Wireline Sonic Waveform Data

ODP logging contractor: LDEO-BRG

Well name: 719B

Leg: 116

Location: Bengal Fan (central tropical S Indian Ocean)

Latitude: 0° 57.646' S

Longitude: 81° 23.967' E

Logging date: August, 1987

Bottom felt: 4748.2 mbrf (used for depth shift to sea floor)

Total penetration: 465.6 mbsf

Total core recovered: none

TOOL USED: SDT (Digital Sonic Tool, also known as Array Sonic)

Recording mode: depth-derived, borehole compensated mode

Remarks about the recording: Originally, the data was recorded up to the sea floor. Some of the proprietary tapes were corrupted, and the customer tapes did not include all the data. For this reason, the logging record is incomplete, and acoustic data are available only below 275 mbsf.

Like in all of the past legs, the sonic waveforms are not on depth with the other logs: they are usually shifted downward by an amount that varies from 15 to more than 30 m. This offset is usually clearly visible at the base of the bottom hole assembly. In hole 719B, however, it is difficult to assess the correct offset amount, as the logs did not reach the drill pipe at 4834 mbrf. Therefore, it is recommended to first process the sonic waveforms and then match the output to the other logs.

Acoustic data have been recorded in LIS format. Each of the four waveforms consists of 500 samples and is recorded at a sampling rate of 0.1524 m. The original data is first loaded on a Sun system using GeoFrame software. The waveform data files are then converted into ASCII and finally binary format.

Each row of the binary file is composed of the entire waveform set recorded at each depth, preceded by the depth. In the general case of 4 waveforms with 500 samples per waveform, this corresponds to 1 + 4x500 = 2001 columns. In this hole, the specifications of the file are:

Number of columns: 2001

Number of rows: 709 (lower main)

All values are stored as ' IEEE floating point numbers' (= 4 bytes).

Any numerical software or programing language (matlab, python,...) can import the files for further analysis of the waveforms.

The following files were converted:

SDT from DIT/SDT/NGT (lower section, bottom hole assembly at ~ 4834 mbrf)

719B_l.bin: 5022.65-5222.44 mbrf

The sonic waveform files are not depth shifted to a reference run or to the seafloor. For depth shift to the sea floor, please refer to the DEPTH SHIFT section in the standard log documentation file.

Also, please refer to the "Remarks about the recording" note above.

NOTE: For users interested in converting the data to a format more suitable for their own purpose, a simple routine to read the binary files would include a couple of basic steps (here in old fashioned fortran 77, but would be similar in matlab or other languages):

The first step is to extract the files dimensions and specification from the header, which is the first record in each file:

  open (1, file = *.bin,access = 'direct', recl = 50) <-- NB:50 is enough to real all fields

  read (1, rec = 1)nz, ns, nrec, ntool, mode, dz, scale, dt

  close (1)

The various fields in the header are:
      - number of depths
      - number of samples per waveform and per receiver
      - number of receivers
      - tool number (0 = DSI; 1 = SonicVISION; 2 = SonicScope; 3 = Sonic Scanner; 4 = XBAT; 5 = MCS; 6 = SDT; 7 = LSS; 8 = SST; 9 = BHC; 10 = QL40; 11 = 2PSA)
      - mode (1 = Lower Dipole, 2 = Upper Dipole, 3 = Stoneley, 4 = Monopole)
      - vertical sampling interval *
      - scaling factor for depth (1.0 = meters; 0.3048 = feet) *
      - waveform sampling rate in microseconds *

All those values are stored as 4 bytes integers, except for the ones marked by an asterisk, stored as 4 bytes IEEE floating point numbers.

Then, if the number of depths, samples per waveform/receiver, and receivers are nz, ns, and nrec, respectively, a command to open the file would be:

  open (1, file = *.bin, access = 'direct', recl = 4*(1 + nrec*ns))

Finally, a generic loop to read the data and store them in an array of dimension nrec × ns × nz would be:

  do k = 1, nz

    read (1, rec = 1+k) depth(k), ((data(i,j,k), j = 1,ns), i = 1,nrec)

  enddo

For further information about the logs please contact:

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia