Multicomponent seismic recording (measurement with vertical-and horizontal-component geophones and possibly a hydrophone or microphone) captures the seismic wavefield more completely than conventional single-element techniques. In the last several years, multicomponent surveying has developed rapidly, allowing creation of converted-wave or P-S images. These make use of downgoing P-waves that convert on reflection at their deepest point of penetration to upcoming S-waves. Survey design for acquiring P-S data is similar to that for P-waves, but must take into account subsurface V P /V S values and the asymmetric P-S ray path. P-S surveys use conventional sources, but require several times more recording channels per receiving location. Some special processes for P-S analysis include anisotropic rotations, S-wave receiver statics, asymmetric and anisotropic binning, nonhyperbolic velocity analysis and NMO correction, P-S to P-P time transformation, P-S dip moveout, prestack migration with two velocities and wavefields, and stacking velocity and reflectivity inversion for S-wave velocities.Current P-S sections are approaching (and in some cases exceeding) the quality of conventional P-P seismic data. Interpretation of P-S sections uses full elastic ray tracing, synthetic seismograms, correlation with P-wave sections, and depth migration. Development of the P-S method has taken about 20 years, but has now become commercially viable.

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Converted seismic waves (P-to-S on reflection) are being increasingly used to explore for subsurface targets. Rapid advancements in multicomponent acquisition methods and processing techniques have led to numerous applications for P-S images. Uses that have arisen include sand/shale differentiation, carbonate identification, definition of interfaces with low P-wave contrast, anisotropy analysis, imaging through gas zones, shallow high-resolution imaging, and reservoir monitoring. Marine converted-wave analysis using 4-C recordings (a three-component geophone plus a hydrophone) has generated some remarkable images. BACKGROUND

Vertical seismic profiling (VSP) techniques provide a method to measure accurately the seismic velocity and lithologic structure near the borehole. The analysis of a VSP survey can also provide insight into seismic‐wave propagation especially when related to sonic measurements. But VSP and sonic log velocities (or traveltimes) are often found to disagree. Recent field evidence of these differences suggests that the VSP traveltimes are delayed with respect to the integrated sonic times, especially in the deep section (>3000 ft), by about 2.0 ms/1000 ft on the average. The VSP has numerous applications in exploration geophysics, such as calibrating the sonic log. It is thus important to understand why the two measurements differ. Differences in the geometries, source frequencies, and instrumental errors of the two surveys are reviewed. More detailed analysis of seismic wave propagation in the VSP shows that short‐path multiples and velocity dispersion can have a significant delaying effect on the seismic traveltimes. One‐dimensional, wide‐band VSP synthetic seismograms are generated in the frequency domain to study these effects. Different parameters (bandwidth, signal‐to‐noise, layer thickness, multiples, attenuation, dispersion) are varied in the synthetic seismograms. A comparative display of synthetic VSP traveltime minus the integrated sonic time is used to view the effects of these parameters on the synthetic traces. Reasonable variation in noise, layer thickness, bandwidth, and picking method have a small effect on traveltimes. Field data from the Anadarko Basin (4 wells) and an East Texas well are examined with the same technique. From the modeling and field examples, it is found that short‐path multiples can cause a seismic pulse delay of up to 2.0 ms/1000 ft with respect to the integrated sonic log in highly cyclically stratified sections. Velocity dispersion associated with attenuation can have a larger effect, causing up to 7.0 ms/1000 ft delay of the VSP traveltimes with respect to the integrated sonic. These wave propagation effects can explain the observed discrepancy between VSP and integrated sonic times in the deep section.

