By Thomas E. Marshall and Gerald L. Finfinger 

UNITED STATES DEPARTMENT OF THE INTERIOR 
Report of Investigations 91 13 

Gas Content Determinations of Salt 

Samples Using Acoustic Responses 

By Thomas E. Marshall and Gerald L. Finfinger 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 

David S. Brown, Acting Director 
Library of Congress Cataloging in Publication Data: 

. 

L 

Marshall, Thomas E. 

Gas content determinations of salt samples using acoustic 

responses. 

(Report of investigations; 91 13) 

Bibliography: p. 9 - 10. 

Supt. of Docs. no.: 128.23: 9113. 

1. Salt mines and mining-Safety measures. 2. Mine gaseeMeasurement. 3. Salt 

domes. I. Finfinger, Gerald L. 11. Title. 111. Series: Report of investigations (United States. 

Bureau of Mines); 91 13. 

TN23.U43 [TN900] 622 s [622'.8] 87-600105 
CONTENTS 

Page 

Abstract ....................................................................... 1 

Introduction ................................................................... 2 

Acknowledgments ................................................................ 2 

Test description ............................................................... 3 

Test apparatus ............................................................... 3 

Test procedure ............................................................... 4 

Test results ................................................................... 5 

Correlation testing .......................................................... 5 

Repeatability of results ..................................................... 7 

Impact of sample size ........................................................ 8 

Application of test results .................................................... 8 

Conclusions .................................................................... 9 

References ..................................................................... 9 

ILLUSTRATIONS 

1 . Portable sound-resistant chamber ........................................... 3 

2 . Small grab sample of rock salt ............................................. 4 

3 . Test setup inside chamber .................................................. 5 

4 . Strip-chart recordings of relative acoustic responses from three samples 

with different gas contents ............................................... 6 

5 . Relationship between gas content and acoustic responses of salt samples .... 7 

TABLES 

1 . Gas content and acoustic response test results ............................. 7 

2 . Repeatability test results ................................................. 8 

3 . Sample size test results .................................................. 8 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

cm centimeter m 

cm/min centimeter per minute mL 

cm3 cubic centimeter min 

dB decibel Pet 

meter 

milliliter 

minute 

percent 

g gram 
GAS CONTENT DETERMINATIONS OF SALT SAMPLES USING ACOUSTIC 

RESPONSES 

By Thomas E. Marshall1 and Gerald L. Finfinger2 

ABSTRACT 

The Bureau of Mines has developed a practical methodology for determining occluded gas contents of domal rock salt samples. The method, 

which is portable and fieldworthy, provides results in 5 min or less and 

does not require a laboratory environment or specially trained personnel 

to obtain meaningful results. The results can be used to identify gasenriched zones, and thus may aid mine operators in production planning 

or ventilation design. 

The test method is based on the relationship between gas content and 

the acoustic response of salt samples as they are dissolved in water. 

The relationship is defined by an equation, and the results are repeatable. The equation was derived by testing a series of rock salt samples 

collected from domal salt mines in Louisiana. 

Engineering technician. 

2~upervisory geologist. 

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

The release of hydrocarbon gases from 

subsurface strata into underground mine 

workings has long been problematic to the 

mineral industry. In particular, methane 

emissions have been directly responsible 

for many disasters which have resulted 

in the loss of life and property (1). 

While methane control guidelines and procedures are well known to the underground 

coal mining industry, workable methods 

for the domal salt industry are still 

being devised. The mechanisms that govern the occurrence and subsequent release 

of in situ gases in domal salt formati~n differ significantly from the environments associated with coal-bearing 

strata. 

Most coal mining operations experience 

a relatively steady emission of methane 

into the underground workings (2-7). As 

a result, the ventilation requirements 

to dilute the methane to safe levels 

can be identified and easily implemented 

during the mining operation. However, 

methane emission in domal salt mines 

is not always a steady continuous flow, 

but instead tends to occur as sudden 

and rapid releases (8). As a result, the 

ventilation needs cannot always be met 

and the potential for hazardous conditions to form is increased. 

