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Measurement of P-wave velocity requires an estimate of the traveltime travel time and an accurate measurement of the ultrasonic P-wave path length through the sample.
Velocity is defined as follows:
velocity = pathlength/traveltime, or
v = dS/dt.
Traveltime measurement is estimated by an algorithm for graphical first arrival pick. An ultrasonic pulser generates a high-impulse voltage, which is applied to the ultrasonic transmitter and thereby induces oscillation of the crystal element within the transducer-specific frequency band. A trigger pulse from the pulser is then applied to the oscilloscope to record the waveform from the receiving transducer.
By measuring the acoustic traveltime of the waveform through both specimen of the same material but variable thickness and instrumentation, linear regression through the dS/dt plot provides the instrumentation-specific time delay. Subtraction of the system delay time (+ liner material propagation time, if required) from the total traveltime gives the traveltime for the ultrasonic pulse through the sample.
Precise length of the sample is derived from the readout of a linear variable displacement transducer (LVDT). An AC linear variable differential transformer converts changes in physical position into an AC electrical output. The LVDT requires calibration, which is performed at the same time that the system delay is derived.
This LVDT functionality makes it mandatory that the system fully close the transducers when the software control program is opened and activated. Therefore, do not place a core section underneath the transducers when the software is started.
The chisel (bayonet) transducers are fixed at 8.2 cm for the z-axis (downhole) and 3.44 cm for the y-axis (IODP axis designation). The x-axis separation is derived from an LVDT and calibration constants.
Traveltimes for sample half sections are calculated as follows:
x-axis = total traveltime – x-system delay time – liner traveltime
y-axis = total traveltime – y-system delay time
z-axis = total traveltime – z-system delay time
Liner traveltime is calculated as the liner thickness (typically 2.7 mm) divided by the published liner material velocity (cellulose butyrate = 2140 m/s).

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Caliper Transducers: Panametrics-NDT Microscan Delay Line Transducers

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Custom

Exlar Linear Actuator

Acuity 1000 Laser Distance Sensor

Microscan MS-4 Ultracompact Imager Barcode Reader

Software

Generally, only the marine technicians should access configurations settings in the velocity software. However, there are some configuration issues that users need to be aware of.

The program icon is shown in Figure 3. Double click to open and follow the prompts to display the program home screen. DO NOT place the sample below the instruments before opening the program, the program will close all transducers upon startup, and can cause severe injury.

Figure 3. Opening Program Desktop icon.

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1. Prepare a water bath by setting ~2 L of distilled water at room temperature for a couple of hours.
2. Once at room temperature, insert the thermocouple/digital thermometer and record the temperature of the water bath.
3. On the Main window, select Bayonet Y (or Z) >then  Station  > then Y (or Z) Bayonet Calibration
4. Enter the temperature of the water bath into the Temperature field on the calibration screen. The screen displays a plot of measured velocity vs. temperature of the water bath.

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1.  Lower the bayonets into the water bath (the black transducer should be covered). An algorithm

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1.  calculates the temperature-corrected velocity of the water bath and displays the result.

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1. Enter Bayonet Separation in the value field ( Y – 31.50,  Z – 82.50 ).
   

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1. The computer calculates the system delay for the y- and/or z-axes and displays the

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1. ultrasonic waveforms on the corresponding graphic display. The software then displays the

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1. velocity measurement of the water. Compare the measured velocity to the theoretical value.

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1. When the graph shows an acceptable waveform click Accept,

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1.  or Cancel, then repeat the calibration

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1. procedure.
   

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1. Once accepted, the system delay times are saved to the configuration file and the

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1. calibration indicator and date time stamps are updated. If results are not acceptable, rerun the bayonet

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1. calibration.
   

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1. Repeat the calibration procedure for the other bayonet.

Calibrating the CALIPER

Caliper calibration is a little bit more involved. Because the distance between the calipers is variable, the system delay is determined using a set of standards of identical velocity but various lengths. The actual velocity of the standard is irrelevant, but is good to know as a quality check. To determine the system delay time, the first arrival time is plotted against distance and the intercept with "zero distance" equals the system delay (Figure 17). As a quality check the the slope will equal the velocity of the material used. This value should be close to the known value and the r-value (fit) should equal 1 (or be very, very close). The calibration should look like the example shown below (note: the delay time will change as the transducers wear).

Figure 17. Caliper System Delay.
In addition to the delay time determination, the LVDT output (volts) must be calibrated to distance in millimeters (Figure 18). The known lengths of the same standards are used and both calibrations are performed at the same time. As standards are measured, the LVDT calibration improves, so previous measurements are constantly recalculated. This is one of the reasons why at least 6 measurements are required before accepting calibration.

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Figure 18. LVDT Calibration.

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Figure 22. Standard Acquisition.

3. To stop the calibration, click the Stop – Return button on the Standard Information box (Figure 21). To resume measurement, click the Measure A Standard button on the Caliper Calibration window (Figure 22). To complete calibration, at least six standards must be measured; however, additional standards can be added or any standard can be measured multiple times.

Editing Calibration Points

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1. Click the Stop – Return button on the Standard Identification box (Figure 21).
2. Locate the Index number of the point containing the wrong information on the Caliper Calibration screen and type it into the Index field (Figure 24).
3. Click Delete by Index to delete data for that point from the calibration.
4. Remeasure the standard using the Measure A Standard button.
5. Other options in the Array section of the Caliper Calibration screen include deleting only the last measurement (Delete Last) or deleting all data points and starting a new calibration (Delete All Data and Start Again) (Figure 24).

Image AddedImage Removed
Figure 24. Array Section of the Caliper Calibration Screen.

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1. Start the Velocity application by clicking the icon (Velocity 2.0.5) on the desktop.
2. Once the application is launched, click the Make a Measurement button.
3. In the Scan Sample Label dialog box, place the cursor in the Scanner String field (Figure 25).

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Figure 25. Scan Sample Dialog Box.

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1. For Section w/liner mode, a Position Sample dialog box opens (Figure 26). Confirm the sample position as measured by the laser (the laser at the end of the track measures the range to the end of the core and calculates the position in the liner based on an offset for that station). Click OK.

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Section-half measurements, in the liner. (X-Axis)Section-half measurements, in the liner. (X-Axis)Discrete sample (MAD cubes) measurements. (Z-Axis)Discrete sample (MAD cubes) measurements. (Y-Axis)Discrete sample (MAD cubes) measurements. (X-Axis)Piece measurements taken out of the section-half. Consolidated material only. (Any-Axis)Section-half measurements, in the liner. (X-Axis)

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