave mode, and is further decomposed into longitudinal waves at a solid-liquid interface for propagation, and finally received by a wave detector on the water surface. However, in the actually received longitudinal wave signal, the wave field energy is weak and is not easy to identify, so that the existing PS wave interferometry is difficult to apply to actual data processing.

The method is based on the application of the existing seismic interferometry in the conventional longitudinal wave data processing, and provides a new data processing mode which is suitable for the data processing acquired by the OBN and high-density OBS exploration methods, so that the application range of the seismic interferometry is further widened.

Disclosure of Invention

The invention aims to solve the technical problems in the background technology and provides an OBN or OBS exploration transverse wave data conversion method based on a seismic interferometry.

In order to solve the technical problems, the technical scheme of the invention is as follows:

an OBN or OBS exploration transverse wave data conversion method based on a seismic interferometry, the method comprising:

s1, performing longitudinal and transverse wave field separation on the acquired seismic data of the original X component and the Z component to obtain PP waves and PS waves;

s2, selecting a sea surface shot point and a submarine detector, and performing cross-correlation operation on direct waves and PP waves and PS waves received by the submarine detector by using a seismic interferometry to obtain virtual source PP wave and PS wave records of shot points on the sea bottom;

And S3, performing second interferometry treatment on the reflected wave based on the obtained virtual source record with the offset point at the seabed, wherein PP+PS=SS, namely combining the PP wave and the PS wave to obtain the virtual source record of the SS wave, and finally obtaining the section of the virtual source SS wave by using a conventional longitudinal wave data processing method.

Further, the step S2 specifically includes:

Performing cross-correlation operation on xAOB and yOB data, eliminating the time of a propagation path of seismic waves in water, and finally obtaining a seismic record with A as a virtual source and B as a received seismic record;

The principle of the method is as follows:

wherein x and y are shots on the water surface, A, B are detectors on the sea floor.

XAOB and yOB represent two seismic data records. Where x and y are shots of the water surface and A and B are detectors of the sea floor. "xAOB" and "yOB" represent recordings of seismic waves emanating from the x and y shots at the two detectors A and B, respectively.

In seismic exploration, shot is the location of a seismic source and a geophone is the device that receives the seismic wave. By analyzing the recordings of seismic waves emanating from different shots at different receivers, information about the subsurface structure may be obtained. In the invention, the cross-correlation operation is carried out on xAOB and yOB data, the time of the propagation path of the seismic wave in water is eliminated, and finally the seismic record with A as a virtual source and B as a received seismic record is obtained. This is a key step in seismic interferometry, which can effectively improve the accuracy of seismic data processing.

Further, the step S3 specifically includes:

The first step, carrying out interference processing on two data of the AOB and the AON, and eliminating an incident wave part AO after cross-correlation operation to obtain BN;

Step two, in the first step, obtaining a record with N as a virtual source point, then taking N as a shot point, taking M as a PS wave NM received by a detector, carrying out cross-correlation operation on BN and NM, eliminating a public path NO, and finally obtaining BM, wherein the result can be regarded as B as the virtual source record with the virtual source point M as the detector;

In the recording, the point B is excited by a transverse wave source, the point M is a received SS wave signal, and the principle formula of the method is shown as follows:

ω_ψS(t,M,B)＝∫∫[ω_PS(t,A,B)*ω_PS(t,N,M)*ω_PP(-t,A,N)]dAdN (2)

wherein M and N are submarine detectors.

AOB and AON represent two seismic data records. Here a is the virtual shot of the seabed and B and N are detectors of the seabed. "AOB" and "AON" represent recordings of seismic waves emanating from point A as virtual shots at both B and N detectors, respectively.

In seismic exploration, virtual shots are virtual source positions obtained by means of seismic interferometry, and detectors are devices that receive seismic waves. By analyzing the recordings of seismic waves emanating from different virtual shots at different detectors, information about the subsurface structure may be obtained. In the invention, two data of AOB and AON are processed by an interference method, and after cross-correlation operation, an incident wave part AO is eliminated to obtain BN. Then, taking N as a shot point, taking M as PS wave NM received by a detector, performing cross-correlation operation on BN and NM, eliminating a public path NO, and finally obtaining BM. This is a key step in seismic interferometry, which can effectively improve the accuracy of seismic data processing.

