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Ultrasonic Velocity and Attenuation Measurements in Geological Samples


Ultrasonic velocity and attenuation measurements in geological samples

Application

Ultrasonic velocity and attenuation measurements in geological samples such as rocks and minerals.

Background

Ultrasonic sound transmission techniques have been utilized to define the mechanical properties of geological materials. Specifically, these techniques have been used to quantify the physical parameters such as hardness, elastic modulus and grain structure of the material. These techniques have also been used to monitor the compaction of soil or sand under laboratory conditions. These tests can typically be performed on prepared specimens with two flat, parallel surfaces, using through-transmission techniques with low frequency transducers as described below. Field tests are normally performed at seismic frequencies well below the ultrasonic range.

Equipment

Prepared geological samples can typically be tested with ultrasonic flaw detectors such as the Olympus EPOCH 600, or ultrasonic pulser/receivers such as the Olympus Model 5077PR. Pairs of low frequency contact transducers are employed in both longitudinal and shear wave modes. The specific transducers used in a given case will depend on the acoustic properties of the test materials and the sample dimensions, but frequencies of 1 MHz and below are most commonly employed. The frequency should be selected to provide undistorted waveforms in addition to adequate penetration.

Procedure

Samples may be prepared in a variety of shapes and sizes as long as they have two flat, parallel surfaces for transducer coupling. Commonly they will be cylinders or blocks with thicknesses ranging from approximately 2” (50 mm) to 10” (250 mm) in the direction of measurement. Sample diameter or width should be larger than the element size of the transducer being used. Through transmission testing with separate transmitting and receiving transducers is normally employed due to the high level of scattering exhibited by most geological materials at ultrasonic frequencies. Set the instrument’s pulser to through mode and adjust gain and filtering for best response. In through transmission tests, the probe zero delay can be determined by touching the two transducers together and using that signal as the zero point. Gel couplant is useful for longitudinal wave testing to minimize absorption into porous samples. Special high viscosity couplant such as Olympus SWC must be used for coupling shear wave transducers.

Pulse transit time can be measured with either a flaw detector or an oscilloscope or digital waveform display connected to a pulser/receiver. Flaw detector software can also calculate velocity and measure relative signal strength in decibels. The examples below show typical waveforms from two setups.

Longitudinal wave
One-way transit time measurement in 3.75” (95 mm) sandstone tested with an Epoch 600 flaw detector, two V102-SB longitudinal wave transducers, and two V152-RB shear wave transducers (both 1 MHz, 1” diameter).
Shear wave
One-way transit time measurement in 3.75” (95 mm) sandstone tested with an Epoch 600 flaw detector, two V102-SB longitudinal wave transducers, and two V152-RB shear wave transducers (both 1 MHz, 1” diameter).

One-way transit time measurement in 3.75” (95 mm) sandstone tested with an Epoch 600 flaw detector, two V102-SB longitudinal wave transducers, and two V152-RB shear wave transducers (both 1 MHz, 1” diameter).

Longitudinal wave one-way transit time measurement in 8” (200 mm) rock salt tested with Model 5077PR pulser/receiver and two V101 transducers (500 KHz, 1” diameter).
Longitudinal wave one-way transit time measurement in 8” (200 mm) rock salt tested with Model 5077PR pulser/receiver and two V101 transducers (500 KHz, 1” diameter).

The measured velocities along with material density can be used to calculate elastic properties as described in this application note. Attenuation is correlated with physical properties like grain structure or micro-cracking experimentally, using reference samples representing known conditions to establish a calibration curve.

Products used for this application


5077PR

The 5077PR manually controlled ultrasonic pulser-receiver with 35 MHz (-3 dB) bandwidth and square wave pulser-receiver is ideal for maximizing response in scattering materials. The square wave pulser is particularly advantageous when testing with transducers of 10 MHz or lower.

EPOCH 1000 Series

The EPOCH 1000 is an advanced conventional ultrasonic flaw detector that can be upgraded with phased array imaging at an authorized Olympus service center. Key features include: EN12668-1 compliant, 37 digital receiver filter selections, and 6 kHz pulse repetition rate for high speed scanning.

EPOCH 600

The EPOCH 600 is mid-level, handheld ultrasonic flaw detector. Weighing only 1.68 kg (3.72 lb.), its horizontal case is built to withstand the rigors of very harsh environments. EN12668-1 plus features such as 400 V PerfectSquare tunable square wave pulser, digital filtering for enhanced signal-to-noise ratio.

EPOCH 650NEW

The EPOCH 650 is a conventional ultrasonic flaw detector with excellent inspection performance and usability for a wide variety of applications. This intuitive, rugged instrument is a continuation of the popular EPOCH 600 flaw detector with additional capabilities.

EPOCH LTC

The EPOCH LTC is a mid-level, handheld ultrasonic flaw detector in a compact 2.12 lbs (0.96 kg) vertical case. It is a full-featured instrument with EN12668-1 compliance and a wide variety of standard features as well as specialized options to meet your inspection needs.
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