How sioseis process geom calculates rp numbers

According the process geom documentation (22 October 2002)

The shot is assigned an X-coordinate by adding (type 1) or multiplying

(type 2) DFLS (distance from the last shot) to the X-coordinate of the

shot-receiver distance to the shot X-coordinate.  The  RP X-coordinate

is calculated by assuming the RP is halfway between the shot and

receiver.  The RP number is the RP X-coordinate divided by DBRPS

(distance between RPs) and truncating to an integer.  The coordinate of

the first shot of the job is the shot number (from the header) times the

distance from the previous shot.  i.e.

xr = xs + rx

xrp = (xr + xs) / 2.

Type 2 geom example where the first shot is shot 9711, dfls = 50,

dbrps = 6.25, range to trace 1 = -6168:

xs = 9711. * 50. = 485550.

rx = -6168.

xr = 485550. -6168. = 479382

xrp = (485550 + 479382.) / 2. = 964932/2 = 482466

rp_number = 482466 / 6.25 = 77194.56 = 77195

Type 9 geom example where the first shot is 33000, dbrps = 6.25, and

range to trace 1 = -6168:

(type 9 geometry sets dfls = 1. for the first shot)

xs = 33000.

rx = -6168.

xr = 33000 -6168 = 26832

xrp = (33000 + 26832) / 2 = 59832 / 2 = 29916

rp_number = 29916 / 6.25 = 4786.56 = 4787

Type 9 geom example where the first shot is 58309, dbrps = 6.25, and

range to trace 1 = -6168:

xs = 58309

xr = 58309. -6168. = 52141.

xrp = (58309. + 52141.) / 2. = 110450. / 2. = 55225.

rp_number = 55225. / 6.25 = 8836