Introduction

The P-wave velocity gantry measures the speed at which ultrasonic sound waves pass through materials that are placed between its transducers. The three orthogonal sets of piezoelectric transducers allow the velocity to be determined in the X-, Y-, and Z-directions (Figure 1) on working-half split-core sections. The P-wave bayonets (PWBs) measure the velocity along the core (Z-direction) and across the split-core face (Y-direction), and a P-wave caliper (PWC) measures the velocity perpendicular to the split-core face (X-direction). A laser measures the position of the top of the split-core section, and using this, along with the known distance to the transducers, calculates the position at which the velocity was measured (this is recorded as Offset in the LIMS Database). For discrete sample cubes, the velocity is measured along each of the three axes separately using the PWC. Mini-cores are measured along the axis of the cylinder (X-direction) using the PWC. For discrete samples cubes and mini-cores, all sample information, including offset, is entered by the user.

Velocity data generated from these sensors are part of the physical properties suite of shipboard sample measurements.

Figure 1. Section half measurement directions.

Method Theory

Measurement of P-wave velocity requires an estimate of the 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 thickness of the sample, or path length, is derived from the readout of an Acuity AR700 laser displacement sensor. The laser offset correction is determined during the calibration process and requires that the system fully close the transducer caliper when the software 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 caliper separation is derived from an Acuity AR700-4 Laser Displacement Sensor and calibration constants.
Traveltimes for sample half sections are calculated as follows:
x-axis = total traveltime – x-system delay time – liner traveltime (section-halves only)
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).

The travel times for any measurement signal is based on either a user selected location on a graphical display of the first arrival wave (manual pick), or a software auto-pick feature, which attempts to determine the first arrival of the measurement signal.  The auto-pick feature will search for the first instance of a signal stronger than a user set threshold value (milli-amps).  Next it will take the absolute value of the signal and determine where the wave-form crosses the graph's x-axis the 2nd time.  It will then subtract an assumed 1/2 of wave length, and display the pick location on the same graphical display.  This should get the travel-time of the measurement signal.  Verification of the signal and pick location by the user is critical.

P-Wave Measurement Overview

A temperature-equilibrated split core section-half  in it's liner is placed on the core track.  The user positions the section half under the sensor and triggers the measurement from the software control panel. The measurement is taken continuously, with the recorded result representing the average of several thousand determinations.  A barcode reader records all relevant sample information, which is used for the data upload into LIMS. A laser sensor measures the distance to the top of the section-half and determines the sampling interval from the known sensor offsets.  Discrete measurements are discussed in the Caliper Measurement section below.

Caliper Measurement

• The sample is placed between 2 flat, 1 inch diameter sensors that squeeze firmly onto the specimen to ensure good contact.  De-Ionized (DI) Water is introduced between the sample and sensors to aid in signal propagation.
• One sensor acts as a transducer and the other as a receiver to record velocity measurements at a rate of 0.5 MHz.
• To measure discrete samples, the sample is measured along each (x, y, and z-) axis between the caliper sensors.
• A series of polycarbonate standards of different thickness are measured to obtain a linear regression transit time vs. distance for calibration. Their industry-calibrated standard sonic velocity is 2.750 m/s.

Bayonet Measurement

• Two pairs of piezoelectric transducers set at 90° to each other are inserted into the unconsolidated or semisoft section-half materials until the sensors are buried in the material to be measured.
• One of each pair of sensors acts as transducer and the other as receiver to measure velocity in two directions simultaneously.
• Calibration is performed by inserting the probes into a container filled with distilled water of known temperature, and therefore known velocity.

Data Quality

Velocity data quality is affected by several variables:

• Quality of the acoustic coupling between the core material and the sensor transducers. Note: Use water to increase the quality of the contact.
• Quality of the coupling between both the transducer and the core liner and between the core liner and sample. Note: Use water to increase the quality of the contact.
• Consolidation of the sediments; noncohesive sediments containing microcracks or gas voids cannot be measured accurately.

Apparatus, Reagents, & Materials

Hardware

The velocity track system consists of the following components (Figure 2):

• Caliper transducers
• Chisel-type transducers (bayonet transducers)
• Linear actuator
• Laser sensors
• Barcode reader
• Aluminum and Acrylic Standards

              Figure 2a. P-Wave Velocity Transducer Components.                                                                  Figure 2b.Velocity Standards.                                                                                             Figure 2c. Range Laser

Caliper Transducers: Panametrics-NDT Microscan Delay Line Transducers

Bayonet Transducers

Custom made.

Exlar Linear Actuator

Acuity 1000 Laser Distance Sensor

Acuity AR700-4 Laser Displacement Sensor

Microscan MS-4 Ultracompact Imager Barcode Reader

Software

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.

CAUTION: When first opening the program, all 3 sensors will go through a communication initiation process where they will move down and up; the program will display a warning message first. However, make sure there are no samples or body parts underneath any of the sensors.

Figure 4, seen below, shows the home screen, which is the main program interface window. There are a number of graphical displays and adjustable settings. These are shown in detail on later figures.

Figure 4. Program home screen.

Configurables: Aquisition Parameters and Hardware Control

The software home screen allows the user to modify a number of setup and acquisition parameters directly, as well as control the motion of the transducers.

Experiment Setup

• Caliper vs. Bayonet and Measurement Axis:  The user selects the type of sample, while simultaneously selecting the measurement axis via the pictured tiles
• Stack Level: maximum ± voltage level that the stacked signal must attain for a successful first pick calculation. The acquisition stacks for 10 seconds to arrive at this value. If after 10 seconds the value has not exceeded 5 V, the measurement is aborted. If this happens, reposition the sample under the transducer. Do not change the Stack Level value.
• Stack Iterations: number of stacks that exceed ±5 V, averaged to reduce noise. Increasing this value will increase acquisition time.
• Threshold Level: value used to detect the initial peak and subsequent first arrival. Do not change this value.
• Peak Width: value used to detect the initial peak and subsequent first arrival. Do not change this value.
• 2 MHz Filter: value used to filter out electrical noise. This value is set at user's discretion.

Configuration Editor: Velocity Set Parameters

The Station Setup window (Figure 5) stores the physical configuration measurements of the caliper and bayonet transducers, as well as the velocity of the core liner and the velocity filter limits.

• Axis Offsets: physical offset from the laser zero point to the center of the transducer pair for the three stations (caliper, bayonet Y, bayonet Z). This measurement will not change unless the station location is physically changed.
• Axis Separation: physical distance between the bayonet transducer pairs. This measurement should never change unless the bayonets holders are physically changed.  The X-Axis is not shown because its separation is determined during calibration and measurement by the displacement Laser.

Figure 5. Configuration Editor: Station Velocity Setup Tab.

Bayonet Calibration Tab

This tab (Figure 8) shows the results of the latest bayonet calibration. Values cannot be changed.

• Y-Bayonet axis system delay: water velocity corrected to temperature for Y-bayonet.
• Z-Bayonet axis system delay: water velocity corrected to temperature for Z-bayonet.

Figure 8. Configuration Editor: Bayonet Calibration Tab.

VISA Resources Tab

This tab (Figure 9) shows the alias locations for the serial ports configured using LabView Measurement and Automation Explorer Program. Only Application Developers can change the values on this tab.

Figure 9. Configuration Editor: VISA Resources Tab.

File Paths Tab

This tab (Figure 10) shows the file paths used by the application to store the configuration file and data files. Only Application Developers can change the values on this tab.

Figure 10. Configuration Editor: File Paths Tab.

Standards

• Laboratory reagent water (distilled)
• Aluminum cylinder of known length and velocity; velocity = 6295 m/s
• Acrylic (as above) half-cylinders of variable thicknesses to accommodate transducer placement on the half-core section. As of August 2010, the velocity is ~2950 m/s but subject to further verification.

Figure 11. P-Wave Standards

Instrument Calibration

The bayonet and caliper transducers must be calibrated and the system delay determined whenever a check standard is out of range, +/-2% of the expected velocity.

• The bayonet transducers are calibrated by back-calculating the system delay from the total traveltime, for the y- and z-axes, from the transducer separation value and theoretical velocity of distilled water at the temperature of the water bath.
• Calibration of the caliper requires the transducer and system delay to be calibrated concurrently. The transducer separation is measured with a displacement laser.

System delay is derived from the separation entered by the operator for the calibration standard and traveltime is derived from a traveltime pick algorithm. As the current transducers have pliable plastic pole faces the system delay is on the order of 25 µs, closing the calipers too hard produces unwanted variation.
The calibration uses the derived velocity of the standard material to test for acceptance or rejection of the data.  Aluminum, the material used as the calibration standard for the caliper, has a published sound speed of 6925 m/s.

Calibrating the CALIPER

Calibration requires correcting the displacement laser's offset and determining the system delay (Figure 7a/7b).  Calibration is completed with an aluminum block of the following approximate dimensions; 50.80 x 76.25 x 30.60mm.

Procedure

1. Place the aluminum block in the caliper and add a little water on the bottom transducer and atop the block.
2. Close the caliper on the aluminum block along an axis of known dimension.  Commonly the 50.80 mm axis is used as it is closest in thickness to a section-half.  Do not over close the transducers as a greater force will vary the physical distances of the caliper system. Observe the graph to verify the system is getting a clean signal.  If you are not getting a clean signal, follow the steps in the trouble shooting section of this guide.
3. Verify the software is picking the first arrival wave in the Raw Stacked graph tab. You can adjust the threshold scale bar to achieve the proper pick location (7b).
   Also, verify the auto-pick location in the Absolute Value tab (ABS or ABS Zoom).  This pick should be at the zero crossing after the first wave form (Figure 7a).
4. Set the Laser Offset Correction by entering or verifying the Aluminum width, i.e. 50.80 mm, and clicking the button.
5. Determine the system delay by entering or verifying the Aluminum Velocity.  Given to be 6295 m/s, and next clicking the button.
6. If the displayed velocity is correct, accept changes.
7. Lastly, as a check on your calibration, measure the velocity of the aluminum block and an acrylic standard to verify it is within an expected error margin, +/- 2.5%.  The assumed Acrylic Velocity as of June, 2019 is 2730 m/s.  It is recommended to use an acrylic standard of a similar thickness to what the user will be         measuring, i.e. a core section or cube sample.

                                                        Figure 7a. Caliper Calibration Window: ABS Zoom Graph Tab.                                                                                                                                                  Figure 7b. Caliper Calibration Window: Raw Graph Tab.                                             

Calibrating the Bayonet Transducers

Because the distance between the bayonets is fixed, only the system time delay needs to be calculated. This is achieved by measuring the velocity in water of known temperature, the difference between the known and measured value is due to system delay.

Procedure

1. Prepare a water bath (Figure 8a) 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 note the temperature of the water bath.
3. On the Main window, select Station  > then Y (or Z) Bayonet Calibration
4. Enter the temperature of the water bath into the Temperature field on the calibration screen. (Figure 8a/8b).  The screen displays a plot of theoretical velocity of water vs. temperature. Lower the bayonets into the water bath (the black transducer should be covered as shown in Figure 9).  Observe the graph to verify the system is getting a clean signal.  If you are not getting a correct pick or clean signal, follow the steps in the trouble shooting section of this guide.
   

5. Verify the software is picking the first arrival wave in the Raw Stacked graph tab. You can adjust the threshold scale bar to achieve the proper pick location (8a) as long as the signal i not too noisy.  Also, verify the auto-pick location in the Absolute Value tab (ABS or ABS Zoom Figure 8b).  This pick should be at the zero crossing after the first wave form (Figure 8b).  An algorithm calculates the temperature-corrected velocity of the water bath and displays the result in the Corrected Velocity tile. 

6. Click the Determine System Delay button.  The computer calculates the system delay based on the separation distance and theoretical velocity of water.  Compare the corrected velocity to the theoretical value.  If in range click Accept, or repeat the calibration procedure.
7. Repeat the calibration procedure for the other bayonet.

                                                        Figure 8a. Y-Bayonet Calibration Window: ABS Zoom Graph Tab.                                                                                                                                                  Figure 8b. Z-Bayonet Calibration Window: Raw Zoom Graph Tab.                         

Figure 9. Bayonet Transducers in Water Bath

Sample Preparation

Two types of samples can be measured using the velocity gantry:

• Section-half core samples
• Discrete samples

Sample Prep Overview

• Before placing the working half in the core tray, make sure the surface is clean (lightly scrape away any material smeared across the cut surface during core splitting).
• Place the core section in the tray and make sure it is as flat as possible.

Instrument Preparation

• Clean transducers of any residue with water and paper towels.
• Before initializing the P-wave software, ensure the caliper transducers and laser beam are not blocked. Note: This step is crucial.

Sample Analysis

Measuring Section-Half Samples

Before measuring samples, be sure the samples are properly prepared and the system is calibrated.

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).

Figure 25. Scan Sample Dialog Box.

4. Scan the barcode on the section half label to populate the fields on the Scan Sample Label dialog box. Enter Operator (user last name) and confirm measuring Station and Mode.

Stations Bayonet Y and Bayonet Z are for soft sediments. To use a Bayonet measuring station, the Mode value must be Section w/liner (material is still in core liner). Note: The orientation assigned to the sample measurement depends on the Station selected.

5. 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.

Figure 26. Position Sample Dialog Box.

6. The bayonet transducers lower into the section half; ensure they are completely inserted into the sample. If contact between the transducers and sediment is poor, add distilled water around the transducers to improve contact with sediment.

7. On the JOG Bayonet screen (Figure 27), monitor the waveform and position the transducer to obtain the best trace. The plot displays the live signal with no stacking. A weak signal is fine, as long as a clean first arrival is present. Once the waveform is acceptable, click Continue to begin sample acquisition.

Figure 27. JOG Bayonet Y Screen.

8. During acquisition, the program adds multiple waveforms until the total (max peak) is greater than ±5 V. This process is repeated and the waveform is averaged.

Measuring Discrete Samples

Discrete samples measured on the Cailper station are hard material that has been cut from the core as cubes, minicores, or slabs. Because the material can be measured in various orientations, the user must select the measurement axis (see Figure Appendix A).
Traveltime is calculated as total traveltime minus x-system delay time. Discrete sample measurements are not corrected for the core liner nor is an offset recorded in the LIMS. The transducer separation is measured with an LDVT as it is for the sample half measurement. For discrete samples the axis of measurement is selected for each measurement.
Retaining the correct "up" direction is critical for axis determination and P-mag orientation; make sure the sample has the "up" direction marked on it (see Figure Appendix A).
Before measuring samples, be sure the samples are properly prepared and the system is calibrated.

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 28).

Figure 28. Scan Sample Label Dialog Box.

4. Scan the barcode on the discrete sample label to populate the fields on the Scan Sample Label screen. Enter Operator (user last name) and confirm measuring Station (Caliper) and Mode (Discrete).

5. Enter information requested in the Additional Discrete Information dialog box (Figure 29): Measurement Axis: X, Y, or Z.

Figure 29. Additional Discrete Information Dialog Box.

6. If the material to be measured is a whole piece removed from the liner (rarely used), select Piece w/o Liner mode in Step 4 and complete the offset (position of the piece in the liner before it was removed) in the Additional Piece Information dialog box (Figure 30).

If the orientation of the piece is unknown, check the Orientation Unknown box; otherwise the measurement is assigned to the X-direction.

Figure 30. Additional Piece Information Dialog Box.

7. Place the discrete sample in the caliper and add distilled water to create a good sample-transducer contact if needed.

8. On either the Additional Discrete Information (discrete samples) or Additional Piece Information (piece samples) box, select OK - AutoClose (for hard samples) or OK – Manual Close (for softer/friable samples).

AutoClose automatically closes the caliper on the sample. The transducer moves to a predefined position or until the actuator stalls.

9. On the JOG Caliper screen (Figure 31), monitor the waveform and position the transducer to obtain the best trace. The plot displays the live signal with no stacking. A weak signal is fine, as long as a clean first arrival is present. Once the sample is properly positioned, click Continue to begin sample acquisition.

Figure 31. JOG Caliper Screen.

10. During acquisition, the program adds multiple waveforms until the total (max peak) is greater than ±5 V. This process is repeated and the waveform is averaged.

The touch-screen Open/Close commands may be sluggish in response. If there is no response, try actuating the button using the mouse.

11. Repeat the measurement process for each axis to be measured.

Accepting Results and Manual Pick

Once the velocity data have been acquired, the results are shown on the Display Results screen (Figure 32).

Figure 32. Display Results Screen.

The Manual Pick Velocity field remains blank until the user clicks the Manual Pick Arrival Time button and sets a manual pick time. The first arrival should be picked in the same manner the system was calibrated. Both Manual Pick and Auto Pick are saved with the sample data.

1. Select the Manual Pick Arrival Time button to open the Manual Pick screen (Figure 33.A), showing the waveform. To help select the first arrival, the plot can be zoomed in using the graph palette zoom function or by entering Zoom Range Minimum and Zoom Range Maximum (27. to 40.) below the waveform plot on the Manual Pick screen. Select the Cursor Movement Tool on the graph palette, then use the mouse to drag the red cursor line to the first arrival, this should be the slope of the first positive peak at zero (Figure 33.B).
2. Click Continue to accept the manual pick and save it to the data file or Cancel to discard the manual pick results.

