IODP Expedition 325, Hole M0042A - Wireline Sonic Waveform Data

IODP-MSP drilling and logging contractor: ESO

Hole: M0042A

Expedition: 325

Location: Great Barrier Reef (tropical SW Pacific)

Latitude: 19 50.63885° ' S

Longitude: 150° 26.8818' E

Logging date: March 8, 2010

Sea floor depth (driller's): 56.32 m DSL

Sea floor depth (logger's): 73.78 m WRF on ASGR; 65 m WRF on all other tools

Total core recovered:  10.94 m (23.58 % of cored section)

Lithologies: carbonate sand, corals, bioclastic sediments

TOOL USED: 2PSA (Full Waveform Sonic Probe)

Recording mode: 25 kHz (compressional waveforms)

The 2PSA-1000 sonic probe of Mount Sopris measures compressional wave velocities of the formation. The 2PSA-1000 is composed of an acoustic transmitter and 4 receivers. The transmitter transmits an acoustic signal that propagates through the borehole fluid to the rock interface where some of the energy is critically refracted along the borehole wall. As a result of wavefront spreading (Huygens principal), some of the refracted energy is transmitted back into the borehole. Where energy is transmitted back into the borehole adjacent to a receiver the receiver picks up the signal, amplifies it, digitizes it and then sends the digitized signal to the surface. The recorded waveforms are examined, and wave arrival times selected (picking). The arrival times are the transit times of the acoustic energy. By measuring the acoustic transit time and knowing the distance between receivers (1 foot), the fluid velocity and borehole diameter, the sonic velocity of the rock is calculated. Consequently, three interval velocities values are generated at each sampling point. The borehole propagates energy in many different modes including the compressional and shear head waves, an infinite series of normal compressional and normal shear modes, as well as the Stoneley wave modes. All wave modes are exited when the source spectrum contains sufficiently high frequencies. By controlling the transmitted frequency band, the complexity of the received waveforms can be reduced. Acoustic waves are highly dependent on borehole conditions. Larger cavities causes the induced wave to scatter and acoustic energy is lost more rapidly.

Normal modes (monopole surveys) are a result of constructive interference in the waveguides (borehole). For each normal mode, there exists a frequency below which the mode cannot be exited (cutoff frequency). The normal modes are highly dispersive, with their phase velocities approaching head wave velocities as frequency approaches the mode cutoff frequency. The optimal frequency band for producing head waves narrowly includes the cutoff frequencies for the first order compressional and shear-normal modes. In this manner, unwanted modes are not exited and received head waves are high. Shear wave arrivals do not occur in monopole surveys when the shear velocity is less than the fluid velocity because critical refractions cannot occur. In this case, shear velocity can be found from the Stoneley velocity (conducted below the cutoff frequency, so that only the Stoneley wave is received while interference from other modes is suppressed). The calibration of the tool is performed either in water (1500 m/s for P-wave) or into a steel pipe (5440 m/s), while running downhole. The precision of acoustic travel time measurements is approximately 5%.

The data was filtered (frequency filter) in such a way that only the energy around the induced frequency (source) was analyzed. Waveform picking was done manually in the WellCad software package to ensure good quality data. Time picks were saved and the acoustic velocities were calculated (using the receiver spacing of 1 ft). All presented acoustic data is accurate. Where no clear first arrivals in the waveform were present in at least two receivers, a value of zero was entered in the database. The downhole measurement spacing interval selected was 0.05 m.

The acoustic data acquired with the tool acquisition software were converted into ASCII format. Each of the four waveforms generally consists of 1024 samples, each recorded every 4 (10 kHz mode) and 8 microsec (1 kHz mode).

Each line consists of the entire waveform, preceded by depth.

In borehole M0042A the 2PSA tool was run at a frequency of 25 kHz.

(There are 526 columns in the output .txt files…...)

The following files have been loaded:

2PSA from Lower Section (pipe at  7 m WMSF, frequency of 25 kHz)

325-M0042A_2psal-25RX1.txt: 32.55 - 38.45 m WMSF

325-M0042A_2psal-25RX2.txt: 32.55 - 38.45 m WMSF

2PSA from Upper Section (pipe at  7 m WMSF, frequency of 25 kHz)

325-M0042A_2psau-25RX1.txt: 7.62 - 32.37 m WMSF

325-M0042A_2psau-25RX2.txt: 7.62 - 32.37m WMSF

Additional information about the drilling and logging operations can be found in the Operations section of the Site Chapter in IODP Proceedings of Expedition 325.

For any question about the data or about the LogDB database, please contact LogDB support: logdb@ldeo.columbia.edu.