Application of Sound Level for Estimating Rock Properties

The process of drilling, in general, always produces sound as a by-product. This sound is generated from the rock-bit interface, regardless of the material the bit is drilling in. The drillability of rock depends on many factors, like bit type and diameter, rotational speed, thrust, flushing and penetration rate. Sound is used as a diagnostic tool for identification of faulty components in the mechanical industry. However, its application in mining industry for estimating rock properties is not much explored. Knowledge of rock properties is essential for mine planning and design. The rock properties such as compressive strength, porosity, density etc. are uncontrollable parameters during the drilling process. The rock properties must be determined at a mine or construction site by testing a sample. There are various techniques for the determination of rock properties in the laboratory and the field. International Society of Rock Mechanics (ISRM) and American Society for Testing and Materials have suggested or standardised the procedure for measuring the rock strength. However, the method is time consuming and expensive. As an alternative, engineers use empirical and theoretical correlations among various physico-mechanical properties of rock to estimate the required engineering properties of rocks. Most of the works in the application of sound levels are in other branches of engineering (Vardhan et al., 2004, 2005, 2006; Vardhan & Adhikari, 2006). A couple of studies in oil and gas industries have proposed a technique called ろSeismic–While–Drillingわ for estimating rock formations. For instance, few studies have proposed the use of noise produced by the bit during drilling as a seismic source for surveying the area around a well and also for formation characterization while drilling (Onyia, 1988; Martinez, 1991; Rector & Hardage, 1992; Miranda, 1996; Asanuma & Niitsuma, 1996; Hsu, 1997; Aleotti et al., 1999; Tsuru & Kozawa, 1998; Hand et al., 1999; Fernandez & Pixton, 2005). A recent study (Stuart et al., 2004) has also reported a method of estimating formation properties by analyzing acoustic waves that are emitted from and received by a bottom hole assembly. It needs to be emphasized that ろSeismic–While–Drillingわ technique is different from the technique of estimating rock properties using sound levels produced during drilling. For rock engineering purpose, very limited publications are available on this subject. The usefulness of sound level in determining rock or rock mass properties has been shown


Introduction
The process of drilling, in general, always produces sound as a by-product. This sound is generated from the rock-bit interface, regardless of the material the bit is drilling in. The drillability of rock depends on many factors, like bit type and diameter, rotational speed, thrust, flushing and penetration rate. Sound is used as a diagnostic tool for identification of faulty components in the mechanical industry. However, its application in mining industry for estimating rock properties is not much explored. Knowledge of rock properties is essential for mine planning and design. The rock properties such as compressive strength, porosity, density etc. are uncontrollable parameters during the drilling process. The rock properties must be determined at a mine or construction site by testing a sample. There are various techniques for the determination of rock properties in the laboratory and the field. International Society of Rock Mechanics (ISRM) and American Society for Testing and Materials have suggested or standardised the procedure for measuring the rock strength. However, the method is time consuming and expensive. As an alternative, engineers use empirical and theoretical correlations among various physico-mechanical properties of rock to estimate the required engineering properties of rocks. Most of the works in the application of sound levels are in other branches of engineering (Vardhan et al., 2004(Vardhan et al., , 2005; Vardhan & Adhikari, 2006). A couple of studies in oil and gas industries have proposed a technique called Seismic-While-Drilling for estimating rock formations. For instance, few studies have proposed the use of noise produced by the bit during drilling as a seismic source for surveying the area around a well and also for formation characterization while drilling (Onyia, 1988;Martinez, 1991;Rector & Hardage, 1992;Miranda, 1996;Asanuma & Niitsuma, 1996;Hsu, 1997;Aleotti et al., 1999;Tsuru & Kozawa, 1998;Hand et al., 1999;Fernandez & Pixton, 2005). A recent study (Stuart et al., 2004) has also reported a method of estimating formation properties by analyzing acoustic waves that are emitted from and received by a bottom hole assembly. It needs to be emphasized that Seismic-While-Drilling technique is different from the technique of estimating rock properties using sound levels produced during drilling. For rock engineering purpose, very limited publications are available on this subject. The usefulness of sound level in determining rock or rock mass properties has been shown The pneumatic drill was firmly clamped at its top and bottom with the help of four semicircular mild steel clamps, which were in turn bolted firmly to four mild steel bushes for frictionless vertical movement of the unit over the two guiding columns of the setup. In order that the top and bottom clamps work as one unit, they were firmly connected with the help of four vertical mild steel strips (1.3 cm thick, 5 cm width and 50 cm length) on each side of the pneumatic drill. For increasing the vertical thrust, two vertical mild steel strips (1.3 cm thick, 5 cm width and 32 cm length) were bolted to the top and bottom clamps. On this strip, dead weights made up of mild steel were fixed with the help of nut and bolt arrangements. For conducting drilling experiments at low thrust level (less than the dead weight of the drill machine assembly), a counter weight assembly was fabricated. For this purpose a steel wire rope (0.65 cm diameter) was clamped to the top of the pneumatic drill unit which in turn passed through the pulley arrangements located at the top plate of the setup. A rigid frame was firmly grouted to the shop floor at a distance of 86 cm from the experimental setup. The steel wire rope from the experimental setup was made to pass over the pulley mounted on the rigid frame. At the other end of the rope, a plate was fixed for holding the counter weights. The dead weight of pneumatic drill machine and accessories for vertical drilling was 637 N. With the help of counter-weight arrangement, it was possible to achieve a desired thrust value as low as 100 N. Similarly, through the arrangement of increasing the thrust level, it was possible to achieve a thrust value as high as 900 N.

