Mission: As the Nation's principal conservation 

agency, the Department of the Interior has responsibility for most of our nationally-owned public 

lands and natural and cultural resources. This 

includes fostering wise use of our land and water 

resources, protecting our fish and wildlife, preserving the environmental and cultural values of 

our national parks and historical places, and providing for the enjoyment of life through outdoor 

recreation. The Department assesses our energy 

and mineral resources and works to assure that 

their development is in the best interests of all 

our people. The Department also promotes the 

goals of the Take Pride in America campaign by 

encouraging stewardship and citizen responsibilityfor the public lands and promoting citizen participation in their care. The Department also has 

a major responsibility for American Indian reservation communities and for people who live in 

Island Territories under U.S. Administration. 
Report of Investigations 9295 

Analysis of Acoustic Responses 

of Domal Salt Mine Samples 

By Roy H. Grau Ill and Thomas E. Marshall 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Manuel Lujan, Jr., Secretary 

BUREAU OF MINES 

T S Ary, Director 
Library of Congress Cataloging in Publication Data: 

Grau, Roy H. 

Analysis of acoustic responses of domal salt mine samples / by Roy EI. Grau I11 

and Thomas E. Marshall. 

p. cm. - (Report of investigations / United States Department of the 

Interior, Bureau of Mines); 9295 

Includes bibliographical references. 

Supt. of Docs. no.: I 28.23:9295. 

1. Rock-salt-Testing. 2. Methane-Analysis. 3. Noise-Measurement. 

I. Marshall, Thomas E. 11. Title. 111. Series: Report of investigations (United 

States. Bureau of Mines); 9295. 

TN23.U43 [TN900] 622 S-dc20 [622'.82] 89-600275 CIP 
CONTENTS 

Page 

Abstract ........................................................................... Introduction ........................................................................ Acknowledgments .................................................................... Theoryandbackground ............................................................... Acoustical-dissolution test .............................................................. Testapparatus .................................................................... Testprocedure ..................................................................... Dataanalysis ....................................................................... Summary .......................................................................... References ......................................................................... Appendix-Salt sample data ............................................................ 

ILLUSTRATIONS 

1 . Experimental apparatus for conducting acoustic test ....................................... 2 . Examples of microcomputer printouts of acoustical test results for normal salt and methaneenrichedsalt ................................................................... 3 . Relationship between average gas content data and average noise levels for two domal salt mines ..... 

TABLES 

1 . Distribution of salt samples tested for gas content and noise ................................. 2 . Results of tests performed on salt samples using laser Raman microprobe ....................... A.1 . Average gas content and decibel levels of salt samples at mine A ............................. A.2 . Average gas content and decibel levels of salt samples at mine B ............................. 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

atm atmosphere, standard kg kilogram 

cm centimeter mL milliliter 

cm3 cubic centimeter min minute 

dB decibel oz ounce 

ft foot ~ct percent 

g gram s second 

Hz hertz V volt 

in inch 
ANALYSIS OF ACOUSTIC RESPONSES 

OF DOMAL SALT MINE SAMPLES 

By Roy H. Grau 111' and Thomas E. ~arshall~ 

ABSTRACT 

The U.S. Bureau of Mines has developed an acoustic test for determining if a rock salt sample is from 

a "normal" nongassy salt face or from a gassy face where an outburst has the potential to occur. The 

test is based on the observation that a salt sample taken near an outburst-prone zone produces an 

audible popping sound when it is dissolved in water. The sound is produced when pressurized gas that 

is trapped in salt samples is released. In the acoustic test, the sound is electronically measured, digitized, 

and transmitted to a microcomputer; the output is an average decibel value that classifies the sample 

as "normal" or "outburst." The acoustic test procedure and equipment are described, and raw test results 

and data analysis are shown. Results are also shown for several salt samples that were analyzed using 

a laser Raman microprobe. 

'~ining engineer. 

2~ngineering technician. 

Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 
INTRODUCTION 

Methane gas (CH,) has long been recognized as a 

hazard in coal mining. In recent years, sporadic methane 

occurrences have been encountered in the domal salt 

mining industry (9,11).~ salt domes and coalbeds differ 

in that salt domes do not have regular macroscopic fractures that transport methane throughout the whole ore 

body; gas occurs in isolated zones because of the impermeable nature of salt. 

In domal salt mines, the standard drill-and-blast method 

is used to form a room-and-pillar configuration. Large 

methane gas volumes are liberated in only a small percentage of blasted faces (2). The methane may be accompanied by a violent expulsion of salt from the mine roof 

and/or wall. The expulsion of salt and methane creates 

cone-shaped voids in the roof or rib termed "outburst 

cones." Since salt is relatively impermeable, a small handsize sample from a gassy zone may contain and hold a 

measurable quantity of methane many months after 

mining; evidence of this can be seen as small suspended 

trapped bubbles within the salt crystals. 

The U.S. Bureau of Mines, in previous studies, has 

found that the occluded methane gas and higher hydrocarbons in a rock salt sample can be measured using a 

laboratory dissolution test, which involves dissolving a rock 

salt sample in water and collecting the gas for gas chromatograph analysis (3,IO). The gas volume can then be 

normalized to the rock salt mass. This report describes 

acoustical response of pressurized gas being released when 

rock salt samples are dissolved in water. With the addition 

of this acoustical information, the test could be an indicator of the outburst potential for the particular face. 

It must be kept in mind that while methane under 

pressure appears necessary for outburst occurrence in the 

gulf coast domal salt mines, this is not the only factor that 

may contribute to outburst formation. Factors such as 

strata weakening due to folding or shearing and 

mechanically weakened salt due to undersized pillar 

overloading may create areas that are susceptible to 

outbursts in high gas pressure zones (7). 

The dissolution test by itself has been used primarily 

as a research tool; it has not been accepted by the salt 

mining industry for daily use because of several inherent 

problems. It is a relatively lengthy procedure and requires the use of selective equipment, including a gas 

chromatograph. Also, methane gas content values for 

normal and outburst salt tend to overlap (10). 

The samples used for the acoustic test were classified 

as "outburst" or "normal" prior to testing. Samples were 

considered outburst if they were within about 100 ft from 

an outburst that already occurred. Values for normal salt 

range from less than 0.0003 to 0.31 cm3 CH, per 100 g 

NaC1, while outburst salt values range from 0.014 to 7.4 

cm3 CH, per 100 g NaCl (10). A previous investigator has 

indicated that domal salt may contain large quantities of 

CO, (I) that may not be detected by the dissolution test 

alone because CO, is exsolved from the brine solution as 

the salinity increases. If some outbursts are caused by 

high-pressure CO, and face samples contain CO,, the 

dissolution test alone may not reveal the gas. This 

indicates that a test that could easily determine if a sample 

contained any type of high-pressure gas would be helpful 

in predicting conditions that might precipitate an outburst. 

In at least two instances, the acoustic test has identified 

samples containing gas prior to mining into gassy salt 

zones in a U.S. gulf coast domal salt mine (5). The test 

provides no indication of the gas composition; it only 

records the acoustic emission of gas escaping under 

excessive pressure. 

During dissolution tests, it was noted that gases are 

consistently liberated with a series of loud "pops" attributed 

to the pressures under which the gases are occluded in the 

salt. Initially, a dosimeter connected to a strip chart recorder confirmed the magnitudes of the pops (4); however, 

this method allowed operator error in analyzing the strip 

chart. Subsequent tests by the Bureau, resulting in the 

acoustical-dissolution test, which uses a dosimeter-data 

logger and a microcomputer, have refined the method. 

The equipment records and analyzes the sound level of the 

pops during dissolution. This provides a concrete numeric 

amplitude value for each salt sample, which can then be 

classified as "normal" or "outburst." This methodology is 

being used by at least one domal salt mine operator as 

standard operating procedures for checking faces before 

blasts. The results show that the test is a reliable method 

for differentiating between the two salt types. 

