CTITLESAMPFKC  -- INVERSE TRANSFORM IMAGE TRACE FROM FREQUENCY TO TIME
C***********************************************************************
C                 COPYRIGHT ATLANTIC RICHFIELD COMPANY 1991            *
C***********************************************************************
CA    AUTHOR      BRUCE VER WEST
CA    DESIGNER    BRUCE VER WEST
CA    LANGUAGE    VS FORTRAN
CA    SYSTEM      IBM / CRAY
CA    WRITTEN     07/01/86
C     REVISED     12-04-87  BJV  INCREASED BUFFER BLOCKING ARRAY        00070009
C                                DIMENSIONS TO 39 AND 19.               00080009
C     REVISED     MM-DD-YY                                              00070009
C     REVISED     02-20-89  JJC  FOR SPARC PRODUCTION.
C     REVISED     04-11-90  LWC  CHANGED FFT991 CALL TO RFFTMLT
C     REVISED     04-30-90  LWC  REPLACE FFT991
C                                                                       00080009
C
CA    SUBROUTINE SAMPFKC(IX,NXTOT,NTBUF,INTRT,
CA                        Q,WORK,WORK2,TRIGST,IFAXW,OTR,IPR)
CA
CA    THIS SUBROUTINE PERFORMS AN INVERSE FOURIER TRANSFORM TO GO
CA    FROM THE FREQUENCY DOMAIN BACK TO THE TIME DOMAIN AND OUTPUT
CA    THE RESULT AS A TIME TRACE AS A FUNCTION OF DEPTH POINT NUMBER
CA
CA**********************************************************************
CA
CA    ARGUMENTS:
CA
CA    IX           INPUT          COUNTER OF THE OUTPUT MIGRATED IMAGE
CA                                DEPTH POINT TRACE.
CA
CA    NXTOT        INPUT          TOTAL OF DEPTH POINT TRACES TO BE
CA                                TRANSFORMMED.
CA
CA    NTBUF        INPUT          NO. OF DEPTH POINT TRACES IN BUFFER
CA                                TO BE TRANSFORMMED.
CA
CA    INTRT        INPUT/         COUNTER OF THE INPUT DEPTH POINT
CA                 OUTPUT         TRACES TO BE TRANSFORMMED.
CA
CA    Q            INPUT          COMPLEX DOWNWARD-CONTINUED FIELD
CA                                FROM WHICH THE IMAGE TRACE IS OBTAINED
CA                                VIA AN INVERSE FOURIER TRANSFORM.
CA
CA    WORK         INPUT          COMPLEX ARRAY WHICH SERVES AS INPUT
CA                                TO THE CRAY ASSEMBLER INVERSE FOURIER
CA                                TRANSFORM ROUTINE CRFFT2 WHICH TAKES
CA                                A CONJUGATE SYMMETRIC COMPLEX INPUT
CA                                RADIAL FREQUENCY ARRAY AND RETURNS TO
CA                                A REAL TIME DOMAIN ARRAY. ITS LENGTH
CA                                MUST BE A POWER OF TWO.
CA
CA    WORK2        INPUT          COMPLEX WORK ARRAY WHICH CONTAINS THE
CA                                TRIG TABLES TO BE USED BY THE CRAY
CA                                FFT ROUTINE CRFFT2.
CA
CA    TRIGST       INPUT          ARRAY WHICH CONTAINS THE TRIG TABLES
CA                                NEED BY THE FFT ALGORITHM CFFT99 FOR
CA                                THE FOURIER TRANSFORMS FROM FREQUENCY
CA                                TO TIME.
CA
CA    IFAXW        INPUT          ARRAY WHICH CONTAINS THE PRIME
CA                                FACTORS FOR THE FFT LENGTH
CA
CA    OTR          OUTPUT         THE OUTPUT TIME TRACE RETURNED FROM
CA                                THE INVERSE TRANSFORM FROM FREQUENCY
CA                                TO TIME.
CA
CA**********************************************************************
C
      SUBROUTINE SAMPFKC(IX,NXTOT,NTBUF,INTRT,
     +                    Q,WORK,WORK2,TRIGST,IFAXW,OTR,IPR)
C
      COMMON /CMPFKC/ IF1,LNT,LW,LW2,IW1,ALPHA,SCALE,CMIN,VMUTE,
     +                AFFR,IKHHI
      COMMON /CMPFKT/ NT,DT,NW,DW,NWD2,NWD21,NWP2
      COMMON /CMPFKX/ NX,DX,NKX,DKX,NKXD2,NKXD21,NKX2,NKXP2
      COMMON /CMPFKH/ NH,DH,NKH,DKH,NKHD2,NKHD21,NKH2,IHBEG
      COMMON /CMPFKB/ IKXBF(39),IWBF(19),MKXBF(39),MWBF(19),
     +                NBF,MBF,NKXBF,NWBF
C
      COMPLEX Q(LNT/2,1),TRIGST(1),WORK(1),WORK2(1)
      DIMENSION OTR(1),IFAXW(1)
C
      IF (IX .LE. INTRT) GO TO 100
C
      CALL ARSET(WORK,NWP2*NTBUF,0)
      I1 = INTRT+1
      INBUF = MIN0(NTBUF,NXTOT-INTRT)
      DO 10 I=1,INBUF
      IND = (I-1)*NWD21+1
   10 CALL CCOPY(LW,Q(1,INTRT+I),1,WORK(IND),1)
      DO 20 I=1,INBUF*NWD21
   20 WORK(I) = CONJG(WORK(I))
      CALL FFT991(WORK,WORK2,TRIGST,IFAXW,1,NWP2,NW,INBUF,+1)
C LWC CALL RFFTMLT(WORK,WORK2,TRIGST,IFAXW,1,NWP2,NW,INBUF,+1)
      INTRT = INTRT+INBUF
C
  100 CONTINUE
C
      IND = (IX-I1)*NWD21+1
      CALL SCOPY(NT,WORK(IND),1,OTR,1)
      CALL SSCAL(NT,SCALE,OTR,1)
C
C
      RETURN
      END