Rock samples used in the investigation
Sound level measurement on pneumatic drill set up was carried out for five different rock samples obtained from the field. These rock samples were gabbros, granite, limestone, hematite and shale. The size of the rock blocks was approximately 30 cm x 20 cm x 20 cm.

Instrumentation for noise measurement
Sound pressure levels were measured with a Larson-Davis model 814 integrating averaging sound level meter. The instrument was equipped with a Larson Davis model 2540 condenser microphone mounted on a model PRM904 preamplifier. The microphone and preamplifier assembly were mounted directly on the sound level meter. The acoustical sensitivity of the sound level meter is checked once a year. For all measurements, the sound level meter was handheld. To determine the noise spectrum, the instrument was set to measure A-weighted, time-averaged one-third-octave-band sound pressure levels with nominal midband frequencies from 25 Hz to 20 kHz. The sound level meter was also set to measure A-weighted equivalent continuous sound levels (Leq). For each measurement, the sound level meter was set for an averaging time of 2 minutes.

Determining the compressive strength and abrasivity of rock specimens
The compressive strength of rock samples was determined indirectly using Protodyakonov's apparatus. Protodyakonov index for estimation of compressive strength of rock samples is an indirect and time-consuming method. However, this method was chosen due to limited availability of any particular type of rock samples in the laboratory. Therefore, first sound level measurement using drilling was carried out. Then the same drilled rock block was used for determining compressive strength and abrasivity. It was difficult to prepare samples for determining uniaxial compressive strength from these drilled rock blocks. Abrasion test measures the ability of rocks to wear the drill bit. This test includes wear when subject to an abrasive material, wear in contact with metal and wear produced by contact between the rocks. For this purpose, Los Angele's abrasion test apparatus was used. The results of the experimental study for the compressive strength and the abrasivity of the rock samples are given in Table 1. It is seen that, with increase in compressive strength of rock samples, the abrasivity decreases. This is due to increase in the resistance of rocks to wear with increase in the compressive strength. Typical thrust values in the field may vary from 150 to beyond 500 N. For each thrust mentioned above, the A-weighted equivalent continuous sound level (L eq ) was measured by holding the sound level meter at 15 cm distance from the drill bit, drill rod and the exhaust for air pressure values of 5.0, 5.5, 6.0 and 7.0 kg/cm 2 . Similarly, the L eq level was measured at the operator's position for each thrust of 160 to 360 N and air pressures of 5 to 7 kg/cm 2 as mentioned above. The operator's position refers to the position of the operator's ear which was at a height of 1.7 m from the ground level and 0.75 m from the center of the experimental set-up. During measurement, all the doors and windows of the room were kept open so as to reduce the effect of reflected sound. For a particular condition, at each microphone location and for the same rock block, 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.