One important part of the Bureau's overall mission is 

to develop technology that provides for a safer work environment. This work was performed with the cooperation 

of personnel at the Cote Blanche Mine, Domtar Industries 

Inc., New Iberia, LA; Weeks Island Mine, Morton Salt 

Co., New Iberia, LA; and Avery Island Mine, International 

Salt Co., Avery Island, LA. 

ACKNOWLEDGMENTS 

The authors thank Timothy McCue, production super- gassy faces and outburst-prone areas. Also, thanks go to 

visor, Cote Blanche Mine for his efforts in identifying Patricia Olivier, chemist, Cote Blanche Mine, who ran the 

acoustic test response at the minesite and provided the 

results. 

3~talic numbers in parentheses refer to items in the list of references 

preceding the appendix at the end of this report. 
The instrumentation used for the tests was originally 

designed for measuring occupational noise exposure; it was 

modified slightly for use with the dissolution test. It consists of a small sensitive microphone, a dosimeter-data 

logger (6), a microcomputer, a soundproof chamber, and 

associated hardware for dissolving the salt sample. A11 

equipment, except the dosimeter-data logger and the 

microcomputer, are inside the soundproof chamber. The 

2- by 2- by 2-ft chamber (inside dimension) reduces ambient noise levels. It provides ample space for the needed 

equipment (fig. 1). An instrument panel, equipped with' 

receptacles, is on the chamber's side with the microphone 

outputs. Permanently attached inside the chamber is a 

round aluminum canister (not shown in figure I), which 

serves two purposes: It provides an additional sound shield 

from ambient noise and it acts as an amplifier for the 

noise emitted by the dissolving salt. The canister has a 

removable cover to provide easy access when the salt is 

added. Inside the aluminum canister is a 2,000-mL glass 

beaker filled with 1,800 mL of normal tap water and an 

inverted 250-mL beaker that holds the test samples. The 

microphone picks up the sound of the dissolving salt, and 

the information is measured, processed, and stored by the 

Figure 1.-Experimental apparatus for conducting acoustic 

test. 
TEST PROCEDURE 

The standard test procedure developed for measuring 

acoustic levels of dissolving domal rock salt is as follows: 

1. A salt sample weighing about 50 g is selected and 

placed in the beaker of water inside the soundproof chamber. Although tests have shown that reliable results can be 

obtained for a sample that varies by 40 g, the sample size 

should be kept as uniform as possible (4). . 2. The dosimeter-data logger test switch is moved from 

the standby to the test position. Verification of test startup 

is then confirmed by observing the indicator lights on the 

bottom of the dosimeter; a red light should flash four 

times per second as sampling takes place. 

3. The timing device is activated and the test is run for 

2 min. 

Normai Sait 

US BUREAU OF MINES 

SALT/SC!UND STUDY 

MINE SALTY 

Vl. I S/N: 7047 

dB-3C) 1 / 14 SIN: 5428 

DYNAM I C RANGE : 40- I03dF 

TEST NUMBER: 248 

LOCAT I ON: SENE 

DATE : (:)2/i:)6/35 

METHANE LEVEL : !I). O!1;.2S 

RUN TIME: 2: 5 

AVERfiGING FERIOC: 1O sec 

PER I nf?S COMPLETED: 12 

TIME HISTORY 

4. After the test is run, the microcomputer is connected to the dosimeter-data logger and the digital sound 

levels are transferred to the computer memory. A commercially available software package immediately performs 

all the averaging functions, with the results provided via a 

printout. 

Figure 2 shows typical computer-generated results for 

a normal salt sample (0.0028 cm3 CH, per 100 g) and for 

an outburst sample (2.1079 cm3 CH, per 100 g). The average sound produced (shown in the last line of figure 2) 

when the normal salt sample was dissolved was 40.1 dB; 

the outburst salt sample averaged 68.4 dB. The first four 

lines of the printout provide user programmable information. Information such as heading, title, mine name, and 

possibly a short note is helpful here. Line 5 identifies 

which particular dosimeter was used, and line 6 provides 

the serial number of the software package used. The 

Outburst Sai t 

US BUREfiU OF MINES 

SALTiSOUNE STUDY 

MINE SALTY 

:Jl . 1 S./N: 7Cj47 

dB-30 1 / 14 SIN: 5423 

DYNAMIC RGNGE: 40-10ZdE 

TEST NUMEEK: 229 

LCCATIGN: ZIFNN 

DATE: [26/07/55 

METHGNE LEVEL: 2.1079 

RUN TIME: 2: 4 

AVERAGING PERIOD: ?O sec 

PERIODS COMPLETED: I2 

TIME HISTDRY 

PER I OD 

NUMBER--LEVEL (dB 1 

1 - &Sm[> 3 

i 

- 63.0 

- .:. - . r:) 

