WO2020033465A1 - Surface wave estimation and removal from seismic data - Google Patents

Abstract

The present method predicts and separates dispersive surface waves from seismic data using dispersion estimation and is completely data-driven and computer automated and no human intervention is needed. The method is capable of predicting and suppressing surface waves from recorded seismic data without damaging the reflections. Nonlinear signal comparison (NLSC) is used to obtain a high resolution and accurate dispersion. Based on the dispersion, surface waves are predicted from the field recorded seismic data. The predicted surface waves are then subtracted from the original data.

Description

SURFACE WAVE ESTIMATION AND REMOVAL FROM SEISMIC DATA

BACKGROUND

[0001] This application claims priority to U.S. Provisional Patent Application No. 62/717,456, entitled“Surface Wave Prediction and Removal from Seismic Data,” filed August 10, 2018, the entire contents of which are hereby incorporated by reference.

[0002] This disclosure pertains to seismic wave and data processing.

[0003] In seismic exploration of conventional and unconventional energy, a seismic source is used to create seismic waves that travel downward into the Earth and then bounce back by reservoir layers. These reflected waves carry important information about the subsurface oil and gas distribution as well as other natural resources. Unfortunately, the same seismic source generates not only reflected waves but also surface waves or ground rolls. The surface waves are viewed as noise in reflection seismic imaging and will degrade the quality of data.

[0004] Surface waves are energetic waves propagating along the Earth’s surface. They are characterized by slow but frequency-dependent propagation velocities. In seismic data processing, surface waves and body waves are usually processed differently. Surface waves are useful for probing medium properties at shallow depths. In the processing of body waves, surface waves are usually considered as coherent noises that are required to be removed. In recent years, seismologists use dense seismic arrays to investigate shallow crustal structure with high resolution. To utilize body waves, suppressing surface waves while maintaining the data quality of body waves is important, such as extracting body waves from ambient-noise data recorded by a dense seismic array. Various approaches have been explored to achieve this goal. These methods fall into four broad categories: frequency filtering, domain transform based methods, coherence-based methods, and other methods.

[0005] Surfaces waves usually have lower frequencies than reflections. One of the most widely used surface wave removal method is to apply a bandpass filter to remove low- frequency surface waves. A similar method is a time derivative filter which suppresses low frequencies and boosts high frequencies. However, some reflection events may have similar frequencies to those of the surface waves and filtering in the frequency domain may damage these reflection events. [0006] Because surface waves are usually characterized by slow apparent velocities and low frequencies, domain transformations were proposed to separate surface waves. These domain transformation methods include the frequency-wavenumber (f-k) transform, radial trace transform, tau-p transform, Radon transform, S transform, wavelet transform, sparse decomposition and local time-frequency decomposition. Many of these methods rely on regular receiver distribution which may not be the case for real data acquisition. These methods also frequently introduce aliasing and artifacts to the processed data.

[0007] Instead of suppressing surface waves in a different domain where they have different features compared to reflections, coherence-based methods aim to utilize the linear moveout to remove surface waves. These methods include interferometric removal, prediction error filters (PEF), plane-wave destruction filters, phase-matched filters, adaptive filters, and the Fourier correlation coefficient filter. Other alternative coherence-based methods try to flatten the reflections before removing the surface waves by various approaches. These approaches include the median filter, singular value decomposition (SVD), Karhunen-Loeve transformation, and coherence analyses. However, these coherence- based methods need auxiliary processing steps to estimate the moveout of reflections or surface waves. Additionally, they only perform effectively on nondispersive surface waves.

[0008] For spatially dense distribution of receivers, stacking can attenuate surface waves. However stacking may not be able to eliminate energetic surface waves recorded by sparsely distributed receivers. For multi-component seismic data, polarization analysis can suppress surface waves. When the dispersion information is available, dispersive surface waves can be un-dispersed into a narrow time window for better muting. However, muting compressed surface waves will still cause aliasing and introduce muting artifacts. If the near surface model is available, surface waves could be computed based on the model and then be suppressed by adaptive subtraction. It has been proposed to suppress surface waves and refractions using a wavefield extrapolation approach. This method performs well when the seismic data have a good acquisition coverage and the near surface velocity model is known. Neural network analysis was also proposed to detect and suppress surface waves. A morphological component analysis has also been used to detect and remove surface waves. These analyses require prior training to learn the pattern of surface waves in seismic data.

SUMMARY [0009] The present disclosure relates generally to processing seismic data. In particular, the present disclosure provides an approach for the extraction and suppression of surface waves while maintaining the quality of the useful reflection events.

