Youshun Sun

Adaptive Moving Window Method for 3D P-Velocity Tomography and Its Application in China

A new tomography method, the adaptive moving window, is introduced and applied to construct the velocity structure of the crust and upper mantle of China and surrounding areas. More than 345,000 high-quality compressional body-wave phase data extracted from the Annual Bulletin of Chinese Earthquakes spanning from 1990 to 1998 are used. The area of interest is represented horizontally by 2338 points with 1° intervals. Each point is assigned a window (a cell or a region centered at each point) whose size is varied depending upon the ray path density. A five-layer 1D model from surface down to uppermost mantle is then determined at each point by performing a Monte Carlo random search where earthquake locations are held constant. Combining and smoothing the obtained 1D models, an equivalent 3D model is achieved. The predicted travel times through the 3D model match very well with the observed ones from local to regional distances. The model has a good correlation with tectonic features and is generally consistent with the existing models constructed by other researchers. Our model gives detailed information about structure and is feasible for application to high-quality earthquake location problems. Manuscript received 24 June 2003.

ML Amplitude Tomography in North China

We have selected 10,899 M L amplitude readings from 1732 events re- corded by 91 stations, as reported in the Annual Bulletin of Chinese Earthquakes (ABCE), and have used tomographic imaging to estimate the lateral variations of the quality factor Q0 (Q at 1 Hz) within the crust of Northern China. Estimated Q0 values vary from 115 to 715 with an average of 415. Q0 values are consistent with tectonic and topographic structure in Eastern China. Q0 is low in the active tectonic regions having many faults, such as the Shanxi and Yinchuan Grabens, Bohai Bay, and Tanlu Fault Zone, and is high in the stable Ordos Craton. Q0 values are low in several topographically low-lying areas, such as the North China, Taikang-Hefei, Jianghan, Subei-Yellow Sea, and Songliao basins, whereas it is high in mountainous and uplift regions exhibiting surface expressions of crystalline basement rocks: the Yinshan, Yanshan, Taihang, Qinlin, Dabie and Wuyi Mountains, and Luxi and Jiaoliao Uplifts. Quality-factor estimates are also consistent with Pn- and Sn-velocity patterns. High- velocity values, in general, correspond with high Q0 and low-velocity values with low Q0. This is consistent with a common temperature influence in the crust and uppermost mantle.

Imaging Poisson's Ratio of the Uppermost Mantle beneath China

We have obtained a Poissons ratio image of the uppermost mantle beneath China by performing tomographic inversion of travel-time differences between Sn and Pn. The arrival pairs were selected from the Annual Bulletin of Chinese Earthquakes from 1985 to 2007 and from the International Seismological Center (ISC) data set between 1985 and 2005. The data include 58,663 arrival pairs from 10,486 earthquakes recorded at 204 stations. The average Poissons ratio is 0.266. The preliminary tomographic results show that (1) the pseudowave velocity is high and the velocity ratio (V(P)/V(S)) and Poissons ratio are low in the stable cratons around the Tibetan Plateau such as the Tarim and Junggar basins, the Ordos craton, and the southern region of the Sichuan basin; (2) a low pseudowave velocity and high velocity ratio and Poissons ratio exist in the central and northern Tibetan Plateau, the North-South Seismic Zone, and north China; and (3) the high velocity ratio and Poissons ratio region in the Tibetan Plateau extends to the surrounding cratons, suggesting that the uplift of the Tibetan Plateau results from a high Poissons ratio or partially melted rocks beneath the plateau and that the softer rocks have intruded into the upper mantle of surrounding cratons.