4 - 7G.(> 

5 - 72.tf 

6 - 69.G 

7 - 66.C) 

&3 - 67.(3 

9 - 64.C) 

10 - 68.(:) 

11 - 67.0 

12 - h7.0 

LEG! CURRENT: 40.1 LEQ CURRENT: h8.4 

Figure 2.-Examples of microcomputer printouts of acoustical test results for normal salt (right) and methane-enriched salt (left). 
seventh line shows the "DYNAMIC RANGE," which is 

the decibel range that can be recorded. The "TEST 

NUMBER" and "LOCATION" are supplied by the user, 

but the "DATE" is provided by the computer's batteryoperated clock. The "METHANE LEVEL is the gas 

content (cubic centimeters per 100 grams) of the sample, 

if known. The test duration is automatically shown as 

"RUN TIME" and is in minutes and seconds. 

"AVERAGING PERIOD" denotes the time increments in 

seconds that will be averaged to produce the time history file, and "PERIODS COMPLETED" shows how 

many time increments were averaged. The actual "TIME 

HISTORY" profile shows two events: First, "PERIOD 

NUMBER" represents the 10-s blocks of time that the 

samples are averaged; second, "LEVEL (dB)" gives the 

average decibel level for the 40 measurements taken 

during that 10-s period. The decibel levels for all of the 

periods are then averaged to produce the "LEQ 

CURRENT." The LEQ is called the average sound level. 

It is defined as the constant sound level in a given situation 

and time period that conveys the same sound energy as 

does an actual time-varying sound during the same time 

period. This is used to determine the relationship between 

decibel levels and gas emitted. 

DATA ANALYSIS 

One hundred and seventy-three salt samples were analyzed. The samples were obtained from ribs and faces 

of known location at two mines identified as "mine A" 

(108 samples) and "mine B" (65 samples). These samples 

had been stored in a Bureau salt sample library, accumulated from this and other studies. The samples do not 

represent a true ratio of normal-to-outburst samples that 

would be taken from a random minewide sampling program. The ratio of normal-to-outburst samples present is 

probably higher than what mine operators would expect 

from daily sample collections at the mined faces. For example, of the 173 samples tested, 25 were outburst. This 

does not imply that, for every 173 faces sampled during 

normal salt production, there are 25 outburst faces. The 

mentioned, different pressures may be required to precipitate an outburst. This could explain why noise levels 

for outburst samples vary between mines but are reliably 

constant within each particular mine. These results show 

general guidelines that a salt mining enterprise could use 

in expanding its own data base concerning salt type and 

acoustic test results. If the methodology were accepted by 

the mining enterprise, a detailed data base could easily be 

formed. 

Tests were conducted by the Bureau to determine if a 

relationship existed between the gas content and the sound 

emitted of salt samples. Each sample was divided into 

two equal parts; one-half was tested using the original gas 

dissolution test, and the other half by the acoustical-disdata are useful, however, because they give a relative range solution test. Figure 3 shows a graphic comparison of the 

of sound level ou~put for normal and outburst-related salt. data from mines A and B. The valuss are in increments 

Distribution of all samples from both mines is shown 

in table 1, by salt type (normal or outburst) and by the 

range of decibels for the specified salt type. From mine A, 

84 pet of all normal salt samples were at a decibel range 

of 40 to 65 dB. The remaining 16 pct of normal samples 

were widely distributed above 65 dB. This suggests that, 

while normal salt may produce as much noise as outburst 

salt, 84 pct of normal salt samples in mine A had lownoise outputs. 

