SQS SYSTEM- STONE QUALITY AND SOUND

(by Giuseppe Lenzi and Stefano Limonta)

(This article appeared in the print edition of Litos in January,99).

1. Not Destructive Tests (N.D.T.) and quality evaluation of the materials

Within a generic industrial process, the final product must be controlled. Usually this control is carried out submitting a product sample to critical tests just to evaluate the performance limits. Some time this approach cannot be applied for different reasons; in the last years the industry developed not critical methods to evaluate the goodness and the performance of its products. In the metallurgic industry for example the welds are often controlled with penetrating fluids or, in particular situations, with X-ray.

The basic assumption of N.D.T. methods is the full knowledge of the reaction of a "good" product to a specific not critical test ("reference reaction"). Another sample product will be "bad" if the its test reaction is different from the "reference reaction".

2. The sonic methods

The sonic methods hold an important position in the N.D.T. landscape. The basic idea of sonic methods is the analysis of the mechanical energy propagation (elastic wave - pressure - sound) accross the materials. If different materials (shape, composition, phisical conditions, ...) are mechanically stimulated, also the energy propagation inside those materials will be different. If the mechanical energy propagation accross a specific material is known, it will be possible to compare the behaviour of any other sample of that material under sonic stimulation.

Before the train departure from the station, the train crew control the iron wheels using an hammer: the presence of a crack can be discovered by listening the emitted sound from the percussion. A "strange" sound can declare a possible wheel failure.

The sonic tests are carried out by the measurement of the necessary time for the sound to cover the distance between the "sound source" and the "ear" (sound receiver). If this distance is exactly known, it is possible to compute the average velocity of sound propagation for a specifi material.

Density and elastic properties of materials affect the sound velocity. The sound "will run" very quickly in a very solid and elastic material.

Different physical factors can change the density and the elastic properties, hence the sound velocity: temperature, pressure, porosity, water content and others. In the cold air the sound is faster than in hot air; infact the cold air is more dense than hot air.

These considerations suggest to apply the sonic method to establish the influence of those physical factors on materials. The following table show the density and the sound velocity for some common materials (J.R.Frederick: Ultrasonic Engineering - John Wiley & Sons, 1965):

3. Application of the sonic method to the quality evaluation of stone blocks

The value of a stone block is also related to its homogeneity level. In general, at the end of the block cutting, the people hope to get uninjured slabs with an uniform color. Hence we have a double problem:

• detection of crack and voids inside the block
• evaluation of homogeneity level of the material

This specific application of the sonic test is simple and profitable. While in other geo-exploration fields the knowledge of the environment to be explored might be very little, for quarry stones the reference status is perfectly known: the material is generally solid and homogeneus. The sonic tests can furthermore be executed along three different directions. In this way it is possible to detect a plane fracture that, although invisible to a sonic control executed along a section which is planar to the defect, can be detected from different sides of the block (fig. 1).

Thanks to the versatility of the sonic method, the SQS equipment can be used to control the quality of different types of civil construction materials.

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Fig. 1 - The sonic method to evaluate the quality of stone blocks

The sonic test of stone blocks can be carried out by different approaches.

A stone block can be qickly controlled with a few measurements of sound velocity along the same direction but in different points of two opposite block sides and along different directions, changing the block sides. If the sound velocity, related to a specific couple of source/receiver, is lower than in the other positions, there is a failure between the source point and the receiver point.

A detailed analysis can be carried out by measurements of sound velocity along profiles: 2 or 3 for each direction of the block, executing horizontal and vertical scans. The fig. 2 show a sonic test on a granite block, along a detailed vertical profile. After block cutting, the central slabs showed the presence of an important fracture in their lower part.

Fig. 2 - An example of the sonic test result

Of course the sound velocity evaluated with this method is the average velocity value along the distance between the source and receiver points.

Voids, cracks or faults can be detected easily with the sonic methods; infact a void is a strong obstacle for the energy sound transmission. Hence a void reduce the velocity and the amplitude of the trnasmitted sound.

The evaluation of the color homogeneity of a stone block is based on the assumption tha a different color is related to a different material. Density or porosity changes for example can be associated to material changes inside the same block. The following table show the connection between density, porosity and sound velocity for some granite types ("Practical HandBook of Physical properties of Rocks and Minerals", CRC Press 1989):

4. The equipment

The SQS equipment - Stone Quality and Sound - is very simple and user friendly, it doesn't need any specific competence. The fig. 3 show the SQS equipment and its components. The sonic signal processing is automatic and after the test you get an immediate quality response.

SQS is simple in its appearence but it involve high technology components for the A/D (Analogic to Digital) conversion of signals, for the recording of data, its processing and for results presentation.

The signal is acquired using a hammer and a sonic receiver and it is automatically shown by the internal display together with the sonic velocity.

Before the sonic test you need to define the geometry of the section (distance between source and receiver points); this information and the travel time knowledge will enable the immediate quality analisys.

The use of the hammer as sound generator guarantees an high power sonic impulse, able to cross the block. The operator can verify directly the quality of the transmitted impulse and the value of sound velocity automatically computed, along a specific direction. The measure will be repeated along different directions and the comparison between the velocity values give us informations about the eventual presence of defects inside the block: high velocity values show homogeneous material, low velocity values are related to an internal failure.

5. Application of geophysical methods to the characterization of stone quarry

Sonic and electromagnetic techniques can be applied to explore the quarry. How much thick is the stone volume ? Where is the beginning of the stone under the ground ? Are there fractures within the quarry stone ?

Some geophysical methods could be used to approach these problems. In this century exploration companies developed complex and powerful methods to discover natural oil deposits. Refraction and reflection seismic techniques use sonic waves to investigate the underground. These waves, produced by explosive or other sources, cross the rock and are detected from a sonic receivers network placed on the earth surface. The analysis of the received sonic signals give detailed information about ground composition differences and discontinuities in the earth. The sonic waves are reflected from the separation surface between the rock and the sand for example, and can be diverted by soil velocity changes.

But refraction and reflection seismic methods are very expansive and time consuming and only some specific application, such as the oil research, can justify large money investments.

GPR (Ground Penetrating Radar) is another geophysical method that can be applied to characterize a quarry. The equipment is usually simple and the exploration times are short. In a few days it is possible to study a large quarry.

GPR generate an high frequency electromagnetic impulse that, by an antenna, penetrate in the rock. When the radar impulse hits a fracture or a discontinuity, it is reflected and return to the same antenna. The analysis of these radar impulse reflections (radar echoes) allow to design the shape of the underground along two dimensional sections. Infact the radar antenna is usually moved along a line on the surface. At each point of this surveying line a radar impulse leaves the antenna and the eventual echo of the same impulse is received. In general the commercial radar equipments collect all the emitted and received impulses to produce an image of the underground.

The data processing and interpretation can be some time quite difficult, but the application of GPR to stone quarry exploration is simple and particularly profitable. Usually a granite or marble quarry present large compact and homogeneus material volumes; the radar targets, fractures and discontinuities, are well visible inside an uniform background.

A big limit to GPR application is the water. Infact it attenuates and absorbs the radar impulses, breaking the penetration in the ground of the elctromagnetic energy.

Authors:

Giuseppe Lenzi and Stefano Limonta have a long time experience in Geophysical exploration and in the Not Destructive Techniques applications to civil structures. They developed the SQS equipment to evaluate the stone blocks quality.