Wireline Sonic Waveform Data

 

ODP logging contractor: LDEO-BRG

Hole: 793B

Leg: 126

Location: Izu-Bonin Forearc (NW Pacific)

Latitude: 31° 6.33' N

Longitude: 140° 53.27' E

Logging date: June, 1989

Bottom felt: 2975.3 mbrf (used for depth shift to sea floor)

Total penetration: 1682 mbsf

Total core recovered: 697.9 m (63.7 %)

 

TOOL USED: SDT (Sonc Digital Tool)

Recording mode: Depth-derived, borehole compensated mode, long spacing

Remarks about the recording: the main characteristic of the sonic data during the recording was the high noise level that interfered with the signal from the formation. This noise is primarily the result of absence of adequate centralization of the sonic tool, a consequence of the small diameter of the drill pipe, which makes the use of a centralizer extremely risky, as the tool string could easily get stuck. This lack of centralization resulted in asymmetrical arrivals at the receivers and 'road noise" caused by occasional direct contact of the tool with the formation.

Like in all of the past legs, the sonic waveforms are not on depth with the other logs: in Hole 793B they show an offset of about 28 m downward. This offset is very clearly visible at the base of the bottom hole assembly in the middle and lower sections. In the upper section, logs such as resistivity show the casing shoe at 3543 mbrf. From the sonic waveforms, however, it is difficult to assess the amount of the offset because they seem to have a similar signature through casing and open hole. It is recommended to process the waveforms first 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 in LIS format has been 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 files are:

Number of columns: 2001

Number of rows: 1130 (upper section)

Number of rows: 1297 (middle section)

Number of rows: 3926 (lower section)

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 (upper section, casing at ~ 3543 mbrf)

793B-u.bin: 3449.1-3688.84 mbrf

 

SDT from DIT/SDT/NGT (middle section, bottom hole assembly at ~ 3749 mbrf)

793B-mid.bin: 3743.4-3940.91 mbrf

 

SDT from DIT/SDT/NGT (lower section, bottom hole assembly at ~ 4009 mbrf)

793B-l.bin: 3959.81-4557.98 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