Pn Tomographic Velocity and Anisotropy beneath the Iran Region

We present tomographic velocity and anisotropy models of the uppermost mantle beneath the Iran region. A total of 74,375 Pn phase readings from 86 stations of the Iranian network and 133 stations of the International Seismological Centre are used in the investigation. The study uses the Pn travel‐time tomography method proposed by Hearn (1996). The tomography results show some interesting anomalies. The average Pn velocity under the Iran region is approximately 8.0  km/s, and the maximum velocity perturbations are approximately 3%–4%. High Pn velocities under the Zagros Mountains and the Caspian Sea may be due to the presence of oceanic crust/lithosphere material. Low Pn velocities were found under the Alborz and Caucasus regions and may be due to higher temperatures or partial melting resulting from volcanoes and mid‐Cenozoic volcanic/plutonic rocks in these regions. The inversion velocities support the idea that the subduction of the Arabian plate into the mantle beneath the Iranian plateau may have resulted in the upwelling of hot material. The well‐resolved Pn anisotropy model is jointly obtained with a velocity model for the areas with good ray‐path coverage. In the plate collision regions (Zagros and Alborz), the fast Pn anisotropy direction is oriented parallel to the collision arc and to large reverse faults due to pure shear deformation from cross‐fault compression and along‐fault extension. Under the Caucasus regions, the Pn anisotropy results indicate that the preferred alignment of olivine crystals is parallel to the plate movement direction; however, the surface fault strike is at an angle of nearly 45° with the crustal movement direction and anisotropy. These differing deformations suggest potential decoupling between the crust and upper mantle. The possible decoupling and differing deformation between the crust and upper mantle are easily enhanced under high temperatures due to volcanoes and supported by low velocities beneath the Caucasus. We validate the existence of Pn anisotropy under these regions by azimuthal averaging of the apparent Pn velocity.

An approach for simultaneously inverting MT data for resistivity and susceptibility

We present a technique for inverting magnetotelluric (MT) data for simultaneous estimates of resistivity and susceptibility. We demonstrate the efficacy of the method by processing a large survey collected in the Dayangshu (DYS) basin of Northeast China. MT is one of the most important geophysical exploration techniques, and has been widely applied to both resource exploration and the determination of deep crustal structure. In most conventional MT inversion algorithms, the subsurface is assumed to be nonmagnetic and only resistivity is estimated. Most normal sedimentary rocks, granite, and a few weakly metamorphic rocks are effectively non-magnetic. However, there are a class of rocks with high levels of magnetism such as Basalt. It has been previously shown that the magnetic properties of rocks significantly effects MT response. If an MT investigation is carried out in a region underlain by highly magnetic materials, conventional inversion techniques generate estimates with anomalous resistivity values. We develop a simultaneous inversion method to deal with this case. Our method considers both electrical and magnetic effects and simultaneously inverts MT measurements for subsurface resistivity and susceptibility. This paper presents the basic theory of the method and a case study taken from Chinas DYS basin. The case study demonstrates that simultaneous inversion can improve the quality of resistivity inversions, and more importantly, obtain information about rock susceptibility. Estimates of susceptibility are a valuable tool for the identification and mapping of volcanic strata with high levels of magnetism.

Identification and quantitative characterization of igneous rocks: Method and application in the north Huimin Sag

Identification and characterization of igneous rocks have been a challenge in seismic exploration in the areas with igneous rocks. In this study, a method has been developed to accurately identify and describe igneous rocks and therefore improve oil and gas reservoir prediction in the igneous region. Based on geology, drilling, seismic, logging and other available information, forward igneous rock models are developed and igneous rock boundaries are identified and characterized using seismic attributes analysis. Thickness, area size, and volume of igneous rocks are estimated using the tuning thickness method and the top-bottom subtraction method. The obtained information of igneous rock distribution is used to characterize hydrocarbon reservoirs. In the north Huimin Sag, drilling results correlate well with the inversion results, indicating that accurate information of igneous rock distribution effectively improves the reservoir prediction.

Effects of Stress-induced Anisotropy On Monopole And Dipole Logging

Anisotropy of the formation surrounding a borehole might be caused by intrinsic characteristics (bedding, mineral orientation, etc.) or the effect of stresses on fractures. In this study, we make monopole and dipole logging measurements in a borehole drilled in an anisotropic rock block. Then we apply uniaxial horizontal stresses parallel and perpendicular to anisotropy symmetry axes and measure velocities with the same logging systems. The experimental results show that the fast and slow shear velocities measured depend on both intrinsic anisotropy and stress-induced anisotropy. A combination of P and S wave velocities obtained from monopole logs and shear velocities obtained from flexural wave recorded by dipole logs can be used to determine intrinsic anisotropy and direction of maximum horizontal stresses.