Noise assessment of pneumatic drill under various test conditions at operator's position
The noise spectrum at the operator's position for test conditions A1 and A2 are shown in Fig. 2. It is seen that the background sound level at the measurement location due to the operation of the air compressor alone is below 82 dB with the nominal one-third-octave midband frequencies from 25 Hz to 20 kHz. Also, the increase in sound level with midband frequencies above 50 Hz is more than 10 dB for test condition A2 relative to that of test condition A1. Therefore, the sound level in the frequency range of 63 Hz to 20 kHz for test condition A2 is unlikely to be affected by the background noise due to the compressor. However, the sound level for test condition A2 may be affected due to test condition A1 with nominal midband frequencies from 25 to 50 Hz as the difference in sound level in this range of frequency is below 9 dB. The noise spectrum at the operator's position for test conditions A2, A3 and A4 are shown in Fig. 3. It is seen that from 50 to 100 Hz, the increase in sound level for test condition A3 relative to that of A2 is from 2.8 to 7.2 dB and that of A4 relative to that of A3 is from 3.2 to 5.9 dB. This shows that drilling operation has increased the sound level with midband frequencies from 50 to 100 Hz. The increase in sound level in this frequency range (50 -100 Hz) is due to impact between the piston and the drill steel and that between the drill steel and the rock. The increase in sound level for test condition A3 relative to that of A2 with midband frequencies from 125 Hz to 2 kHz is in the range of 1.0 to 11.7 dB and that of A4 relative to that of A3 is in the range of 1.6 to 6.0 dB. The noise in this frequency range (125 Hz -2 kHz) is due to the exhaust of the drill machine. The combination of drilling noise and exhaust noise has resulted in increase of sound level in this frequency range (125 Hz -2 kHz). There is significant increase in sound level of the order of 6.6 to 14.2 dB from 2.5 to 20 kHz for test condition A3 relative to that of A2 and 4.0 to 7.7 dB for test condition A4 relative to that of A3. This increase in sound level is due to resonance of the steel parts of the drill steel due to rock drilling. The L eq level at the operator's position for different rocks of varying strength at various thrusts and air pressures are given in Table 3. In this table, the compressive strengths of rocks are given in increasing order i.e., shale has the lowest compressive strength and the highest abrasivity whereas gabro has the highest compressive strength and the lowest abrasivity. At an air pressure of 5 kg/cm 2 and thrust of 160 N, the difference in A-weighted sound level for different rocks was of the order of 0.8 dB, which varied from 0.8 to 1.4 dB with an increase in the thrust from 160 to 360 N. At an air pressure of 5.5 kg/cm 2 , and a thrust of 160 N, the difference in A-weighted sound level for different rocks was 0.9 dB. At this air pressure (5.5 kg/cm 2 ), an increase in the thrust from 160 to 360 N caused an increase in the sound level by 1.6 dB. Similar results were observed at air pressures of 6 and 7 kg/cm 2 with an increase in the thrust from 160 to 360 N. The effect of air pressure on sound levels at constant thrust of 160 N for different rock samples at operator's position is shown in Fig. 4. An increase in sound level is observed with increasing air pressure values. With an increase in air pressure by 2 kg/cm 2 , i.e., from 5 to 7 kg/cm 2 and at a thrust of 160 N, the sound level of block-1 increased by 1.6 dB. Similar results were shown by other rock samples too. The increase in sound level for different rocks (Block-1 to Block-5) with an increase in the air pressure by 2 kg/cm 2 at a thrust of 160 N is 1.9, 2.1, 2.2 and 2.4 dB respectively. The effect of compressive strength of rock on sound level at operator's position for a constant thrust of 160 N and for different air pressure values is shown in Fig. 5 The above result shows that an increase in the compressive strength and a decrease in the abrasivity of rocks increase the sound level. It is worth mentioning that, to maintain optimum penetration rate, the thrust and air pressure must be increased in rocks having higher compressive strength and lower abrasivity, which in turn results in higher sound levels.  Table 3. L eq level at the operator's position for different rocks at various thrust and air pressures The L eq level at exhaust for different rocks of varying strength at various thrusts and air pressures are given in Table 4. A significant increase in the sound level with an increase in the compressive strength and a decrease in the abrasivity is observed for different rocks. For instance, the difference in A-weighted sound level for block-1 and block-5 is 2.2 dB at constant air pressure and thrust of 5 kg/cm 2 and 160 N respectively. The variation of sound levels in all the five blocks, each with a different compressive strength and abrasivity, at an air pressure of 5 kg/cm 2 and thrust varying from 160 to 360 N is shown in Fig.6. It can be seen that, with an increase in the compressive strength and a decrease in the abrasivity of rocks, the L eq level increased near the exhaust at each thrust level for a constant air pressure of 5 kg/cm 2 . Similar results can be seen from  Table 4. L eq level at exhaust for different rocks at various thrust and air pressures The L eq level near the drill rod for rocks having varying compressive strength and abrasivity at various thrusts and air pressures is given in Table 5. Maximum increase in the sound level with an increase in the compressive strength and a decrease in the abrasivity was observed near the drill rod compared to that of other positions.  Fig. 8. The L eq level near the drill bit for rocks having varying compressive strength and abrasivity at various thrusts and air pressures is given in Table 6. In general, an increase in the sound level is observed at each thrust and air pressure with an increase in the compressive strength and a decrease in the abrasivity of the rocks. The difference in the sound level at an air pressure of 5 kg/cm 2 and with an increase in the 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/cm 2 , this sound level difference in different rocks varied from 1.2 to 2.1 dB. This shows that an increase in the compressive strength and a decrease in the abrasivity of rock increase the sound level significantly. In this case also, both air pressure and thrust were observed to have a significant effect on the sound level. For example, an increase in the air pressure by 2 kg/cm 2 at a constant thrust of 160 N indicated an increase in the sound level of 1.7 dB for block-1 and 1.0 dB for block-2 to block-5.  Table 6. L eq level near the drill bit for different rocks at various thrust and air pressures