S U M M A R YSeismic migration and inversion are closely related processes for obtaining images of the subsurface. Both techniques attempt to infer petrophysical and structural parameters from seismic data, and both are driven by an underlying mathematical model for wave propagation in the earth. In this sense, migration can be regarded as the first step in' a general linearized-inversion scheme; we adopt this viewpoint here in formulating a joint migration/inversion method for anisotropic elastic media. Our derivation is based on the distorted-wave Born approximation, where approximate Green's functions for the background medium are determined using asymptotic ray theory. We also make use of a stationary-phase correction to account for out-of-plane scattering in a 2-D earth model. The inversion is cast as a discrete, generalized l2 optimization problem, which we regularize using a priori model variances. An approximate solution is obtained in one or more iterations using a quasi-Newton technique. Our implementation is tailored for weakly anisotropic elastic media possessing transversely isotropic (TI) symmetry, and so is well suited for investigations of earth models characterized by a singe set of parallel fractures or periodic thin layering.Several TI model parametrization schemes are evaluated for a vertical-incidence narrow-aperture recording configuration. By applying singular-value decomposition to the approximate Hessian operator used in the inversion, we find that when the symmetry axis of the medium is vertically oriented the condition number is minimized by choosing vertical impedance parameters, density and Thomsen's (1986) anisotropy parameters. Elastic stiffnesses and density may be a better parametrization choice if the symmetry axis is either horizontal or unknown. The small effective rank (2-3) of the Hessian in our narrow-aperture tests implies that migration inversion (without a priori constraints) of surface-reflection data may be inadequate to fully characterize a TI medium, which would require six parameters. However, this result does not in itself justify the use of simpler isotropic inversion schemes, since the principal eigenvectors determined here represent a blend of 'isotropic' and 'anisotropic' information about the earth.A suite of point-diffractor tests shows that individual perturbation amplitudes recovered in the inversion are smaller, but cover a larger region of the image, than those used in the original model, consistent with the band-limited nature of the input signal. Parameter cross-coupling problems affect the inversion results for all parameters, but are more prominent in the inversion images for anisotropy parameters. In our tests, the linearized-inversion results converge to a local minimum after three iterations, and the scatterer distributions in the inversion images are essentially fixed after a single iteration.

A tomographic technique (traveltime inversion) has been developed to obtain a two‐ or three‐dimensional velocity structure of the subsurface from well logs, vertical seismic profiles (VSP), and surface seismic measurements. The earth was modeled by continuous curved interfaces (polynomial or sinusoidal series), separating regions of constant velocity or transversely isotropic velocity. Ray tracing for each seismic source‐receiver pair was performed by solving a system of nonlinear equations which satisfy the generalized Snell’s law. Surface‐to‐borehole and surface‐to‐surface rays were included. A damped least‐squares formulation provided the updating of the earth model by minimizing the difference between the traveltimes picked from the real data and calculated traveltimes. Synthetic results indicated the following conclusions. For noise‐free cases, the inversion converged closely from the initial guess to the true model for either surface or VSP data. Adding random noise to the observations and performing the inversion indicated that (1) using surface data alone allows reconstruction of the broad velocity structure but with some inaccuracy; (2) using VSP data alone gives a very accurate but laterally limited velocity structure; and (3) the integration of both data sets produces a more laterally extensive, accurate image of the subsurface. Finally, a field example illustrates the viability of the method to construct a velocity structure from real data.

The devastating 2010 Haiti earthquake (M w 7.0) was caused by rupture of the Léogâne, blind, thrust fault located 5 km north of the 1,200-km-long, left-lateral, Enriquillo-Plantain Garden fault zone (EPGFZ). Unexpectedly, the EPGFZ remained largely quiescent or slightly reactivated during the 2010 earthquake. Nevertheless, the EPGFZ still formed a major, crustal boundary between a coseismically uplifted lowland north of the EPGFZ and a subsided area in the highlands south of the fault.Here we use high-resolution sonar data from two Haitian Lakes that straddle the EPGFZ to demonstrate the presence of a 10-to 15-km-wide, 120-km-long, late Holocene fold-thrust belt which deforms clastic, lowland basins along the northern edge of the EPGFZ. In the eastern part of the study area, sonar results from Lake Azuey show that the linear trace of the EPGFZ cutting the Holocene lake bed is more deeply buried and less active than the adjacent, newly discovered, northwest striking, northeast dipping Jimani thrust fault that is part of the adjacent, transpressional belt of en echelon thrusts and folds. This structural relationship between a less active EPGFZ and more recently active, transpression-related Jimani thrust is remarkably similar to the 2010 epicentral area 70 km to the west between the less active EPGFZ and seismogenic, northeast-dipping, Léogâne thrust during the 2010 Haiti earthquake. In this complex transpressional zone, we propose that coseismic deformation alternates at recurrence intervals of centuries between oblique, transpression-related structures (Léogâne, Jimani, and Trois Baies thrusts) and the main strike-slip, plate boundary fault zone (EPGFZ).

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