Xander Campman

Tutorial on seismic interferometry: Part 1 — Basic principles and applications

Seismic interferometry involves the crosscorrelation of responses at different receivers to obtain the Green’s function between these receivers. For the simple situation of an impulsive plane wave propagating along the x-axis, the crosscorrelation of the responses at two receivers along the x-axis gives the Green’s function of the direct wave between these receivers. When the source function of the plane wave is a transientas in exploration seismology or a noise signalas in passive seismology, then the crosscorrelation gives the Green’s function, convolved with the autocorrelation of the source function. Direct-wave interferometry also holds for 2D and 3D situations, assuming the receivers are surrounded by a uniform distribution of sources. In this case, the main contributions to the retrieved direct wave between the receivers come from sources in Fresnel zones around stationary points. The main application of direct-wave interferometry is the retrieval of seismic surface-wave responses from ambient noise and the subsequent tomographic determination of the surfacewave velocity distribution of the subsurface. Seismic interferometry is not restricted to retrieving direct waves between receivers. In a classic paper, Claerbout shows that the autocorrelation of the transmission response of a layered medium gives the plane-wave reflection response of that medium. This is essentially 1D reflected-wave interferometry. Similarly, the crosscorrelation of the transmission responses, observed at two receivers, of an arbitrary inhomogeneous medium gives the 3D reflection response of that medium. One of the main applications of reflected-wave interferometry is retrieving the seismic reflection response from ambient noise and imaging of the reflectors in the subsurface. A common aspect of direct- and reflected-wave interferometry is that virtual sources are created at positions where there are only receivers without requiring knowledge of the subsurface medium parameters or of the positions of the actual sources.

Reflection images from ambient seismic noise

One application of seismic interferometry is to retrieve the impulse response (Greens function) from crosscorrelation of ambient seismic noise. Various researchers show results for retrieving the surface-wave part of the Greens function. However, reflection retrieval has proven more challenging. We crosscorrelate ambient seismic noise, recorded along eight parallel lines in the Sirte basin east of Ajdabeya, Libya, to obtain shot gathers that contain reflections. We take advantage of geophone groups to suppress part of the undesired surface-wave noise and apply frequency-wavenumber filtering before crosscorrelation to suppress surface waves further. After comparing the retrieved results with data from an active seismic exploration survey along the same lines, we use the retrieved reflection data to obtain a migrated reflection image of the subsurface.

A Novel Application of Time Reversed Acoustics: Salt Dome Flank Imaging Using Walkaway VSP surveys

In this paper we present initial results of applying Time-Reversed Acoustics (TRA) technology to saltdome flank, seismic imaging. We created a set of synthetic traces representing a multilevel, walkaway VSP for a model composed of a simplified Gulf of Mexico vertical-velocity gradient and an embedded salt dome. We first applied the concepts of TRA to the synthetic traces to create a set of redatummed traces without having to perform velocity analysis, moveout corrections, or complicated processing. Each redatummed trace approximates the output of a zero-offset, downhole source and receiver pair. To produce the final salt-dome flank image, we then applied conventional, poststack, depth migration to the zero-offset section. Our results show a very good image of the salt when compared to an image derived using data from a downhole, zero-offset source and receiver pairs. The simplicity of our TRA implementation provides a virtually automated method to estimate a zero-offset, seismic section as if it had been collected from the reference frame of the borehole containing the VSP survey.

Imaging and suppressing near-receiver scattered surface waves

When traveling through a complex overburden, upcoming seismic body waves can be disturbed by scattering from local heterogeneities. Currently, surface-consistent static and amplitude corrections correct for rapid variations in arrival times and amplitudes of a reflector, but these methods impose strong assumptions on the near-surface model. Observations on synthetic and laboratory experiments of near-surface scattering with densely sampled data suggest that removing noise from near-receiver scattering requires multichannel approaches rather than single-channel, near-surface corrections. In this paper we develop a wavefield-based imaging method to suppress surface waves scattered directly beneath the receivers. Using an integral-equation formulation, we account for near-surface heterogeneities by a surface impedance function. This impedance function is used to model scattered surface waves, excited by upcoming wavefronts. The final step in our algorithm is to subtract the scattered surface waves. We succes...

Redatuming through a salt canopy and target-oriented salt-flank imaging

We describe a new shortcut strategy for imaging the sediments and salt edge around a salt flank through an overburden salt canopy. We tested its performance and capabilities on 2D synthetic acoustic seismic data from a Gulf of Mexico style model. We first redatumed surface shots, using seismic interferometry, from a walkaway vertical seismic profile survey as if the source and receiver pairs had been located in the borehole at the positions of the receivers. This process creates effective downhole shot gathers by completely moving surface shots through the salt canopy, without any knowledge of overburden velocity structure. After redatuming, we can apply multiple passes of prestack migration from the reference datum of the bore-hole. In our example, first-pass migration, using only a simple vertical velocity gradient model, reveals the outline of the salt edge. A second pass of reverse-time, prestack depth migration using full two-way wave equation was performed with an updated velocity model that consisted of the velocity gradient and salt dome. The second-pass migration brings out dipping sediments abutting the salt flank because these reflectors were illuminated by energy that bounced off the salt flank, forming prismatic reflections. In this target-oriented strategy, the computationally fast redatuming process eliminates the need for the traditional complex process of velocity estimation, model building, and iterative depth migration to remove effects of the salt canopy and surrounding overburden. This might allow this strategy to be used in the field in near real time.

