Wireline Sonic Waveform Data
ODP logging contractor: LDEO-BRG
Well name: 766A
Location: Exmouth Plateau (tropical SE Indian)
Latitude: 19° 55.925' S
Longitude: 110° 27.243' E
Logging date: October, 1988
Bottom felt: 4008 mbrf (depth used onboard by Schlumberger engineer to shift the data to sea floor)
Total penetration: 527.2 mbsf
Total core recovered: 347.5 m (66 %)
TOOL USED: SDT (Digital Sonic Tool, also known as Array Sonic)
Recording mode: Depth-derived, borehole-compensated mode
Remarks about the recording: Like in all of the past legs, the sonic waveforms are not on depth with the other logs: in Hole 766A they show an offset of about 31 m downward. This offset is clearly visible at the base of the bottom hole assembly.
Acoustic data were 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 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: 1468
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 (bottom hole assembly at ~ 244 mbsf)
766A.bin: 238.81-462.38 mbsf
The sonic waveform files were block-shifted to the seafloor by the Schlumberger operator onboard the ship. According to the log header, the amount of this shift was 4008 m. The sonic waveform files are not depth shifted to a reference run. 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
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)
For further information about the logs please contact:
E-mail: Cristina Broglia