Laboratory investigations using CNC machine
Compressor was one of the major sources of noise in the laboratory investigation explained in section 2.1. To overcome this, and also to nullify background noise, another investigation was carried out using Computer Numerical Controlled (CNC) machine with carbide drill bit setup. Further, the main aim of this investigation was to find out the relationship of rock properties with sound level produced during drilling.

Experimental setup
In the laboratory, rock drilling operations were performed on BMV 45 T20, Computer Numerical Controlled (CNC) vertical machining centre. The experimental set-up was in a fibre and glass-paned room of 5 m width, 6 m length and 9 m height. The important specifications of the CNC machine used were: - Table size 450 mm x 900 mm -Recommended optimum air pressure -6 bar. -Power supply -415V, 3Phase, 50Hz Carbide drill bits of shank length 40 mm and diameters of 6, 10, 16 and 20 mm were used for drilling operation. The machine was set to drill 30 mm drillhole length. Since the drilling method affects the sound produced, an attempt was made to standardize the testing procedure. Throughout the drilling process a relatively constant rotation speed (RPM), and penetration rate (mm/min) were provided in order to obtain consistent data.

Rock samples used in the investigation
For this investigation, different igneous rocks were collected from different localities of India taking care of representation of variety of strength. During sample collection, each block was inspected for macroscopic defects so that it would provide test specimens free from fractures and joints. The different igneous rocks used in the investigation and their properties are given in Table 7.

Instrumentation for noise measurement
The instrument used for sound measurement was a Spark 706 from Larson Davis, Inc., USA. The instrument was equipped with a detachable 10.6 mm microphone and 7.6 cm cylindrical mast type preamplifier. The microphone and preamplifier assembly were connected by an integrated 1.0 m cable. A Larson Davis CAL 200 Precision Acoustic Calibrator was used for calibrating the sound level meter. Before taking any measurement, the acoustical sensitivity of the sound level meter was checked using the calibrator.

a. Uniaxial compressive strength
Compressive strength is one of the most important mechanical properties of rock material, used in blast hole design. To determine the UCS of the rock samples, 54 mm diameter NXsize core specimens, having a length-to-diameter ratio of 2.5:1 were prepared as suggested by ISRM. Each block was represented by at least three core specimens. The oven-dried and NX-size core specimens were tested by using a microcontroller compression testing machine. The average results of uniaxial compressive strength values of different rocks are given in Table 7

b. Dry density
Density is a measure of mass per unit of volume. Density of rock material varies, and often related to the porosity of the rock. It is sometimes defined by unit weight and specific gravity. The density of each core sample was measured after the removal of moisture from it. The moisture was removed by placing the samples in an electric oven at about 80 0 C for one hour and they were dried at room conditions. The density data of dry samples was obtained from the measurements of bulk volume and mass of each core using the following formula.
(/ ) Mass of sample gc c Volume of sample   Each test was repeated five times and the average values were recorded. The average results of dry densities of different rocks are given in Table 7.