[0010] Certain features of the present disclosure pertain to methods for processing seismic data recorded by dense seismic arrays for reflection seismology. The present methods are data-driven and involve first estimating high resolution surface wave phase velocities from recorded data using multi-channel nonlinear signal comparison (MNLSC). This enables the prediction of surface waves at each receiver location. Numerical prediction of the surface waves uses the recorded seismic traces based on the estimated phase velocities. The predicted surface waves are then subtracted or separated from the input seismic data.

[0011] Existing surface wave removal technologies commonly introduce fake signals which damage the other useful signals and have limited ability to handle irregular sampled data. The present methods include algorithms that can predict and remove surface waves using MNLSC, which can automatically produce reliable phase velocities of surface waves with high resolution. The methods are entirely data-driven. The present methods greatly remove surface waves without damaging other useful signals. The algorithms included in the present methods are flexible for use with irregular sampled data.

[0012] The present approach effectively predicts and separates surface waves from seismic data without damaging reflections.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 shows (a) a Multi-layered model with Vp (P- wave velocity), Vs (shear wave velocity) and density, (b) recorded vertical-component displacements of the multimode Rayleigh waves, (c) a dispersion map created using Nonlinear signal comparison (NLSC), and (d) a comparison of the original input surface waves and predicted surface waves.

[0014] FIG. 2 shows (a) a multi-layered model with Vp, Vs and density, (b) a full- wave elastic synthetic shot gather containing surface waves and other types of waves, (c) predicted surface waves including the fundamental mode and first overtone Rayleigh waves, and (d) the shot gather after removing the surface waves. [0015] FIG. 3 shows (a) a comparison of predicted surface waves and modeled surface waves for fundamental mode Rayleigh wave, (b) a comparison of predicted surface waves and theoretical surface waves for first overtone Rayleigh wave, and (c) a dispersion map created using NLSC.

[0016] FIG. 4 shows (a) a recorded raw seismic shot gather, (b) the shot gather after removing the surface waves, (c) the location of source and receivers, and (d) the offset for each trace, indicating a gap in the distribution of receivers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0017] The present disclosure relates to methods for processing seismic data to remove surface waves. Preferred embodiments include a method to predict and separate dispersive surface waves based on dispersion estimation that is completely data-driven. Nonlinear signal comparison (NLSC) is used to obtain a high resolution and accurate dispersion. Then based on the dispersion, surface waves are predicted from the input data using phase shift. The predicted surface waves are then subtracted from the original data.

[0018] Preferred embodiments described herein include the use of a dispersion measurement based on NLSC to estimate frequency-dependent phase velocities from seismic data. The dispersion measurement considers two time (t)-domain seismic traces,

and

, recorded by two geophones, the i-th and the j-th geophones. The distance between

X

the two geophones is . The high-resolution dispersion map is obtained based on the nonlinear signal comparison (NLSC) described as:

V^(m) S ^ where ^ and ^> ' ' are the frequency and phase-velocity, respectively; is the normalized dispersion map using the zth and /th traces; fT is a nonnegative parameter to control the resolution. As s ® the NLSC becomes the traditional crosscorrelation.

In the above equation, and are the unnormalized dispersion map and the reference value for normalization, respectively. They can be represented as:

where Io is the modified Bessel function of the zero-th order. d '_: and d^<, are two traces normalized by its variance, which can be represented as:

(T {J

where and are the variances of the data defined as:

where T is the length of the measured time window.

[0019] From the first equation above, the disc range is from 0 to 1. Under

t 00 ^L,ϋίΐ _wj]] reduce to the traditional

. The present

JvZc^sC ]_{ias a un}iform resolution over a wide band of frequencies and the resolution can be controlled by a single parameter T . To apply the above dispersion analysis on the active

^SL·

surface seismic data with multiple channels, ^ _s averaged from all possible pairs of receivers to obtain the final dispersion map.

[0020] Importantly, the present methods produce uniform high resolution dispersion at both low and high frequencies. The traditional cross-correlation based dispersion measurement technique is a special use of the NLSC method. The NLSC method allows for the estimation of phase velocities using algorithms by first picking the local maximum at each frequency. Once the phase velocities have been picked, the surface waves can be estimated at each receiver location using phase shift and local stacking. For simplification, it is assumed that the receivers are distributed along a line in the x-direction. The surface wave is predicted using:

where

is the predicted surface wave at the receiver located at x in the l _Q lix dx^ml x+dx, frequency domain; ' is the recorded seismic trace at location which includes both surface waves and body waves; L is a local spatial window size around the phase shift operator to correct for surface wave propagation

effect; is the estimated phase velocity from NLSC dispersion measurement; and

£3-

^! is a weighting factor that can be referred as the local wave reconstruction operation. Using the equation above, the surface waves can be predicted at each receiver location using its neighboring traces. Finally, the predicted surface waves are subtracted from the original data.

