Hongchuan Sun

2‐D wavepath migration

Prestack Kirchhoff migration (KM) is computationally intensive for iterative velocity analysis. This is partly because each time sample in a trace must be smeared along a quasi‐ellipsoid in the model. As a less costly alternative, we use the stationary phase approximation to the KM integral so that the time sample is smeared along a small Fresnel zone portion of the quasi‐ellipsoid. This is equivalent to smearing the time samples in a trace over a 1.5‐D fat ray (i.e., wavepath), so we call this “wavepath migration” (WM). This compares to standard KM, which smears the energy in a trace along a 3‐D volume of quasi‐concentric ellipsoids. In principle, single trace migration with WM has a computational count of O(N1.5) compared to KM, which has a computational count of O(N3), where N is the number of grid points along one side of a cubic velocity model. Our results with poststack data show that WM produces an image that in some places contains fewer migration artifacts and is about as well resolved as the KM ...

Separation of signal and coherent noise by migration filtering

A key issue in wavefield separation is to find a domain where the signal and coherent noise are well separated from one another. A new wavefield separation algorithm, called migration filtering, separates data arrivals according to their path of propagation and their actual moveout characteristics. This is accomplished by using forward modeling operators to compute the signal and the coherent noise arrivals. A linearized least‐squares inversion scheme yields model estimates for both components; the predicted signal component is constructed by forward modeling the signal model estimate. Synthetic and field data examples demonstrate that migration filtering improves separation of P-wave reflections and surface waves, P-wave reflections and tube waves, P-wave diffractions, and S-wave diffractions. The main benefits of the migration filtering method compared to conventional filtering methods are better wavefield separation capability, the capability of mixing any two conventional transforms for wavefield sepa...

Three-Dimensional Velocity Images of the Crust and Upper Mantle beneath the North-South Zone in China

We have determined the three-dimensional P-wave velocity structure beneath the north-south tectonic belt between Tibet and Eastern China by simulta- neously inverting local, regional, and teleseismic data. Our data set is composed of 45,028 P-wave arrival times from 602 local and regional earthquakes and 985 travel times from 102 teleseismic events. With the LSQR (sparse linear equations and least squares) algorithm, the P-wave velocity perturbations were estimated by the simul- taneous inversion of hypocenters and medium parameters from the surface to a depth of 200 km. We tested the stability and the resolution of our inverted results with a checkerboard test and found that the models are well resolved up to a depth of about 50 km for most parts of the studied region. Results show that the north-south tectonic belt is characterized by a significant lateral heterogeneity in velocity both in the crust and in the upper mantle. Correlating these velocity images with the main tectonic features, we find that (1) the shallow velocity distribution above 3 km is consistent with the topographic features and the basin distribution; (2) the middle-lower crustal velocities from 20 km to 50 km characterize a mechanically weak north-south tec- tonic belt, because it bears a relatively lower-velocity perturbation over a large re- gion; (3) the upper mantle velocities from 85 km to 120 km delineate the eastern Tibetan boundary, but changes in some subzones may reflect the effects of several tectonic events, including paleorifting, the Cenozoic convergence between Tibet and southeastern China and other tectonic episodes.

Lower Velocities beneath the Taihang Mountains, Northeastern China

We used regional P -wave arrival times to invert for 3D velocity structures beneath the Taihang Mountains, which are bordered by the intensely active zones of the Shanxi rift to the west and the eastern plain basin to the east in the North China Block. P -wave velocities show that low velocities (−1.0 to −3.0%) are focused at depths of ∼15–20 km beneath the Taihang Mountains, similar to those beneath the active zones of the Shanxi rift and the eastern plain basin, If the lower velocity is representative of weakness in the underlying crust, there our results indicate that the Taihang Mountains might be potentially active, because the structure patterns are similar to their bounding active zones. Future seismic research, therefore, should strongly focus on the Taihang Mountains.

Reverse Time Migration of Multiples

Multiple reflections have different wave propagation paths from primary reflections and thus can be used to complement the illumination where primary reflections from beneath the salt are not available. We propose to modify conventional reverse time migration (RTM) so that multiples can be used as constructive reflection energy for subsalt imaging. This new approach replaces the impulsive source wavelet with the recorded data containing both primaries and multiples and uses predicted multiples as the input data. In the RTM process, multiples recorded on the surface are extrapolated backward in time to each depth level, and the observed data with both primaries and multiples are extrapolated forward in time to the same depth level, followed by a cross-correlation imaging condition. A numerical test on the Sigsbee2B dataset shows that a wider coverage and a more balanced illumination of the subsurface area can be achieved by migration of multiples compared with conventional migration of primary reflections.

Autocorrelogram Migration: IVSPWD test

We present the results of applying autocorrelogram migration to inverse vertical seismic‐while‐drilling (IVSPWD) profile data collected in the Austin Chalk formation. The seismic reflections were excited by a drill bit moving along a horizontal well at a depth of 2800 m. The data were recorded by a receiver array on the surface. There were 609 twenty‐second traces recorded at each of 10 three‐component stations. After preprocessing, the traces were autocorrelated and migrated. Two imaging conditions were examined. The ghost‐reflection imaging condition produced a reflectivity section that agreed with a nearby common‐depth‐point section. The migration section obtained with a primary‐reflection imaging condition produced a reflectivity section inferior in quality to that of the ghost image. A possible explanation is that the drill‐bit location was not precisely known, which can be shown to induce weak errors in the ghost‐imaging condition but stronger errors in the primary‐reflection imaging condition. Ghos...

Multiple subtraction using statistically estimated inverse wavelets

Surface-related multiple elimination (SRME) typically consists of two steps: The first step is prediction and the second step is subtraction. In subtraction, it is important to effectively attenuate multiple events and preserve primary events. When multiples cross with or overlap on primaries, least-square subtraction usually cannot subtract multiples effectively and may also damage the primaries. When multiples overlap with primaries, least-square subtraction cannot always subtract multiples accurately and often damages the primaries. To remedy this problem, we propose to statistically estimate the inverse source wavelet, correct for errors in the estimate of the inverse wavelet, and then use the corrected inverse wavelets for multiple subtraction. Synthetic tests and real data examples show that the proposed method can effectively attenuate multiples, while they also preserve the continuity of reflection events and successfully avoid amplitude distortion. The proposed method is characterized by low computational costs and ease of implementation.

The application of primary‐only imaging condition to SMAART data

The primary only imaging condition (POIC) is tested on 2-D SMAART JV data associated with a complicated velocity model. This data set is one of the most difficult for any demultiple algorithm. Results indicate that POIC is effective in attenuating some free surface and peg-leg multiples. Images show effective attenuation of strong multiple energy, but, POIC does not work well for multiples with weak energy such as interbed multiples and the multiples associated with the dipping bottom boundary of the salt bodies. The next step is to adjust POIC to handle these weaker events.

Crosswell imaging by 2‐D prestack wavepath migration

Prestack wavepath migration (WM) is applied to 2-D synthetic crosswell data, and the migrated images are compared to those from constrained prestack Kirchhoff migration (KM). Preliminary results show that WM can effectively locate the fault boundary, and slightly improve the image resolution compared to the KM method. However, both the KM and WM images are suitable for interpretation. It is shown that the WM images contain fewer migration artifacts, but the signal strength in the WM images is typically weaker than that in the KM images.