An Experimental Study and Analytical Approach for Estimating Rock Properties Using Sound Levels from Drilling
a report by Harsha Vardhan and Badal Yadav Department of Mining Engineering, National Institute of Technology Karnataka
Knowledge of rock properties, such as compressive strength and density, is extremely important for effective blast design in mines, quarries and other construction projects. The estimation of rock properties is also useful in deep oil well and exploratory drilling to select suitable drill tools for enhanced output and better monitoring of drilling by the crew. Furthermore, drilling parameters such as thrust, revolutions per minute (RPM), etc., can be controlled for an optimum penetration rate and longer life of the drilling tools. Rock properties for deep oil well drilling, exploration, mining and construction sites are not readily available. Therefore, blocks of rock need to be sent to an established national laboratory, which is time consuming. Testing chargers also need to be paid for their services. As the rock types and properties may change as they are excavated, properties need to be re- determined for these rocks. Testing on a continual basis may be difficult and the results may not be available in a timely manner.
There are various techniques for determining rock properties in the laboratory and field. However, the concept of determination of rock properties using sound levels produced during drilling has only been shown clearly in one recent publication,1
to the knowledge of the
authors. Most of the works on the application of sound levels are in other branches of engineering.1
Although a couple of studies in oil and gas
industries have proposed the ‘seismic while drilling’ technique for estimating rock formations, it is quite different from the technique of estimating rock properties using sound levels during drilling.1
Very few
publications are available on this subject for rock engineering purposes. The usefulness of sound levels in determining rock or rock mass properties has only been shown in two publications,2,3
and the need for
further work in this area has been highlighted. It is anticipated that the sound level when drilling in rocks of different physicomechanical properties will be different for the same type of drill machine. Bearing this in mind, the present research work was undertaken.
Study Objectives
Noise measurement for the same type of drill machine varies from strata to strata, thus variations in sound level can indicate the type of rock. The objective of this research work was therefore to investigate the sound level from drilling in rocks with varying properties through laboratory experiments. An analytical approach to the same has been introduced in this paper for estimating rock properties.
Laboratory Investigations
In the laboratory, all of the sound level measurements were conducted on a pneumatic drill machine operated by compressed air. A percussive drill set-up1
to drill vertical holes was used to carry out the
drilling experiments for sound level measurement on a laboratory scale. Sound level on a pneumatic drill set-up was measured for five different rock samples – gabbros, granite, limestone, hematite and shale – measuring approximately 30x20x20cm. Sound levels were
© TOUCH BRIEFINGS 2011 Study Results
The sound level near the drill bit with varying rock properties is discussed here as it is the crux of this paper.
The results of the experimental study for the compressive strength and abrasivity of the rock samples are given in Table 2. It can be seen that, with the increase in compressive strength of rock samples, the abrasivity decreases. This is due to an increase in the resistance of rocks to wear with an increase in compressive strength. The noise spectrum at the operator’s position for test conditions A1 and A2 are shown in Figure 1, whereas the spectrum for test conditions A2, A3 and A4 are shown in Figure 2. The detailed analysis of Figures 1 and 2 as well as the effect of rock properties on the sound level of a pneumatic drill at the operator’s position, near the drill rod and at the exhaust, is discussed in detail by Vardhan et al.1
The effect of compressive strength and abrasivity of rock on the sound level near the drill bit at various levels of thrust and air pressure is given in Table 3. In general, an increase in sound level was observed at each increase in thrust and air pressure, with an increase in compressive strength and a decrease in abrasivity of the rocks. The difference in the sound level at an air pressure of 5.0 kg/cm2 and with an increase in thrust from 160 to 360 N varied from 0.9 to 1.9 dB. At air pressures of 5.5, 6.0 and 7.0 kg/cm2, this sound level difference in different rocks varied from 1.2 to 2.1 dB. This shows that an increase in compressive strength and a decrease in abrasivity of rock significantly increase the sound level.
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measured with a Larson-Davis model 814 integrating-averaging sound-level meter. Both the noise spectrum and equivalent continuous sound levels were determined1
using this equipment. The compressive
strength of rock samples was determined indirectly using Protodyakonov’s apparatus,1
using the International Society of Rock Mechanics standards.4
A set of test conditions was defined for the measurement of sound spectra in granite,1
whereas the abrasivity was determined
which is given in Table 1. For the test conditions A2,
A3 and A4 (which are the same as those mentioned in Table 1), the air pressure was constant at 6.0 kg/cm2. For measuring the variation in sound level while drilling in rocks of different compressive strengths and abrasivities, the rock samples were kept beneath the integrated drill rod of the pneumatic drill. Sound level measurements were carried out for thrust values of 160, 200, 300 and 360 N on each rock sample. For each thrust mentioned above, the A-weighted equivalent continuous sound level was measured by holding the sound level meter at a 15 cm distance from the drill bit for air pressure values of 5.0, 5.5, 6.0 and 7.0 kg/cm2. For a particular condition at each microphone location and for the same rock sample, the sound level was determined five times in relatively rapid succession. The arithmetic average of the A-weighted sound pressure levels from each set of five measurements was computed to yield an average A-weighted sound level for a particular condition.
Geology
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