[0021] Preferred embodiments of the data-driven surface-wave removal method include three steps. First, extract surface wave phase velocities using NLSC technique. Second, predict the surface waves at each receiver location using the estimated phase velocities and neighboring traces from the original seismic data. Third, subtract the predicted surface waves from the original data.

EXAMPLE 1

[0022] This example utilized a synthetic seismic shot gather containing only surface waves. In this example, the near-surface velocity model (Xia et al. 1999) was used to show the performance of embodiments of the present method in the prediction of surface waves. FIG. 1(a) shows a Multi-layered model with Vp (P- wave velocity), Vs (shear wave velocity) and density. The synthetic seismic shot gather was generated using the method by Herrmann (2013), which allows for modeling of only surface waves. FIG. 1(b) shows recorded vertical-component displacements of the multimode Rayleigh waves. There were 46 traces recorded on the surface every 2 m between offsets from 10 m to 100 m. The source was an explosive source, located at a horizontal distance of 0 m and at a depth of 10 m. The recording time was 1.024 s, sampled at 1 ms. The frequency band was from 3 Hz to 70 Hz. The modeled seismic data contained multimode surface waves, including the fundamental mode and overtones. The dispersion analysis was performed using the NLSC. FIG. 1(c) shows the dispersion map using the traces in FIG. 1(b) via the NLSC (with s = 0.01). The three dash lines are the theoretical dispersion curves for the fundamental mode (mode 0), the first overtone (mode 1), and the second overtone (mode 2), respectively. The black dot at each frequency shows the picked local maximum of the dispersion map, which indicates the phase velocity. The black bar shows the range of points with values above 95% of the picked local maximum. From the dispersion map (FIG. 1(c)), phase velocities can be picked at different frequencies. Based on the measured phase velocities from the NLSC dispersion map, the surface waves were predicted using the equation above. The predicted surface waves were in excellent agreement with the recorded surface waves in both phase and amplitude. FIG. 1(d) shows a comparison of the original input surface waves and predicted surface waves. From this example, it was observed that that the present methods can predict the dispersive surface waves, including multiple modes, by using only the recorded data.

EXAMPLE 2

[0023] This example utilized a synthetic seismic shot gather using elastic full wavefield. The second synthetic data was modeled using the spectral element method (SEM) (e.g., Komatitsch and Vilotte 1998, Komatitsch and Tromp 2002) by solving the full elastic wave equation. The computational model and shot gathers are shown in FIG. 2(a) and 2(b), respectively. FIG. 2(a) shows a multi-layered model with Vp, Vs and density. At shallow depths, there were several thin layers to approximate a velocity gradient. At depths of 500 m and 1000 m, two strong reflective interfaces exist. In FIG. 2(b), both surface waves as well as reflected/refracted/converted waves are observed. FIG. 2(b) shows a full-wave elastic synthetic shot gather containing surface waves and other types of waves. FIG. 3(c) shows a dispersion map of the traces in FIG. 2(b) via the NLSC (with s = 0.002). The two solid lines are theoretical dispersion curves for the fundamental mode (mode 0) and the first overtone (mode 1), respectively. FIG. 2(c) shows the predicted surface waves including the fundamental mode and first overtone Rayleigh waves. The surface waves in FIG. 2(c) were predicted by using dispersion measured for both the fundamental mode and the first overtone (Zheng and Hu, 2017) then following the procedure described above. FIG. 2(d) shows the shot gather after removing the surface waves. There were 101 surface receivers from offset 500 m to 1500 m at 10 m spacing and a depth of 10 m. The recording time length was 5 s. The time sampling interval was 1 ms. The source was an explosive source with a 15 Hz Ricker wavelet, located at a horizontal distance of 300 m and a depth of 30 m. Two circles in FIG. 2(b) and 2(d) indicate where surface waves interfered with reflections for comparison. After removing the predicted surface waves (as in FIG. 2(d)), it is noted that reflection signals were not damaged by the present method, even at places where the reflections interfered with surface waves. Reflections become more visible after surface wave removal, which is noted by comparing the waveforms within the red circles in FIG. 2(b) and 2(d). [0024] To verify the fidelity of the predicted surface waves, surface waves were modeled for the fundamental mode and first overtone using the method by Herrmann (2013) and the results were compared to the surface waves predicted using the current methods. FIG. 3(a) shows a comparison of predicted surface waves and modeled surface waves for fundamental mode Rayleigh wave. FIG. 3(b) shows a comparison of predicted surface waves and theoretical surface waves for first overtone Rayleigh wave. From FIG. 3(a) and 3(b), it is noted that predicted surface waves well match the modeled surface waves in both the fundamental mode and the first overtone. From this full elastic synthetic data test, it is noted that the present method can correctly predict the surface waves from data containing other types of waves. Furthermore, the present method can remove surface waves but without damaging useful signals.