Compared with the prior art, the invention has the advantages that:

(1) The conventional interferometry processing method cannot be applied to data processing when the offset points are not located on the same plane. The method can be applied to high-density OBS exploration and OBN exploration data processing, and can eliminate the influence of a sea water layer, so that the application scene of the seismic interferometry is wider.

(2) Both OBS and OBN are four-component detectors, specifically comprising a P component that receives longitudinal wave information, a Z component that primarily receives longitudinal waves, and two X and Y components that receive transverse wave information. In the existing interferometry, P-component data is mainly processed, and the other three components of data are not utilized. The invention uses the X component and the Z component to process, fully utilizes the PS wave data, and has more accurate result.

(3) By using the method, the virtual source record similar to the result of the longitudinal wave interferometry can be obtained, so that in the subsequent processing, the conventional longitudinal wave data processing flow can be used for processing, the method is convenient and quick, the imaging can be performed quickly, and the reference is provided for the subsequent data analysis and interpretation work. Compared with the existing PS wave interferometry, the virtual source record generated by the method provided by the invention has the greatest difference that the result shows that the virtual source S wave excites the S wave to be received, namely, the transverse wave information is completely shown in the obtained virtual source record, so that the constructed S wave speed can be directly obtained, and the speed iteration process in conventional processing is omitted.

Drawings

FIG. 1, prior art PPSP wave and PS wave interferometry;

FIG. 2 shows a PS wave seismic interferometry method according to the present invention;

FIG. 3, PS wave interferometry process flow;

FIG. 4, build geologic model 1;

FIG. 5, virtual source record correspondence model;

FIG. 6, model forward Z component (a) and X component (b) data;

fig. 7, PP wave (a) and PS wave (b) after separation;

FIG. 8, (a) a PP wave virtual source record obtained by a first interferometry, (b) a PP wave record obtained by model forward modeling;

FIG. 9, (a) PS wave virtual source record obtained by a first interferometry, (b) PS wave record obtained by model forward modeling;

FIG. 10, build a horizontal model;

fig. 11, model forward PP wave (a) and PS wave (b);

FIG. 12, virtual source SS wave seismic record;

FIG. 13, virtual source SS wave superposition profile;

FIG. 14, cut-away post-refractive X component data;

FIG. 15, cut-away post-refractive Z-component data;

FIG. 16, ps wave after longitudinal and transverse wave separation;

fig. 17, PP wave after longitudinal and transverse wave separation;

FIG. 18 shows the result of the first interferometry operation of the direct wave and the separated PP wave;

FIG. 19 shows the result of the first interferometry operation of the direct wave and the separated PS wave;

And (5) processing actual data to obtain a virtual source SS wave record.

Detailed Description

The following describes specific embodiments of the present invention with reference to examples:

It should be noted that the structures, proportions, sizes and the like illustrated in the present specification are used for being understood and read by those skilled in the art in combination with the disclosure of the present invention, and are not intended to limit the applicable limitations of the present invention, and any structural modifications, proportional changes or size adjustments should still fall within the scope of the disclosure of the present invention without affecting the efficacy and achievement of the present invention.

Also, the terms such as "upper," "lower," "left," "right," "middle," and "a" and the like recited in the present specification are merely for descriptive purposes and are not intended to limit the scope of the invention, but are intended to provide relative positional changes or modifications without materially altering the technical context in which the invention may be practiced.

