SUBROUTINE DISC(Y,DEP)
C
C     DETERMINATION OF DEPTH OF 3D INTERFACES AND ITS DERIVATIVES
C     FOR BICUBIC POLYNOMIAL APPROXIMATION
C
      DIMENSION Y(2),DEP(6),B(16,2),DX(2),DY(2)
C
      COMMON /AUXI/  IANI(20),INTR,INT1,IOUT,KRE,IREFR,LAY,NDER,IPRINT,
     1 MPRINT,NTR,ISQRT,NAUX,ISOUR,MAUX,MREG,MDIM,IPOL,MSCON,LOUT,
     2 IAMP,MTRNS,ICOEF,IAD,IRHO,ISHEAR,IAC,IRT,mori
      COMMON /AUXX/  MMX(20),MMY(20),MMXY(20)
      COMMON /INTRF/ Z(1000),SX(350),SY(350),NX(20),NY(20),BRD(4),NINT,
     1   XINTA
      COMMON/ZCOEF/ A02(1000),A20(1000),A22(1000)
C
      IBB=0
      LB=2
      IF(INTR.EQ.LAY)LB=1
      MX=NX(INTR)
      MY=NY(INTR)
      DO 1 I=2,MX
      K=MMX(INTR)+I-1
      IF(Y(1).LT.SX(K))GO TO 2
    1 CONTINUE
    2 I1=K
      DO 3 I=2,MY
      K=MMY(INTR)+I-1
      IF(Y(2).LT.SY(K))GO TO 4
    3 CONTINUE
    4 J1=K
      IF(MAUX.EQ.0) GOTO 8
      IF(LB.EQ.2) GOTO 5
      IF(I1.EQ.IU.AND.J1.EQ.JU.AND.LLAY.EQ.LAY) GOTO 10
      GOTO 8
    5 IF(I1.EQ.IL.AND.J1.EQ.JL.AND.LLAY.EQ.LAY) GOTO 10
      IL=I1
      JL=J1
      GOTO 9
    8 IU=I1
      JU=J1
    9 LLAY=LAY
      I=I1-MMX(INTR)
      J=J1-MMY(INTR)
      DX(LB)=SX(I1-1)
      DY(LB)=SY(J1-1)
      MM=MMXY(INTR)-1
      K=MM+(I-1)*MY+J
      B20=A20(K)
      B02=A02(K)
      B22=A22(K)
      B00=Z(K)
      K=MM+I*MY+J
      C20=A20(K)
      C02=A02(K)
      C22=A22(K)
      C00=Z(K)
      K=MM+(I-1)*MY+J+1
      D20=A20(K)
      D02=A02(K)
      D22=A22(K)
      D00=Z(K)
      K=MM+I*MY+J+1
      E20=A20(K)
      E02=A02(K)
      E22=A22(K)
      E00=Z(K)
      HX=SX(I1)-DX(LB)
      HY=SY(J1)-DY(LB)
      XA=3.*HX
      YA=3.*HY
      XB=HX/3.
      YB=HY/3.
      D32=(E22-D22)/XA
      D30=(E20-D20)/XA
      B30=(C20-B20)/XA
      B32=(C22-B22)/XA
      D12=(E02-D02)/HX-XB*(E22+2.*D22)
      D10=(E00-D00)/HX-XB*(E20+2.*D20)
      B10=(C00-B00)/HX-XB*(C20+2.*B20)
      B12=(C02-B02)/HX-XB*(C22+2.*B22)
      B03=(D02-B02)/YA
      B13=(D12-B12)/YA
      B23=(D22-B22)/YA
      B33=(D32-B32)/YA
      B01=(D00-B00)/HY-YB*(D02+2.*B02)
      B11=(D10-B10)/HY-YB*(D12+2.*B12)
      B21=(D20-B20)/HY-YB*(D22+2.*B22)
      B31=(D30-B30)/HY-YB*(D32+2.*B32)
      MAUX=1
      B(1,LB)=B00
      B(2,LB)=B01
      B(3,LB)=B02
      B(4,LB)=B03
      B(5,LB)=B10
      B(6,LB)=B11
      B(7,LB)=B12
      B(8,LB)=B13
      B(9,LB)=B20
      B(10,LB)=B21
      B(11,LB)=B22
      B(12,LB)=B23
      B(13,LB)=B30
      B(14,LB)=B31
      B(15,LB)=B32
      B(16,LB)=B33
      IBB=1
   10 AX=Y(1)-DX(LB)
      AZ=Y(2)-DY(LB)
      IF(IBB.EQ.1) GOTO 11
      B00=B(1,LB)
      B01=B(2,LB)
      B02=B(3,LB)
      B03=B(4,LB)
      B10=B(5,LB)
      B11=B(6,LB)
      B12=B(7,LB)
      B13=B(8,LB)
      B20=B(9,LB)
      B21=B(10,LB)
      B22=B(11,LB)
      B23=B(12,LB)
      B30=B(13,LB)
      B31=B(14,LB)
      B32=B(15,LB)
      B33=B(16,LB)
   11 AUX1=((B33*AZ+B32)*AZ+B31)*AZ+B30
      AUX2=((B23*AZ+B22)*AZ+B21)*AZ+B20
      AUX3=((B13*AZ+B12)*AZ+B11)*AZ+B10
      AUX4=((B03*AZ+B02)*AZ+B01)*AZ+B00
      DEP(1)=((AUX1*AX+AUX2)*AX+AUX3)*AX+AUX4
      IF(NDER.EQ.0) RETURN
      DEP(2)=(3.*AUX1*AX+2.*AUX2)*AX+AUX3
      IF(NDER.EQ.1)GO TO 7
      DEP(4)=6.*AUX1*AX+2.*AUX2
    7 AUX1=(3.*B33*AZ+2.*B32)*AZ+B31
      AUX2=(3.*B23*AZ+2.*B22)*AZ+B21
      AUX3=(3.*B13*AZ+2.*B12)*AZ+B11
      AUX4=(3.*B03*AZ+2.*B02)*AZ+B01
      DEP(3)=((AUX1*AX+AUX2)*AX+AUX3)*AX+AUX4
      IF(NDER.EQ.1) RETURN
      DEP(5)=(3.*AUX1*AX+2.*AUX2)*AX+AUX3
      AUX1=3.*B33*AZ+B32
      AUX2=3.*B23*AZ+B22
      AUX3=3.*B13*AZ+B12
      AUX4=3.*B03*AZ+B02
      DEP(6)=2.*(((AUX1*AX+AUX2)*AX+AUX3)*AX+AUX4)
      RETURN
      END