c. Tensile strength
Rock material generally has a low tensile strength. The low tensile strength is due to the existence of micro cracks in the rock. The existence of micro cracks may also be the cause of rock failing suddenly in tension with a small strain. ISRM. The cylindrical surfaces were made free from any irregularities across the thickness. End faces were made flat to within 0.25 mm and parallel to within 0.25°. The specimen was wrapped around its periphery with one layer of the masking tape and loaded into the Brazil tensile test apparatus across its diameter. Loading was applied continuously at a constant rate such that failure occured within 15-30 seconds. Ten specimens of the same sample were be tested. The average results of Brazilian tensile strength of different rocks are given in Table 7.  For each combination of drill bit diameter, drill bit speed and penetration rate, a total of 64 sets of test conditions were arrived at (drill bit diameter of 6, 10, 16 and20 mm; drill bit speed of 150, 200, 250 and 300 RPM; penetration rate of 2, 3, 4and 5 mm/min). A-weighted equivalent continuous sound level (Leq) was recorded for all 64 different drill holes of 30 mm depth on each rock block. For all measurements, the sound level meter was kept at a distance of 1.5 cm from the periphery of the drill bit (Fig. 10). For a particular condition and for the same rock block, the sound level was determined five times in relatively rapid succession. It was found that the recorded equivalent sound levels were almost consistent. The arithmetic average of each set of five measurements was computed to yield an average A-weighted equivalent sound level for a particular condition. For 15 minutes, the sound level was measured at 1.5 cm from the drill bit without drilling. The equivalent sound level of 65.2 dB was recorded without drilling which was mainly due to the noise of the CNC machine.

Drill Bit
Rock sample

OBA Microphone
It may be argued that sound produced from the CNC machine itself may affect the sound level measurement during rock drilling. It is important to mention here that if the sound level difference between two sources is more than 10 dB, then the total sound level will remain the same as that of the higher source. Further, taking the measurement very close to the source will reduce the effect of sound produced from other sources.

Regression modelling and analysis of variance (ANOVA)
The results of the measurements of rock properties (UCS, dry density, tensile strength) and range (maximum and minimum) of A-weighted equivalent sound level recorded during drilling of igneous rocks are given in Table 7. These results were analysed using Multiple Regression and Analysis of Variance (ANOVA) technique. For analysis Minitab 15 software for windows was used.
To obtain applicable and practical predictive qualitative relationships it is necessary to model the physico-mechanical rock properties and the drill process variables. These models will be of great use during the optimization of the process. The experimental results were used to model the various responses using multiple regression method by using a nonlinear fit among the responses and the corresponding significant parameters. Multiple regression analysis is practical, relatively easy for use and widely used for modelling and analyzing the experimental results. The performance of the model depends on a large number of factors that act and interact in a complex manner. The mathematical modelling of sound level produced during drilling is influenced by many factors. Therefore a detailed process representation anticipates a second order model. ANOVA was carried out to find which input parameter significantly affects the desired response. To facilitate the experiments and measurement, four important factors are considered in the present study. Then the response surface contains linear terms, squared terms and cross product terms.
In order to compare all reasonable regression models, a backward elimination procedure was used as the screening procedure. Then the independent variable having the absolute smallest t statistic was selected. If the t statistic was not significant at the selected  (to test the significance, one needs to set a risk level called the alpha level. In most cases, the ''rule of thumb'' is to set the alpha level at 0.05, i.e., 95% confidence interval) level, the independent variable under consideration was removed from the model and the regression analysis was performed by using a regression model containing all the remaining independent variables. If the t statistic was significant, the model was selected. The procedure was continued by removing one independent variable at a time from the model. The screening was stopped when the independent variable remaining in the model could not be removed from the system. For UCS the p-values for all the independent variables are less than 0.05 showing statistical significance. In addition to this the p value of D 2 term and interaction terms related to A, B, C with D are less than 0.05 which establishes the experimental results. Experimental analysis also shows that for igneous rocks, as the UCS increases the sound level produced during drilling also increases. Fig. 11 shows the variation between experimentally measured UCS with the UCS calculated from the developed regression model for test data. For Dry density the p-values for independent variables A and D are less than 0.05 showing statistical significance. In addition to this the p value of D 2 term and interaction terms related to A with D are less than 0.05 which establishes the experimental results. Fig. 12 shows the variation between experimentally measured density with the density calculated from the developed regression model for test data.