Abbreviations and key term definitions:

OBN is a submarine node;

OBS, ocean bottom seismograph;

CMP, common center point;

PS wave-converted transverse wave;

P wave, longitudinal wave;

s wave, transverse wave;

Example 1:

In the prior art, when the seismic longitudinal wave signal propagates, the seismic longitudinal wave signal can propagate in a medium in any solid-liquid-gas state, so that the seismic interferometry PP wave data processing has more mature application. However, in the data application of PS waves, since S waves cannot propagate in the fluid, and at the same time, the incident angle and the reflected angle are no longer equal when the waves are reflected at the interface, the seismic interferometry is relatively small in the application of PS waves. The following figure shows a treatment scheme proposed by Ma in 2022, and the principle of the method mainly used is shown in fig. 1 below.

In the figure, S1 and S2 are shots at the sea surface, and R is a detector at the sea surface. When the S1 shot point is excited, the seismic wave firstly propagates in the sea water in a P waveform mode, and after reaching the sea bottom, the seismic wave still propagates downwards in a transmission mode in a P waveform mode. After encountering the reflection interface, the reflection is carried out at the interface in the form of converted transverse waves, and finally the reflection reaches the seabed-seawater interface, so that the seismic waves continue to be converted and propagate upwards in the form of P waves. Finally, reflected signals are received by the shots at the sea surface, and the wave field composition of the signals comprises P-P-S-P waves. When the S2 shot is excited, the wave propagates in the sea and is reflected at the sea bottom, and the final signal is received by a detector, whose wave field composition comprises P-P waves.

When the method is used for processing, the signals excited by the S1 and the S2 seismic sources received by the detectors are subjected to cross-correlation operation, so that the time corresponding to the wave propagation path in water can be eliminated, and the virtual source PS wave record of which the shot point and the detector point are both positioned on the seabed is obtained. And (5) completing other shot point operations in sequence, and obtaining a complete virtual source record.

In the above mentioned prior art methods, the following disadvantages are mainly present:

(1) In the method, the shot points are all positioned at the sea surface and are suitable for marine streamer seismic data processing, and OBS and OBN data in marine seismic exploration cannot be processed, so that the method has certain limitation;

(2) During processing, it is not possible to determine whether a converted transverse wave exists in the data. In the observation system corresponding to the method, the received seismic signals usually mainly comprise longitudinal wave data, and the signals such as P-P-S-P are difficult to identify. The X and Y components of the detector may receive converted transverse wave data as compared to OBS and OBN surveys.

(3) As can be seen from the schematic diagram, the result obtained by the method is a single-channel virtual source record, a complete seismic record of a virtual source point cannot be obtained, and in the subsequent processing, the conventional processing flow cannot be used for processing. The obtained result of the existing method is still conventional PS wave, the transverse wave speed of the stratum cannot be directly obtained, and the method is consistent with the existing PS wave processing method in the subsequent data processing.

Based on the existing application method and the application of the combined seismic interferometry in marine longitudinal wave data processing, a new PS wave seismic interferometry is provided. The method principle is as shown in fig. 2:

in the figure, x and y are shots on the water surface, A, B, M and N are detectors on the sea bottom, P represents downward-traveling longitudinal waves, and S1 and S2 represent converted transverse waves. The method is divided into two steps during data processing.

The first step is to process the primary reflection and the direct wave by using an earthquake interferometry, and the purpose is to obtain a virtual source record of the shot points all located on the sea bottom. The cross-correlation operation is carried out on two data of xAOB and yOB in the graph, according to the principle, the time of a propagation path of seismic waves in water can be eliminated, and finally the seismic record with A as a virtual source and B as a received seismic record is obtained. The principle of the method is as follows:

And carrying out second-step operation on the basis of the first step. The points of the offset are all located on the sea floor. In the figure, A is a submarine detector, and after the first step of operation, the point A is also a virtual shot point in a virtual source record. The virtual source record at the point A comprises PS waves shown by AOB and PP waves shown by AON. A new processing method, pp+ps=ss, is used in the subsequent operations. The principle is that the virtual source record of the SS wave is obtained by combining the PP wave and the PS wave. The specific process flow comprises two parts. The first part is to perform interference processing on the AOB data and the AON data, and eliminate the incident wave part AO after the cross-correlation operation to obtain BN. In the first step of calculation, a record can be obtained with N as a virtual source point, and then, with N as a shot point, there is a PS wave NM received by M as a detector. And performing cross-correlation operation on BN and NM, eliminating a public path NO, and finally obtaining BM, wherein the result can be regarded as B as a virtual source record of which the virtual source point M is a detector. In the recording, it can be considered as the SS wave signal received by the point M, excited by the B-point transverse wave source. The principle formula of the method is as follows:

ω_ψS(t,M,B)＝∫∫[ω_PS(t,A,B)*ω_PS(t,N,M)*ω_PP(-t,A,N)]dAdN (2)

the above is a new PS wave interferometry proposed in the present invention. In contrast to the methods mentioned in the prior art, the method can be applied in OBS data processing or in high density OBS data processing.

Example 2:

the embodiment provides a PS wave interferometry processing flow:

in the process of processing, through understanding the principle, programming the related processing, the following processing flow is formulated, the data processing is realized, and the processing imaging is carried out on the obtained virtual source record. The process flow is shown in fig. 3.

The PS-wave interferometry process flow for OBS or OBN data is shown. The method processes the acquired X component and Z component data. Because in actual collection, the attitude of the detector can also change due to the influence of seawater flow when the detector is submerged, so that the detector is disturbed in recording, a vertical component can record a little transverse wave, and a horizontal detector can record Xu Zongbo. Therefore, it is necessary to perform longitudinal and transverse wave separation on the data of the two components, thereby obtaining PP waves and PS waves. Subsequently, seismic interferometry operations are performed on the data.

The operation of the seismic interferometry is mainly divided into two steps, namely, the direct wave, the PP wave and the PS wave are respectively operated by the interferometry to obtain virtual source PP wave and PS wave records with offset points all positioned on the sea bottom, and the aim of the step is to eliminate the influence of a water layer. And after all recording operations are completed, performing a second interferometry process. In this operation, the direct wave is not the processing target of this time, and only the reflected wave is processed. And selecting proper detectors and virtual source shots, performing two times of interferometry operation to finally obtain a single-channel virtual source record, thus completing other detectors, and finally obtaining a virtual source SS wave record corresponding to the data. Because the virtual source record shows that the transverse wave source excites the transverse wave to receive, the transverse wave can be processed by using a conventional longitudinal wave data processing flow, and finally the section of the virtual source SS wave is obtained.

Example 3:

before application to actual data, the feasibility of the method needs to be verified. It is therefore necessary to first process the model data. During the demonstration, the verification was performed by two simple small models. The two models correspond to the two processing steps, respectively. Wherein model 1 is shown in figure 4 below.

401 Cannons are arranged in the model, the cannon spacing is 12.5 meters, 201 detectors are arranged on the seabed, and the spacing is 12.5 meters. Two reflective interfaces are provided in total below the sea floor. In this model, the first step in the method, namely the processing using direct waves with PP and PS waves, is mainly demonstrated. When processing model data, the data is firstly subjected to longitudinal and transverse wave separation and then is subjected to interferometry processing. After the processing, the virtual source records with the shot points all located on the seabed are obtained, and the model corresponding to the virtual source records is changed into a model shown in fig. 5.

The model forward data in fig. 3 is first processed. Fig. 6 (a) is forward Z component data and (b) is X component data, and then the data is subjected to longitudinal and transverse wave separation and offset conversion, so that subsequent processing is facilitated. Fig. 7 (a) shows PP waves after separation, and (b) shows PS waves.

And picking up the direct wave from the separated data, and performing first interferometry operation. This step aims to obtain a record of the virtual source where the offset points are all located on the sea floor. Fig. 7 is the result of the first interferometry operation, and the forward result is compared with the model shown in fig. 4 above in order to better verify the accuracy of the method. Fig. 8 (a) is a PP wave virtual source record obtained after the first interferometry processing, and (b) is a PP wave record obtained after forward modeling and decomposition of the model of fig. 5. Fig. 9 (a) shows a PS-wave virtual source record obtained after the first interferometry treatment, and (b) shows a PS-wave record obtained after forward modeling and decomposition of the model of fig. 5. By comparing the two results, it can be found that the reflections of the two interfaces except the direct wave are basically consistent in time, so that the result of the first interferometry can be considered to be relatively accurate, and the principle of the method of the step is basically established.