Figure 33.A Manual Pick Screen. 

Data Handling

From the Display Results screen one of 4 data handling choices can be selected:

• Exit Save Data: saves data and returns to the Sample Information dialog box for the next sample.
• Cancel Without Saving Data: does not save the data but returns to the Sample Information dialog box. Use this option if there is a data quality warning or if the waveform for the sample was unacceptable.
• Repeat Without Saving Data and Keep ID: does not save data but returns to the JOG screen where the user can reposition the transducer and take another measurement of the same sample.
• Save Data and Repeat with Same ID: saves the data and returns to the JOG screen where the same sample can be run again. This option allows replicate measurements.

LIMS Integration

Sample/Analysis Attributes and Components

Data Upload Procedure

Before running a sample or completing a calibration, you must start the Data Uploader so the data uploading process will run in the background looking for files to upload and every section measured on the logger will automatically upload to LIMS.

1. Open the LIMS Uploader icon on the desktop.
2. Click Start Monitoring to save data files and automatically upload calibration files to LIMS.
3. When data collection is completed, click Stop Monitoring.

Health, Safety, & Environment

Safety

• Keep extraneous items and body parts away from the moving parts.
• The track system has a well-marked yellow and red emergency stop button (Figure 34) to halt the system if needed.
• Do not look directly into the laser light source (class 2 laser product).
• Do not direct the laser beam at other people.
• Do not attempt to work on the system while a measurement is in progress.
• Do not lean over or on the track.
• Do not stack anything on the track.
• This analytical system does not require personal protective equipment.

Figure 34. Emergency Stop Button.

Maintenance & Troubleshooting

Common Issues

The following are common problems encountered when using the P-wave Velocity Gantry and their possible causes and solutions. For information about the laser Measurement and Operation software, see the Vp GIESA Gantry Laser Operation manual.

 Actuator Utilities

The actuator utilities can be used to test the caliper and bayonet actuators (move them up and down) and view the waveform detected by the transducers. There are also utilities to test the home position of the actuators and to check the laser function. This utility is useful when configuring the station offsets.

To change the behavior and/or force of the actuators, the Exlar utility must be used.

Open the Actuator utility (Figure 35) from Main > Actuator Utility and then select one of the JOG screens, Home All, or Read Laser.

Figure 35. Actuator Utilities.

Scheduled Maintenance

After Every Sample

Clean contact sensors with tissue, cloth, or paper towel and water.

Daily

Clean dust from the laser sensor lens using compressed air or delicate tissue wipes. Do not use organic cleaning solvents on the sensor lens.

Weekly

Check set screws on piezoelectric transducers to make sure they are firmly held in position when embedding in sample.

Annually

Coat (not pack) the following parts with grease:

• Angular contact thrust bearings
• Roller screw cylinder
• Roller screw assembly in the actuators

Examine the cable management system for abraded cables or other indications of wear.

Vendor Contact/Consumable Parts

Panametrics transducer

Olympus (www.olympusndt.com)

• Transducer: NDT M2008

Transducer couplant

• Couplant A
• Couplant B

Laser displacement sensor (AR1000)

Acuity Laser Measurement
www.acuitylaser.com
702-616-6070

Barcode reader (MS-4)

Microscan
www.microscan.com
helpdesk@microscan.com
800-251-7711

Actuator (TLM 20)

Tritex
www.exlar.com

Related Documentation and Links

• Vp GIESA Gantry Laser Operation manual
• Olympus Panametrics-NDT Ultrasonic Transducers
• Acuity Laser Measurement AccuRange AR1000™ Laser Distance Sensor User Manual
• Acuity Laser Measurement AR1000 Laser Distance Sensor: Quick Start Guide
• Acuity Laser Measurement AR1000 Laser Distance Sensor Data Sheet
• Exlar Tritex Linear & Rotary Actuator Installation, Operation, and Service Manual
• Exlar Tritex Technical Notification 4/12/07

Credits

This document originated from Word document P-Wave Velocity Gantry: User Guide (see Archived Versions below for a file copy) that was written by T. Cobine (2009-01-17), reviewed by H. Barnes and T. Gorgas, and approved by D. J. Houpt. Credits for subsequent changes to this document are given in the page history.

Archived versions

• P-Wave Velocity Gantry: User Guide: Word file written by T. Cobine (2009-01-17), reviewed by H. Barnes and T. Gorgas, and approved by D. J. Houpt