Wireline Sonic Waveform Data Processing
Operator and logging contractor: LDEO-BRG
Hole: 16A (step-out site from Holes 7A/D)
Expedition: NGHP-1
Location: Krishna-Godavari Basin, Eastern India (Bay of Bengal)
Latitude: 16° 35.5986' N
Longitude: 82° 41.0070' E
Logging date: July 16-17, 2006
Sea floor depth (drillers'): 1264.5 mbrf
Sea floor depth (loggers'): 1266 mbrf
Total penetration: 1481.5 mbrf (217 mbsf)
Total core recovered: 165.39 m (72.2 % of cored section)
Oldest sediment cored: n/a
Lithology: Mostly nannofossil and foraminifer-bearing clays with some sandy intervals (from Hole 7D)
ACOUSTIC TOOL USED: DSI (Dipole Sonic Imager)
Recording mode: Monopole P&S, Upper and Lower Dipole, Stoneley mode (Pass 1), Monopole P&S and Cross Dipole (Pass 2).
Remarks about the recording: none.
MONOPOLE P&S MODE: measures compressional and shear slowness. The monopole transmitter is excited by a high-frequency pulse, which reproduces conditions similar to previous sonic tools.
UPPER DIPOLE MODE: measures shear wave slowness using firings of the upper dipole transmitter.
LOWER DIPOLE MODE: measures shear wave slowness using firings of the lower dipole transmitter.
CROSS-DIPOLE MODE: uses alternate firings of upper and lower dipole transmitter, thus allowing acquisition of orthogonally polarized data for anisotropy studies.
transmitter, driven by a low-frequency pulse, generates the Stoneley wave.
STONELEY MODE: measures low-frequency Stoneley wave slowness. The monopole transmitter, driven by a low-frequency pulse, generates the Stoneley wave.
Acoustic data are recorded in DLIS format. Each of the eight waveforms geerally consists of 512 samples, each recorded every 10 (monopole P&S) and 40 microsec (dipolemodes), at depth intervals of 15.24 cm (6 inches).The original waveforms in DLIS format are first loaded on a virtual PC machine using Schlumberger's Techlog log analysis package. The packed waveform data files are run through a module that applies a gain correction. After they are exported from Techlog in LAS format they are converted into binaryand GIF format (images) are cconverted using in-house software. Each line is composed of the entire waveform set recorded at each depth, preceded by the depth value. In the general case of 8 waveforms with 512 samples per waveform, this corresponds to 1 + 8x512 = 4097 columns. In this hole, the specifications of the files are:
Number of columns: 4097
Number of rows: 821 (Pass 1)
Number of rows: 453 (Pass 2)
The following files have been loaded:
DSI from FMS/DSI/GPIT/SGT (Pass 1, open hole)
16A-mono_p1.bin: 86.24-211.06 mbsf
16A-ldip_p1.bin: 86.24-211.06 mbsf
16A-udip_p1.bin: 86.24-211.06 mbsf
16A-st_p1.bin: 86.24-211.06 mbsf
DSI from DSI/SGT (Pass 2, open hole)
16A-mono_p2.bin: 49.06-117.79 mbsf
16A-cd_ldip_il_p2.bin: 49.06-117.79 mbsf
16A-cd_ldip_cl_p2.bin: 49.06-117.79 mbsf
16A-cd_udip_il_p2.bin: 49.06-117.79 mbsf
16A-cd_udip_cl_p2.bin: 49.06-117.79 mbsf
All values are stored as '32 bits IEEE float'.
Any image or signal-processing program should allow to import the files and display the data.
The sonic waveform files were depth-shifted to the seafloor (-1266 m) but they were not depth-matched to the reference run. Please refer to the "DEPTH SHIFT" section in the standard processing notes for further information.
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 any question about the data or about the LogDB database, please contact LogDB support: logdb@ldeo.columbia.edu.