Test results of mine B samples show that all normal 

samples had acoustic levels below 55 dB, and outburst 

samples all had levels above 55 dB. Assuming the sound 

levels are related to gas pressures, this could be a 

significant finding. ~epend:ln~ upon factors previously 

Table 1.-Distribution of salt samples tested for 

gas content and noise 

Mine A Mine B 

Range . . dB . . 40-65 6585 Total a55 5585 Total 

Normal 

samples . . . . 79 15 94 54 0 54 

Outburst 

samples . . . . 0 14 14 0 11 11 

Total.. . . . . 79 29 108 54 11 65 

of 5 dB, with the corresponding average gas content. The 

data are also shown in tables A-1 and A-2 in the appendix, sorted by sound emitted. A large increase in gas content from both mines occurs at 55 to 65 dB. The diagram 

proves only relative relationships between the sound and 

the gas content exist. Variances do exist and may be due 

to a number of reasons, including the presence of CO, in 

the sample, the inhomogeneous nature of gas occlusions in 

the sample, and varying gas pressure. A numerical value 

for CO, contents from the dissolution test lacks meaning, 

since CO, is exsolved from the brine in the dissolution 

chamber as its salinity increased. 

The acoustic test results proved to be a good indicator 

of a potential outburst face. However, several questions 

remained. Why did samples from the two mines differ? 

Is the difference in the frequency of pops caused by inclusions distributed in a nonhomogeneous manner? Was 

the loudness of the pops caused by one large inclusion or 

many small ones together? Could differences in results be 

attributed to higher levels of CO,? 

To answer these questions, tests were conducted on 

seven samples (four outburst, three normal) from the two 

mines, using a laser Raman microprobe. The Raman microprobe can determine the total internal gas pressures 
aeage ose ees oto doasat es

pg, 

(4) CO, appears insignificant within the inclusions themselves, but this does not rule out the presence of CO, in 

fractures within the salt mass itself. The CO, within inclusions could be an indicator of such a condition. 

Table 2.-Results of tests performed on salt samples using laser Raman microprobe 

Mine CH, content, Average Peak coz Sample Average 

and cm3 per pressure, pressure, content, type sound, 

sample 100 g atm atm PCt dB 

Mine A: 

1 .... 8.6 145 155 2.5 0 74.2 

2.... 8.2 70 90 3.4 0 74.1 

3.... 7.5 103 110 2.5 0 75.5 

4.... .003 N D ND ND N 40.0 

5.... .001 ND ND ND N 41.3 

Mine 8: 

6.... 2.4 161 200 ND 0 77.9 

7.... .008 ND N D ND N 40.0 

N Normal. 

ND Not detected. 

0 Outburst. 
SUMMARY 

One hundred and seventy-three salt samples from two 

different mines were analyzed for methane gas content and 

noise output. Results showed that for mine A, 84% of 

low-gas salt samples produced a decibel range of 40 to 65. 

The remaining 16% of normal salt samples produced noise 

levels that were widely distributed above 65 dB. All outburst samples from mine A produced a noise greater than 

67 dB. All normal salt samples from mine B produced 

noise levels below 55 dB, and all outburst samples had 

levels above 55 dB. 

In order to determine why the samples produced a 

range of acoustic noise, tests were performed on seven 

samples using the laser Raman microprobe. Results from 

these tests lead to several hypothetical conclusions: (1) 

The acoustic noise level is related to the gas content and 

the gas pressures, (2) gas inclusions are not homogeneously distributed through the samples, (3) no methane 

inclusions are present in low-gas-content salt, and (4) CO, 

appears in~i~cant within the inclusions themselves. 

Gas inclusions are not homogeneously distributed 

through the salt; this could explain why there is a variation 

between results of the acoustic test and the dissolution test 

on outburst samples. Also, although the CO, appears 

insignif~cant within the inclusions themselves, the CO, 

within inclusions could be an indicator of the presence of 

CO, in large fractures within the salt mass itself. 