In the proposed method, two seismic interferometry operations are required. The principle of the first-step interferometry has been demonstrated above, followed by the beginning of the second-step interferometry. To this end, a horizontal model is reconstructed, as shown in fig. 10. The number of shots 201, the number of detectors 201, the two positions are consistent, and the distance is 10 meters. The model simulates three reflective interfaces of four strata altogether, each stratum having a thickness of 1000 meters. The longitudinal wave velocity is 1800m/s, 2000m/s, 2200m/s and 2400m/s in turn from top to bottom, and the transverse wave velocity is 1000m/s, 1100m/s, 1200m/s and 1300m/s in turn.

After forward modeling, processing of the model data is started. Before the second interferometry operation, longitudinal and transverse wave field separation is performed first, and the direct wave is not needed in the operation, so that the direct wave also needs to be cut off. After the pre-treatment in the early stage, PP wave and PS wave required for calculation are obtained as shown in fig. 11.

And after the data is preprocessed in the earlier stage, starting a second interferometry operation, sequentially selecting different shots and detection points, and calculating by a two-step correlation method to finally obtain the SS wave virtual source record shown in fig. 12. The recording may be considered as the case where the excitation is performed using a transverse wave source and the detector receives transverse waves. In the calculation process, due to the influence of the ray paths, the generated virtual source records are limited, and only the virtual source records of the intermediate detector data can be obtained, and the situation is basically consistent with theory.

The reflection time of the middle channel in fig. 12 is sequentially 1.97s, 3.78s and 5.42s, the actual SS wave propagation time is calculated to be 2s, 3.81s and 5.47s according to the speed and the depth in the model, and the comparison shows that the time difference between the two is not more than 0.05s, and the result is relatively accurate. And selecting a proper range of the detectors, and completing the operation of the data received by the remaining detectors. And finally, performing conventional processing by using the virtual source SS wave record to obtain the superposition profile of the virtual source data. As shown in fig. 13. The structure in the section is basically consistent with the stratum in the model, the same phase axis is horizontal, and the depth corresponding to each reflection axis is obtained by calculating the propagation time and is also basically consistent with the model. From this, the principle of the second-step seismic interferometry is basically established.

From the demonstration of the two model results, the PS wave interferometry proposed in the present time can be basically established, and can be applied to the data processing of OBN or high-density OBS exploration. By using the method, not only is the influence of a water layer eliminated, but also the data of the X component and the Z component are applied, and finally the SS wave information of the area can be obtained quickly, so that a reference is provided for the follow-up fine processing explanation.

Example 4:

Based on the model data, the method provided by the invention is applied to the actually measured OBN data. During processing, a gun line and a receiving line are selected for processing. The total of 314 cannons are arranged in the cannon line, the average cannon distance is 32.4 meters, the total of 225 detectors are arranged in the receiving line, and the average cannon distance is 45.2 meters. The data processing mainly uses an X component and a Z component, and in the original record, the refractive wave energy is too strong, so that the refractive wave part in the record is firstly cut off, and the record is shown in fig. 14 and 15 after the cutting off. Then, the data is subjected to longitudinal and transverse wave separation to obtain PP waves and PS waves, and the separated data are used for data processing in the method as shown in figures 16 and 17.

Firstly, picking up a direct wave, and carrying out seismic interferometry processing on the direct wave, the separated PP wave and PS wave to obtain a virtual source record of which the offset points are all positioned on the sea floor after the first operation. As shown in fig. 18 and 19 below.

And then performing a second operation on the virtual source record obtained by the previous operation according to the model data processing flow to obtain the virtual source SS wave seismic record. As shown in fig. 20.

