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.  Each receiver is assigned an X-coordinate by adding 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.
      xs = FLOAT(lhead(3)) * dfls
      rx = FLOAT(lhead(10))
      xr = xs + rx
      xrp = (xr + xs) / 2.
      lhead(6) = NINT( xrp/dbrps )



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