ishot - an array specifying the directions rays are to be traced from the shot points listed in the arrays xshot and zshot; the following code is used: (1) 0 - no rays are traced (2) -1 - rays are traced to the left only (3) 1 - rays are traced to the right only (4) 2 - rays are traced to the left and right (default: 0) iraysl - use the array irayt to select which ray groups listed in the array ray are to be traced for a particular shot point (default: 0) irayt - an array selecting those ray groups listed in the array ray which are active for a particular shot point listed in the arrays xshot and zshot. If there are n ray groups listed in the array ray, then the jth ray group for the ith shot is referred to in the (2n(i-1)+j)th element of irayt for rays traced to the left , and in the (2n(i-1)+n+j)th element of irayt for rays traced to the right. You must specify values of irayt for each group and for each direction regardless of the values of ishot (default: 1) ifast - use a Runge-Kutta routine without error control to solve the ray tracing equations and a look-up/interpolation routine to evaluate certain trigonometric functions; with ifast=0, a Runge-Kutta method with error control and the intrinsic trigonometric functions are used. Using ifast=1 is about 30-40% faster and provides essentially the same accuracy as ifast=0 (default: 1) Note: ifast=1 is intended to be used for all routine modelling and ifast=0 only to test a final model or if ifast=1 obviously fails i2pt - perform two-point ray tracing, i.e., determine the ray take- off angles that connect sources and observed receiver locations within each ray group; the receiver locations are obtained from the file tx.in for those picks that have the same integer code as specified in the array ivray for the ray group; if i2pt=2, only rays which reflect off the appropriate floating reflector specified in the array frbnd (if frbnd>0) are traced (default: 0) iturn - an array corresponding to the ray groups listed in the array ray to trace rays only to their turning or reflection point in the search mode; if a refracted or head wave ray group is to contain a reflection(s) before it turns (specified using nrbnd and rbnd), then iturn=0 must be used (default: 1) isrch - search again for the take-off angles of a ray group if the same ray code is listed more than once for the same shot in the array ray; isrch=1 must be used if the take-off angles of two or more ray groups with the same ray code are different because of multiple reflections and/or conversions specified by nrbnd, rbnd, ncbnd,and cbnd (default: 0) istop - stop tracing any ray which reflects off a boundary not specified in the arrays ray or rbnd; if istop=2, rays are also stopped if they enter a layer deeper than that specified by the ray code in the array ray, i.e., the layer number is greater than L and the ray code is L.n, where n=0, 1, 2, or 3 (default: 1) idiff - continue to trace headwaves along a boundary even if the velocity contrast across the interface is no longer positive, in which case headwaves emerge parallel to the boundary (idiff=1); if idiff=2, initiate a headwave ray group even if there is no critical point (or it is not found) to model diffractions along the top of a low-velocity layer (default: 0) ibsmth - apply a simulation of smooth layer boundaries (1) or apply the simulation and plot the smoothed boundaries (2); ibsmth=2 has no effect if isep>1 (default: 0) insmth - an array listing the layer boundaries for which the smooth layer boundary simulation is not to be applied, or only applied outside the model distances xminns and xmaxns, when ibsmth=1 or 2, (default: 0) imodf - use the velocity model in the file v.in instead of the model in part (5) of the file r.in (default: 0)

xshot - an array containing the x-coordinates (km) of the shot points (default: 0.0) zshot - an array containing the z-coordinates (km) of the shot points (default: a very small distance below the model surface) ray - an array containing the ray groups to be traced; the following code is used: (1) L.1 - rays which refract (turn) in the Lth layer (2) L.2 - rays which reflect off the bottom of the Lth layer (3) L.3 - rays which travel as head waves along the bottom of the Lth layer (4) L.0 - ray take-off angles supplied by the user in the arrays amin and amax nray - an array containing the number of rays to be traced for each ray group in the array ray (default: 10; however, the default is the first element of nray for all ray groups if only one value is specified) space - an array which determines the spacing of take-off angles between the minimum and maximum values for each ray group in the array ray. For space=1, the take-off angles will be equally spaced; for space>1, the take-off angles will be concentrated near the minimum value; for 0<space<1, the take-off angles will be concentrated near the maximum value (default: 1; however, space=2 for a reflected ray group if specified as L.2 in the array ray) amin, amax - arrays containing minimum and maximum take-off angles (degrees); measured from the horizontal, positive downward and negative upward (for rays traveling left to right or right to left); used for ray groups specified by ray=1.0 nsmax - an array containing the maximum number of rays traced when searching for the take-off angles of the ray groups in the array ray (default: 10; however, the default is the first element of nsmax for all ray groups if only one value is specified) n2pt - maximum number of iterations during two-point ray tracing (i2pt>0); this is the maximum number of rays traced for each receiver to determine the take-off angle of the ray that connects the source and receiver (default: 5) x2pt - distance tolerance (km) for two-point ray tracing; less than n2pt rays will be traced for a particular receiver if a ray end point is within x2pt of the receiver location (default: (xmax-xmin)/2000) crit - head waves are generated if a down-going ray in the search mode has an angle of incidence at the bottom of the Lth layer within crit degrees of the critical angle when ray=L.3 (default: 1) hws - the spacing (km) of rays emerging upward from the bottom of the Lth layer when ray=L.3 (default: (xmax-xmin)/25) nhray - maximum number of rays traced for a head wave ray group (default: pnrayf) aamin - minimum take-off angle (degrees) for the refracted ray group in the first layer (default: 5) aamax - maximum take-off angle (degrees) for reflected ray groups specified as L.2 in the array ray (default: 85) stol - if a ray traced in the search mode is of the correct type and its end point is within stol (km) of the previous ray traced in the search mode, then the search for that ray type is terminated; a value of stol=0 will ensure that nsmax rays are always traced in the search mode (default: (xmax-xmin)/3500) xsmax - for reflected ray groups specified as L.2 in the array ray, determine the minimum take-off angle using the search mode so that the maximum range for this ray group is xsmax (km) if iturn=0 or the maximum offset of the reflection point is xsmax/2 (km) if iturn=1; for head wave ray groups specified as L.3 in the array ray, the maximum offset of the point of emergence from the head wave boundary is xsmax (km); xsmax=0 will ensure that the take-off angle of the reflected ray which grazes off the bottom of the Lth layer is determined and head waves are traced along the Lth boundary until the edge of the model is reached (default: 0.0) nrbnd - an array containing the number of reflecting boundaries for each ray group in the array ray (default: 0) rbnd - an array containing the reflecting boundaries specified in the array nrbnd; the following code is used: (1) L - ray traveling downward is reflected upward off the bottom of the Lth layer (2) -L - ray traveling upward is reflected downward off the top of the Lth layer ncbnd - an array containing the number of converting (P to S or S to P) boundaries for each ray group in the array ray (default: 0) cbnd - an array containing the converting boundaries specified in the array ncbnd; the following code is used: (1) i - ray will convert from its present wave type (P or S) at the ith layer boundary encountered (2) 0 - ray will leave the source as an S-wave frbnd - an array containing the floating reflecting boundaries for each ray group in the array ray; the values of frbnd correspond to the order in which the reflectors are listed in the file f.in (default: 0) pois - an array containing the value of Poisson's ratio for each model layer; a value of 0.5 signifies a water layer with a corresponding S-wave velocity of zero (default: 0.25; however, the default is the first element of pois for all layers if only one value is specified) poisl, poisb - arrays specifying the layers and block numbers, respectively, of model trapezoids within which Poisson's ratio is modified over that given by pois using the array poisbl; for poisb, the trapezoids with a layer are numbered from left to right poisbl - an array containing the value of Poisson's ratio for the model trapezoids specified in the arrays poisl and poisb overriding the values assigned using the array pois npbnd - number of points at which each layer boundary is uniformly sampled for smoothing if ibsmth=1 or 2 (default: 100) nbsmth - number of applications of a three-point averaging filter to each layer boundary if ibsmth=1 or 2 (default: 10) xminns, xmaxns - minimum and maximum model distance over which the layer boundaries listed in the array insmth are not to be smoothed using ibsmth=1 or 2 (defaults: smooth boundaries between xmin and xmax) step - controls the ray step length in the solution of the ray tracing equations according to the relationship step length (km) = step*v/(|vx|+|vz|) where v is velocity and vx and vz are its partial derivatives with respect to x and z (default: 0.05) smin, smax - minimum and maximum allowable ray step length (km) (defaults: (xmax-xmin)/4500, (xmax-xmin)/15)

Sat Mar 7 19:13:54 EST 1998