REFERENCES 

1. Cybulski, W., C. Gorol, T. Gotkowski, and J. Sobala. 

Charakterystyka Zagrozenia Metanowega w Polskich Kopalniach Soli 

(Characteristic Features of Methane Hazard in Polish Salt Mines). Pr. 

GI. Inst. Gorn., Komun., No. 567, 1972, 20 pp. Saf. Mines Res. Establ. 

Trans. 6344. 

2. Grau, R H. III., T. E. Marshall, and G. L. Finfinger. Evaluation 

of Methane Liberations From Production Blasts in Domal Salt Mines. 

Paper in Proceedings of Fourth International Mine Ventilation Congress, Brisbane, Australia, July 3-6, 1988. Australas. Inst. Min. and 

Metall., 1988, pp. 229-234. 

3. Hyman, D. M. Methodology for Determining Occluded Gas 

Contents in Domal Rock Salt. BuMines RI 8700, 1982, 11 pp. 

4. Marshall, T. E., and G. L. Finfinger. Gas Content Determinations of Salt Samples Using Acoustic Responses. BuMines RI 9113, 

1987, 10 pp. 

5. McCue, T. (production supenisor, Cote Blanche Mine). Private 

communication, 1986; available upon request from T. Marshall, BuMines, Pittsburgh, PA. 

6. Metronics, Inc. Metrologger Instruction Handbook dB-301/301P. 

Undated, 35 pp. 

7. Molinda, G. M. Investigation of Methane Occurrence and Outbursts in the Cote Blanche Domal Salt Mine, Louisiana. BuMines 

RI 9186, 1988, 21 pp. 

8. Pasteris, J. (associate professor, Dep. Earth and Planetary Sci., 

WA Univ.). Private communication, 1987; available upon request from 

G. Molinda, BuMines, Pittsburgh, PA. 

9. Plimpton, H. G., R K. Foster, J. S. Risbeck, R P. Rutherford, 

F. King, G. L. Buffington, and W. C. Traweek. Final Report of Mine 

Explosion Disaster, Belle Isle Mine, Cargill, Inc., Franklin, St. Mary 

Parish, Louisiana. MSHA Accident Invest. Rep., 1980, 156 pp. 

10. Schatzel, S. J., and D. M. Hyman. Methane Content of Gulf 

Coast Domal Rock Salt. BuMines RI 8889, 1984, 18 pp. 

11. Thoms, R L., and J. D. Martinez. Blowouts in Domal Salt. 

Paper in Fifth Symposium of Salt (Hamburg, Federal Republic of 

Germany, May 29-June 1, 1978). North. OH Geol. Soc., Cleveland, 

OH, V. 1, 1980, pp. 119-134. 
APPENDIX.-SALT SAMPLE DATA 

Table A-1.-Average gas content and decibel levels of salt samples at mine A 

2.... N 

3.... N 

4.... N 

5.... N 

6.... N 

7.... N 

8.... N 

9.... N 

10 ... N 

11 ... N 

12 ... N 

13 ... N 

14 ... N 

15 ... N 

16 ... N 

17 ... N 

18 ... N 

19 ... N 

M... N 

21 ... N 

22 ... N 

23 ... N 

24 ... N 

25 ... N 

26 ... N 

27 ... N 

28 ... N 

29 ... N 

30 ... N 

31 ... N 

32 ... N 

33 ... N 

34 ... N 

35 ... N 

36 ... N 

N Normal. 

Salt CH, content, Sound, 

Sample type cm3 per dB 

100 g 

73 ... N 0.08 16 60.1 

Salt CH, content, Sound, 

Sample type cm3 per dB 

100 0 

I.... N 0.ooOo 40.0 

0 Outburst. 

Salt CH, content, Sound, 

Sample type cm3 per dB 

100 g 

37 ... N 0.0057 40.1 
Table A-2.-Average gas content and decibel levels of salt samples at mine B 

33 ... 

N Normal. 

Salt CH, content, Sound, 

Sample type cm3 per dB 

100 g 

1 .... N 0.0000 40.0 

0 Outburst. 

Salt CH, content, Sound, 

Sample t~ Pe cm3 per dB 

100 g 

34 ... N 0.01 65 40.0