Sponsored by National Nuclear Security Administration

Models of geophysically important properties of the Earth, such as seismic velocity, Q and density, can become large and complex when those properties vary in three dimensions within the model. We have developed a system to represent the distribution of seismic properties in the Earth that can accommodate a wide range of local to global scale 3D Earth models with spatially variable resolution. A 2D grid of nodes is tessellated using either triangles or quadrilaterals and a profile is defined at each 2D grid node that extends from the center of the Earth to the surface. The surface of the model corresponds with the topographic/bathymetric surface of the Earth, which is referenced to the surface of the GRS80 ellipsoid. Each profile can be separated into a number of layers defined by interfaces across which geophysical properties may be discontinuous. Within the layers between interfaces, the vertical distribution of geophysical properties may be defined by a number of continuous sublayers, by arbitrary order polynomials or by various types of splines. Layer thicknesses can vary laterally and zero thickness layers and layer pinch-outs are accommodated. The distribution of nodes is very flexible, allowing model resolution to vary over a wide range in three dimensions. In this paper, we present detailed descriptions of the software algorithms used to construct, store and interpolate these models