Imaging scattered seismic surface waves

Surface-wave analysis is a key tool for seismologists, ranging from near-surface characterization in geotechnical applications to global seismology. Even in exploration seismology, where surface waves are regarded as a kind of noise, the fact that they typically represent the bulk of the recorded energy makes an understanding of surface-wave propagation important. On the other hand, the heterogeneity of the near surface can make such analyses difficult since the heterogeneity is responsible for scattering and mode conversion. Here, we show how multichannel seismic records of scattered surface waves can be used to obtain spatial images of the heterogeneity. We discuss both data processing and imaging and illustrate our method on laboratory-scale data. Further, synthetic examples show that we can locate individual scatterers accurately, even when many scatterers produce interfering surface waves. Our laboratory results show that the method has the potential to locate near-surface heterogeneities in the field.

Characteristics of Seismic Noise: Fundamental and Higher Mode Energy Observed in the Northeast of the Netherlands

We study seismic noise recorded in the northeast of the Netherlands by beamforming and by using empirical Greens functions obtained by seismic interfero- metry. From beamforming we found differences in noise directions in different fre- quency bands. The main source region for primary microseisms (0.05-0.08 Hz) is in the west-northwest direction, while the secondary microseisms (0.1-0.14 Hz) have a west-southwest back azimuth. Furthermore, we observed a fast (∼4 km=s) arrival corresponding to the Rayleigh wave overtone. This arrival is also in the secondary microseism band (between 0.15 and 0.2 Hz), but has a west-northwest back azimuth. Both arrivals in the secondary microseism band gain in strength during winter, as does the average wave height in the North Atlantic. We measured phase velocity dispersion curves from both beamforming and noise cross-correlations, as well as group velocity from the latter. These are then jointly inverted for an average 1D S-wave model. The results show how the combination of different methods leads to a more complete char- acterization of the propagation modes and an improved knowledge of the subsurface, especially as the group velocity measurements increase the upper frequency limit of analysis, providing valuable information of the shallowest subsurface.

Suppressing near-receiver scattered waves from seismic land data

Upgoing body waves that travel through a heterogeneous near-surface region can excite scattered waves. When the scatteringtakesplaceclosetothereceivers,secondarywaves interferewiththeupcomingreflections,diminishingthecontinuity of the wavefront. We estimate a near-surface scattering distribution from a subset of a data record and use this scattering distribution to predict the secondary waves of the entire data record with a wave-theoretical model for near-receiver scattering. We then subtract the predicted scattered waves from the record to obtain the wavefield that would have been measured in the absence of near-surface heterogeneities. We apply this method to part of a field data set acquired in an area with significant near-surface heterogeneity. Themainresultofourprocessingschemeisthatweeffectively remove near-surface scattered waves. This, in turn, increases trace-to-trace coherence of reflection events. Moreover,applicationofourmethodimprovestheresultsobtained from just an application of a dip filter because we remove parts of the scattered wave with apparent velocities that are typically accepted by the pass zone of the dipfilter. Based on these results, we conclude that our method for suppressing near-receiver scattered waves works well on densely sampled land data collected in areas with strong near-surface heterogeneity.

Basin Delineation with a 40‐Hour Passive Seismic Record

Several geophysical methods exist to delineate the lower interface of a sedimentary basin. Most popularly employed are gravity and magnetic surveys and surface-wave inversion. While all three methods are successful overall in estimating an average basin depth, they fail to find a more detailed depth variation. As an alter- native, we consider three passive seismic techniques, using especially body waves. We analyze 40 hours of data, recorded with 110 stations installed over the Abu Gharadig basin in Egypt. In an earlier study we found the frequency band of 0.09-1.0 Hz to be dominated by body waves. As a first method we apply body-wave seismic interfero- metry (SI). Using body-wave noise, we extract PP and SS reflections from the basin floor. We estimate the depth of the basin to be around 4.8 km. As a second technique we estimate the resonance spectra of the basin, using the horizontal-to-vertical (H/V) spectral ratio. Using surface-wave noise, we find an extremum that is probably related to the complete sedimentary package. Using this peak, we find a basin depth of 5.4 km. Using S-phase arrivals, we find two extrema in the H/V, which are probably related to the S-wave resonances of two distinct layers in the basin. As a third method we compute receiver functions (RFs). Based on the RFs, we can confirm the presence of a large interface in the upper crust, but we cannot well constrain its depth.

Imaging dipping sediments at a salt dome flank - VSP seismic interferometry and reverse-time migration

Summary We present results of applying seismic interferometry to image dipping sediments abutting a salt dome. We create a set of synthetic traces representing a multi-level, walk away Vertical Seismic Profile (VSP) for a model composed of a simplified Gulf of Mexico vertical-velocity gradient and an embedded overhanging salt dome. The sediment reflectors in the model dip up towards the salt dome flank. To process these data, we create a set of redatummed traces using seismic interferometry. This is done without having to perform any velocity analysis or moveout corrections. Each of these redatummed traces mimics the output of a downhole source and down-hole receiver pair. The linear v(z) gradient enables the redattumed data set to illuminate and capture reflections from both the salt-dome flank and the upward turning sediments. We then apply pre-stack depth migration to these traces to produce the final image of the beds and the salt dome flank. The final migrated results demonstrate that the reflected turning ray energy from both the salt flank and sediments are adequate to create structurally correct images using the combination of seismic interferometry and prestack depth migration.