Field investigations
An attempt was also made to experimentally determine the UCS in the field during drilling blast holes. The Medapalli Open Cast Project (MOCP), belonging to M/S Singareni Colliery Company Limited, situated in the state of Andhra Pradesh in India was used for the field investigations. The rock stratum at the MOCP consists primarily of sandstone, carbonaceous shale, sandy shale, coal, shale, shaly coal, carbonaceous sandstone, and carbonaceous clay. There were a total of five coal seams in that area. Out of these five seams, four coal seams from the top had already been extracted. Borehole data near the investigation area are shown in Fig. 14, which were obtained from the Geology section of the mine. The lithological details from the 4 th to 5 th seam are also indicated in Fig. 14 (Fig. 15). Sound levels were recorded for 16 different drill holes. At each second, the equivalent continuous A-weighted sound levels were recorded by the dosimeter. To determine the sound level spectrum, the one-third-octave-band analyser was set to measure A-weighted, time-averaged one-third-octave-band sound levels with nominal mid-band frequencies from 25 Hz to 20 kHz. For each measurement, the one-third-octave-band analyser was set for an averaging time of 2 min. The data recorded during field measurements using the dosimeter and one-third-octave-band analyser were downloaded to the computer for analysis. Some critical observations, such as colour change of flushing dust and the exact time during colour change were also recorded. For the same drill diameter and type, penetration rate and weight on bit, the sound levels were measured for various drilled holes consisting of strata of different compressive strengths. For about 3 min, the sound level at about 1.5 m from the drill rod was measured without drilling. The sound level measured without drilling was mainly due to the compressor operating near the drilling machine.

Sound measurement
Field investigation of the sound levels produced during drilling was carried out on the rotary drill machine described in Section 3.1. All the measurements were carried out while drilling blast holes. During field investigation, bit type and diameter, blast hole length, weight on bit, compressed air pressure, net drilling time and rpm of the drill bit were recorded. The penetration rate (m/min) was calculated from the drilled hole length (metres) and the net drilling time (minutes). Blast holes were drilled between the 4 th and 5 th seams at each classified strata (Fig. 14). Depending on the blast design, the blast hole length was limited to 6.0 m, whereas at other places it was only 3.0 m. For 3.0 m long blast hole, the weight on the bit was 12.0 kg, whereas for the 6.0 m long holes, the weight on the bit was 8.0 kg. The exploratory borehole data were collected from the Geology section of the mine. The UCS, density, tensile strength, young's modulus and impact strength of various strata were collected from the exploratory borehole data near the blast hole drilling as given in Table 11.

Results of investigation using dosimeter
Using dosimeter, L eq was measured for each second. Drill bit penetration rate in m/sec was calculated. The time taken to drill 3.0 m deep hole was noted down. Then L eq vs drill hole depth was plotted and is as shown in Fig. 16 and Fig. 17.   Fig. 16 it is observed that for the first 45.0 cm depth of drilling, the difference in sound level for Blast hole -1 and Blast hole-12 is as much as 6.7 dB. By neglecting the first 45.0 cm depth, it is observed that for increase in compressive strength by 8.14 MPa (UCS of Blast hole-1 and Blast hole-12), L eq level increases up to 4.0 dB.   This clearly indicates that as the compressive strength increases, the L eq level produced during drilling also increases. However, this increase in L eq level also depends on the weight on the bit which is indirectly related to the compressor pressure used. It is also observed that between depths of 75.0 cm to 125.0 cm and 150.0 cm to 175.0 cm, the L eq levels measured at Blast hole-6 and Blast hole-2 were somewhat similar whereas Blast hole-14 had an increase in L eq value of up to 8.0 dB for depths between 75.0 cm and 125.0 cm and up to 5.3 dB for depths between 150.0 cm and 175.0 cm. This is because of the coal www.intechopen.com Application of Sound Level for Estimating Rock Properties 293 present in Blast hole-6 and Blast hole-2 between these depths which was confirmed on observing the coal dust flushing out of the drill holes at these depths.

