EPP

Irregular surface flattening, which is based on a boundary conforming grid and the transformation between curvilinear and Cartesian coordinate systems, is a mathematical method that can elegantly handle irregular surfaces, but has been limited to obtaining first arrivals only. By combining a multistage scheme with the fast-sweeping method (FSM, the method to obtain first-arrival traveltime in curvilinear coordinates), the reflected waves from a crustal interface can be traced in a topographic model, in which the reflected wave-front is obtained by reinitializing traveltimes in the interface for upwind branches. A local triangulation is applied to make a connection between velocity and interface nodes. Then a joint inversion of first-arrival and reflection traveltimes for imaging seismic velocity structures in complex terrains is presented. Numerical examples all perform well with different seismic velocity models. The increasing topographic complexity and even use of a high curvature reflector in these models demonstrate the reliability, accuracy and robustness of the new working scheme; checkerboard testing illustrates the method’s high resolution. Noise tolerance testing indicates the method’s ability to yield practical traveltime tomography. Further development of the multistage scheme will allow other later arrivals to be traced and used in the traveltime inversion.

The Emeishan large igneous province (hereafter named by its acronym ELIP) is the first accepted large igneous region in China. The current study tries to reconstruct the density structure of the crust in this region. For this purpose, we conducted the gravity survey along an 800-km-long profile, which stretched laterally along the latitude 27°N from Lijiang (Yunnan province) to Guiyang (Guizhou province). The fieldwork included 338 gravity measurements distributed from the inner zone to the outer zone of the mantle plume head. After a series of gravity reductions, we calculated the Bouguer gravity anomaly and then constructed the density model for ELIP by iterative forward modeling from an initial density model tightly constrained by wide-angle seismic reflection data. The topography of the Moho, here physically interpreted as a density discontinuity of ~0.4 g·cm^–3, gradually rises from the inner zone (~50 km deep) to the outer zone (~40 km), describes a thicker crust in the inner zone than in any other segment of the profile and largely reproduces the shape of the Bouguer gravity anomaly curve. Both the Bouguer gravity and the density structure show significant differences with respect to the inner zone and the other two zones of ELIP according to the commonly accepted partition of the Emeishan area. A thicker and denser middle-lower crust seems to be the main feature of the western section of the profile, which is likely related to its mafic magmatic composition due to magmatic underplating of the Permian mantle plume.

In the adjoint-state method, the forward-propagated source wavefield and the backward-propagated receiver wavefield must be available simultaneously either for seismic imaging in migration or for gradient calculation in inversion. A feasible way to avoid the excessive storage demand is to reconstruct the source wavefield backward in time by storing the entire history of the wavefield in perfectly matched layers. In this paper, we make full use of the elementwise global property of the Laplace operator of the spectral element method (SEM) and propose an efficient source wavefield reconstruction method at the cost of storing the wavefield history only at single boundary layer nodes. Numerical experiments indicate that the accuracy of the proposed method is identical to that of the conventional method and is independent of the order of the Lagrange polynomials, the element type, and the temporal discretization method. In contrast, the memory-saving ratios of the conventional method versus our method is at least N when using either quadrilateral or hexahedron elements, respectively, where N is the order of the Lagrange polynomials used in the SEM. A higher memory-saving ratio is achieved with triangular elements versus quadrilaterals. The new method is applied to reverse time migration by considering the Marmousi model as a benchmark. Numerical results demonstrate that the method is able to provide the same result as the conventional method but with about 1/25 times lower storage demand. With the proposed wavefield reconstruction method, the storage demand is dramatically reduced; therefore, in-core memory storage is feasible even for large-scale three-dimensional adjoint inversion problems.