EXAMPLE 3

[0025] This example utilized field data, namely a field shot gather from a land acquisition survey. FIG. 4(a) shows the recorded raw seismic shot gather. FIG. 4(b) shows the shot gather after removing the surface waves. There were 35 traces with offsets from 356 m to 1529 m on the surface. The recording time was 1.5 s and the time sampling interval was 2 ms. The red ellipse indicates regions where surface waves interfered with the reflections. The distribution of trace has a gap at l7th trace. There was a lateral discontinuity in the surface wave moveout resulting from the irregular distribution of geophones. FIG. 4(c) shows the location of source and receivers. FIG. 4(d) shows the offset for each trace, indicating a gap in the distribution of receivers. FIG. 4(b) shows that the surface waves have been effectively removed (e.g., data within the circle). The surface wave removal algorithm can handle the irregularity because the surface wave removal method is locally performed. Because this method can effectively remove surface waves from field data with irregular geometry, it provides many potential benefits for processing real data.

[0026] The examples above show the successful application of the data-driven surface wave removal approach on three datasets, including two synthetic shot gathers and one field shot gather. All of these examples show that the present method is capable of predicting and suppressing surface waves from the data without damaging the reflections.

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Claims

WHAT IS CLAIMED IS:

1. A method for processing seismic data to remove interference from surface waves, comprising:

obtaining and recording input multi-channel seismic data from a plurality of receivers, wherein the input multi-channel seismic data comprises a plurality of recorded seismic traces at a plurality of locations near the receivers;

generating estimated surface wave phase velocities from the input multi channel seismic data using nonlinear signal comparison (NLSC);

generating predicted surface waves for each receiver by using the estimated surface wave phase velocities and performing a phase shift and local stacking analysis for seismic traces at locations near each receiver; and

subtracting the predicted surface waves from the input multi-channel seismic data to generate seismic data lacking interference from surface waves.

2. The method of claim 1, wherein the step of generating estimated surface wave phase velocities comprises using the following equation:

,

¾ {®) .

is phase- velocity,

|_{s a} normalized dispersion map using zth and /th traces,

: is a nonnegative parameter to control resolution,

S^,J

is an unnormalized dispersion map represented by:

wherein T is the length of the measured time window,

4 ‘ and are recorded seismic traces normalized by variance and represented as:

<T s

wherein ‘ and "^* are variances defined as:

is a reference value for normalization represented by:

b

wherein s ϋ¹T and Io is a modified Bessel function of zero- th order.

3. The method of claim 1, wherein the step of generating predicted surface waves for each receiver comprises using the following equation:

x; fi?)

wherein is the predicted surface wave at a receiver

{)!

located at x in frequency domain,

a

, wherein is a weighting factor and L is a local spatial window size around x,

Ct/f X i- 0x_<

is a recorded seismic trace at location

including surface waves and body waves,

phase shift operator to correct for surface wave propagation effect, and

n_^\w_.

is estimated surface wave phase velocity generated using nonlinear signal comparison (NLSC).

4. A method for predicting surface waves in seismic data to facilitate their removal from the seismic data, comprising:

obtaining and recording input multi-channel seismic data from a plurality of receivers, wherein the input multi-channel seismic data comprises a plurality of recorded seismic traces at a plurality of locations near the receivers;

generating estimated surface wave phase velocities from the input multi channel seismic data using nonlinear signal comparison (NLSC); and

generating predicted surface waves for each receiver by using the estimated surface wave phase velocities and performing a phase shift and local stacking analysis for seismic traces at locations near each receiver.

5. The method of claim 4, wherein the step of generating estimated surface wave phase velocities comprises using the following equation:

wherein

frequency, phase- velocity,

s a normalized dispersion map using z^'th and /th traces, s is a nonnegative parameter to control resolution,

S'^J

is an unnormalized dispersion map represented by:

wherein T is the length of the measured time window, d ¾

’ and are recorded seismic traces normalized by variance and represented as:

<J_: s

wherein and are variances defined as:

¹ is a reference value for normalization represented by:

wherein

and Io is a modified Bessel function of zero-th order.

6. The method of claim 4, wherein the step of generating predicted surface waves for each receiver comprises using the following equation:

u

wherein s‘u_^n \x:a

is the predicted surface wave at a receiver

&

located at x in frequency domain,

, wherein <¾ ' is a weighting factor and L is a local spatial window size around x,

is a recorded seismic trace at location x÷ax including surface waves and body waves,

phase shift operator to correct for surface wave propagation effect, and

is estimated surface wave phase velocity generated using nonlinear signal comparison (NLSC).