In the processing of the actual data, the wave field is more complex than the model data, and thus the signal-to-noise ratio in the resulting results is relatively low. In general, the method can be applied to data processing for marine OBN or dense OBS surveys.

It will be appreciated that the method referred to in this invention uses both the X component and the Z component data in processing. The X component mainly represents the transverse wave information of the stratum, so that compared with the existing method, the transverse wave field is clearer, and the processing result is more accurate. The method provided by the invention further improves the utilization rate of the data and can obtain more underground structure information.

It can be appreciated that in the data processing, in the virtual source record obtained by the processing of the method, the offset points are all positioned at the seabed, so that the influence of a sea water layer is eliminated, and the virtual source record can be processed by a conventional land seismic exploration data processing method. Meanwhile, compared with the conventional PS wave data processing, the virtual source record obtained by the method is an SS wave record, which represents the condition that the transverse wave source excites the transverse wave to receive, so that the PP wave data processing method can be directly used for processing and imaging. The obtained virtual source record can be processed to obtain the transverse wave speed of the work area by directly using a speed analysis method, and compared with the conventional PS wave data processing, the method saves a lot of time. When the data of the PS wave is processed conventionally, the positions of the common conversion points need to be iterated repeatedly, the speed of the iterated PS wave is continuously updated through the speed of the PP wave in the area, the time is relatively time-consuming, and the workload is huge. The method can be used for processing, so that the transverse wave speed of the work area can be simply, conveniently and quickly obtained, and the transverse wave section of the work area can be obtained, so that references can be provided for subsequent refinement processing.

While the preferred embodiments of the present invention have been described in detail, the present invention is not limited to the above embodiments, and various changes may be made without departing from the spirit of the present invention within the knowledge of those skilled in the art.

Many other changes and modifications may be made without departing from the spirit and scope of the invention. It is to be understood that the invention is not to be limited to the specific embodiments, but only by the scope of the appended claims.

Claims (3)

1. An OBN or OBS exploration transverse wave data conversion method based on a seismic interferometry method is characterized by comprising the following steps:

s1, performing longitudinal and transverse wave field separation on the acquired seismic data of the original X component and the Z component to obtain PP waves and PS waves;

s2, selecting a sea surface shot point and a submarine detector, and performing cross-correlation operation on direct waves and PP waves and PS waves received by the submarine detector by using a seismic interferometry to obtain virtual source PP wave and PS wave records of shot points on the sea bottom;

And S3, performing second interferometry treatment on the reflected wave based on the obtained virtual source record with the offset point at the seabed, wherein PP+PS=SS, namely combining the PP wave and the PS wave to obtain the virtual source record of the SS wave, and finally obtaining the section of the virtual source SS wave by using a conventional longitudinal wave data processing method.

2. The method for converting the OBS or OBS exploration shear wave data based on the seismic interferometry according to claim 1, wherein the step S2 specifically comprises:

Performing cross-correlation operation on xAOB and yOB data, eliminating the time of a propagation path of seismic waves in water, and finally obtaining a seismic record with A as a virtual source and B as a received seismic record;

The principle of the method is as follows:

wherein x and y are shots on the water surface, A, B are detectors on the sea floor.

3. The method for converting the OBS or OBS exploration shear wave data based on the seismic interferometry according to claim 1, wherein the step S3 specifically comprises:

The first step, carrying out interference processing on two data of the AOB and the AON, and eliminating an incident wave part AO after cross-correlation operation to obtain BN;

Step two, in the first step, obtaining a record with N as a virtual source point, then taking N as a shot point, taking M as a PS wave NM received by a detector, carrying out cross-correlation operation on BN and NM, eliminating a public path NO, and finally obtaining BM, wherein the result can be regarded as B as the virtual source record with the virtual source point M as the detector;

In the recording, the point B is excited by a transverse wave source, the point M is a received SS wave signal, and the principle formula of the method is shown as follows:

ω_ψS(t,M,B)＝∫∫[ω_PS(t,A,B)*ω_PS(t,N,M)*ω_PP(-t,A,N)]dAdN (2)

wherein M and N are submarine detectors.