Bortfeld approximation then revised by Richard-Frasier (1976) and by Richard-Aki (1980). Approximation of Richard-Frasier is giving a simple equation because it is written in a clear three forms, namely the right-hand side includes the first P wave velocity, the second node density and the last wave velocity. | Bortfeld approximation then revised by Richard-Frasier (1976) and by Richard-Aki (1980). Approximation of Richard-Frasier is giving a simple equation because it is written in a clear three forms, namely the right-hand side includes the first P wave velocity, the second node density and the last wave velocity. |