CTITLESADRIV -- FUNCTION TO CALCULATE DISTANCE WRT THETA1               00010001
C***********************************************************************
C                 COPYRIGHT ATLANTIC RICHFIELD COMPANY 1991            *
C***********************************************************************
C                                                                       00020000
CA    AUTHOR       ARCO                                                 00030000
CA    DESIGNER     ARCO                                                 00040000
CA    LANGUAGE     FORTRAN H                                            00050000
CA    WRITTEN      09-01-84                                             00060000
C     REVISED      MO-DA-YR  BY PROGRAMMER                              00070000
C     REVISED      04-02-87  JMP - PUT IN PRODUCTION SPARC.             00080001
C                                                                       00090000
CA    CALLING SEQUENCE:                                                 00100000
CA    X =  SADRIV ( ZTHETA, ZALPHA, ZDEPTH)                             00110001
CA                                                                      00120000
CA    IN/OUT ARGUMENT TYPE DESCRIPTION                                  00130000
CA                                                                      00140000
CA    IN     ZTHETA   R8   LOCAL VARIABLE FOR THETA                     00150000
CA    IN     ZALPHA   R8   LOCAL VARIABLE FOR ALPHA OF CURRENT LAYER    00160000
CA    IN     ZDEPTH   R4   LOCAL VARIABLE FOR THICKNESS OF CURRENT      00170000
CA                         LAYER                                        00180000
CA                                                                      00190000
CA    PURPOSE OF PROGRAM:                                               00200000
CA    THIS FUNCTION IS USED TO CALCULATE THE DERIVATIVE OF THE          00210000
CA    DISTANCE WITH RESPECT TO THETA-1.  THIS IS SIMPLY THE DERIVATIVE  00220000
CA    OF THE DIST FUNCTION AND IT IS SUMMED IN THE MAIN PROGRAM TO      00230000
CA    GET DXR.                                                          00240000
CA                                                                      00250000
CA    ROUTINES CALLED:  NONE                                            00260000
CA                                                                      00270000
C---------------------------------------------------------------------  00280000
      REAL FUNCTION SADRIV (ZTHETA, ZALPHA, ZDEPTH)                     00290001
C                                                                       00300000
C____________________________________________________________           00310000
C                                                                       00320000
C     LOCAL VARIABLES                                                   00330000
C                                                                       00340000
C____________________________________________________________           00350000
C                                                                       00360000
      REAL     *4  ZDEPTH                                               00370000
      REAL     *8  ZTHETA                                               00380000
     2          ,  ZALPHA                                               00390000
C                                                                       00400000
C     MAKE SURE WE DON'T TRY TO TAKE THE SQUARE ROOT OF A NEGATIVE.     00410000
C                                                                       00420000
      IF ((ZALPHA * DSIN(ZTHETA)) .GE. 1.0)                             00430000
     1              ZTHETA = 2 * DASIN(1 / ZALPHA) - ZTHETA             00440001
C                                                                       00450000
      SADRIV = 2 * DCOS(ZTHETA) * ZALPHA * ZDEPTH /                     00460001
     1             (DSQRT(1 - (ZALPHA * DSIN(ZTHETA)) **2) **3)         00470000
      RETURN                                                            00480000
      END                                                               00490000