Results of investigation using one-third-octave band analyzer 3.4.1 Comparison of drilling noise with machine noise
The A-weighted sound level spectrum at the measurement location with 8.0 kg weight on bit for Blast hole-1, Blast hole-12 and machine noise is shown in Fig. 18. It is seen that the maximum sound level at measurement location for Blast hole-1 is 96.4 dB, Blast hole-12 is 92.9 dB with nominal one-third-octave midband frequency of 63 Hz. Similarly, A-weighted sound level at the measurement location with 12.0 kg weight on bit for Blast hole-2, Blast hole-6 and Blast hole-14 is shown in Fig. 19. It is seen that the maximum sound level at measurement location for Blast hole-2 is 100.3 dB, Blast hole-6 is 99.8 dB and Blast hole-14 is 104.1 dB with the nominal one-third-octave midband frequency from 25 Hz to 20 kHz. In both the cases, the increase in sound level with midband frequencies above 50 Hz is more than 10.0 dB during drilling relative to that of machine noise without drilling. Therefore, the sound level in the frequency range of 63 Hz to 20 kHz, during drilling is unlikely to be affected by the background noise due to the compressor. However, the sound level produced during drilling may be affected due to machine noise with nominal midband frequencies from 25 Hz to 50 Hz as the difference in sound level in this range of frequency is below 10.0 dB. From Fig. 18, it is seen that from 25 Hz to 50 Hz, the increase in sound level for Blast hole-1 relative to that of machine noise is from 6.7 dB to 9.6 dB and that of Blast hole-12 relative to that of machine noise is from 3.9 dB to 7.4 dB. Similarly, from Fig. 19 it is seen that from From Fig. 18, the increase in sound level for Blast hole-1 relative to that of machine noise with midband frequencies from 63 Hz to 2 kHz is from 10.8 dB to 22.1 dB and that of Blast hole-12 relative to that of machine noise is from 10.2 dB to 20.3 dB. Similarly from Fig. 19, the increase in sound level for Blast hole-2 relative to that of machine noise with midband frequencies from 63 Hz to 2 kHz is from 10.9 dB to 20.4 dB, for Blast hole-6 relative to that of machine noise is from 10.1 dB to 20.2 dB and for Blast hole-14 relative to that of machine noise is from 14.4 dB to 22.6 dB. Also from Fig. 18, it can be observed that there is a significant increase in sound level of the order of 24.3 dB to 45.7 dB from 2.5 kHz to 20 kHz for Blast hole-1 relative to that of machine noise and 22.5 dB to 44.8 dB for Blast hole-12 relative to that of machine noise. Similarly, from Fig. 19, within frequency range of 2.5kHz to 20 kHz, the increase in sound level relative to machine noise for Blast hole-2 is from 18.9 dB 29.9 dB, for Blast hole-6 relative to that of machine noise is from 16.8 dB to 25.5 dB and for Blast hole-14 relative to that of machine noise is from 21.5 dB to 31.9 dB. This increase in sound level is due to resonance of the steel parts of the drill steel due to rock drilling.

Comparison of drilling noise with rock properties
With 8.0 kg weight on bit, the increase in sound level of Blast hole-1 (UCS of 36.49 MPa) compared to that of Blast hole-12 (UCS of 28.35 MPa), with midband frequencies from 25 Hz to 50 Hz, was of the order of 2.0 dB to 3.8 dB. The increase in sound level, with midband frequencies from 63 Hz to 2 kHz, was of the order of 0.3 dB to 6.9 dB. The increase in sound level, with midband frequencies from 2.5 kHz to 20 kHz, was of the order of 0.8 dB to 5.2 dB.

Conclusions
The laboratory study using portable pneumatic drilling equipment indicated that the sound level near the drill rod is relatively higher than that of the exhaust, the drill bit and the operator's position for all the rock samples tested. Both the thrust and air pressure were found to have a significant effect on the sound level produced by pneumatic drill at all the measurement locations i.e., at operator's position, exhaust, drill rod and the drill bit. The laboratory study using CNC machine was carried out to evaluate the empirical relation between various rock properties and sound level produced during drilling considering the effects of drill bit diameter, drill bit speed and penetration rate. The empirical relationship developed is not aimed at replacing the ISRM suggested testing methods, but rather as a quick and easy method to estimate the physico-mechanical properties of rock. The results of this study could be used to predict the physico-mechancial properties of igneous rocks. In the field investigation, results of frequency analyser shows that the sound level in the frequency range of 63 Hz to 20 kHz, during drilling is unlikely to be affected by the background noise because above 50 Hz the sound level produced is more than 10 dB during drilling relative to that of machine noise without drilling. However, the sound level produced during drilling maybe affected due to machine noise with nominal midband frequencies from 25 Hz to 50 Hz as the difference in sound level in this range of frequency is below 10 dB. Results from both laboratory and filed investigations show that there is a possibility to establish relationship between rock properties and sound level produced during drilling. The present investigations lead to further research in this direction.