To achieve a safer work environment for 

the underground domal salt mining industry, the Bureau of Mines has been 

conducting research to develop predictive 

techniques for identifying gas-enriched 

zones and outburst-prone areas. Earlier 

Bureau investigators have determined 

methods for quantifying gas-enriched 

zones and have correlated the zones to 

observed physical conditions such as salt 

quality, banding, and impurities (9-13). -- 

Based on data collected from three domal 

salt mines in southern Louisiana, a correlation has been established between 

increased gas content and outburst occurrences (=). The gas content measurements were obtained using a dissolution 

and gas chromatography technique (9). - 

This technique is quite simple, but 

requires a laboratory environment and 

trained personnel to obtain accurate 

results. 

Additional studies were undertaken to 

provide the industry with a rapid and 

fieldworthy method for estimating gas 

contents. It was observed during field 

studies and dissolution testing that a 

popping or cracking sound was discernible 

as the salt samples were dissolved in 

water. A laboratory test was conducted 

to determine if the emitted sounds were 

related to the gas content of the salt 

sample. Results from the test indicated 

an excellent correlation between the two 

parameters. Procedures and results of 

the test are discussed in this report. 

ACKNOWLEDGMENTS 

The authors wish to express their appreciation to the personnel of the Cote 

Blanche, New Iberia, LA, Bell Isle, 

Franklin, LA, Weeks Island, New Iberia, 

LA, and Avery Island, New Iberia, LA, 

domal salt mines for their assistance and 

support. They provided samples from 

active mining areas and assisted in 

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

the end of this report. 

verifying the test. Special recognition 

is given to Timothy McCue, production 

supervisor, and Patricia Olivier, chemist, of the Cote Blanche Mine. Mr. McCue 

was instrumental in identifying outburst 

areas and collecting salt samples from 

underground production faces. Ms. 

Olivier performed all of the acoustic 

tests at the Cote Blanche Mine site and 

provided the results. Without their 

dedication and cooperation, the project 

goals couls not have been achieved. 
FIGURE 1.-Portable sound-resistant chamber. 
primarily small grab samples (fig. 2) 

collected from within the mine. The 

chamber is equipped with a magnetic door 

seal and has an instrumentation receptacle built in for strip-chart recorder 

and microphone outputs. 

A cylindrical aluminum canister is 

bolted inside the chamber to provide additional insulation from outside noise 

interferences. A 2,000-mL glass beaker, 

filled with approximately 1,800 mL of 

water, is placed inside the canister. An 

inverted 2501nL glass beaker is placed 

inside the larger beaker and is used as a 

platform to hold the salt sample during 

testing (fig. 3). Sound emissions are 

monitored by a noise dosimeter attached 

to the aluminum canister. The dosimeter 

is connected to the inside jack of the 

instrumentation receptacle. A stripchart recorder is connected to the dosimeter through the outside jack of the 

instrumentation receptacle. The recorder 

operates at a chart speed of 3 cm/min. 

TEST PROCEDURE 

A collected salt sample (approximately 

50 g) is submerged in the water-filled 

2,000-mL beaker. The chamber door is 

closed and sealed, and the strip-chart 

recorder is activated. Sound emissions 

are recorded for five min. The relative 

gassiness of the sample can be readily 

determined by examining the strip-chart 

recording at the end of the test. Visual 

inspection of the strip-chart recordings 

does not provide gas content values, but 

does provide relative values of gassiness . such as low, medium, and high. Figure 4 

shows the recorded output from three 

salt samples with different amounts 

of occluded gas. The gas contents of 

the three samples tested were no gas 
FIGURE 3.-Test setup inside chamber. 

(fig. 4A), cm3 per 100 g (fig. 4B), 8 cm3 could be used to ascertain actual gas 

per 100 g (fig. 4C), as determined by the content values. The results of this 

dissolution method. testing are detailed in the following 

Additional testing was performed to section. 

determine if the acoustic test method 

TEST RESULTS 

A series of tests was conducted to CORRELATION TESTING 

determine the degree of correlation 

between the gas content of domal salt Correlation testing was accomplished by 

samples and the acoustic response of the collecting 20 grab samples from within 

samples as they are dissolved in water. operating domal salt mines in southern 

The tests included an analysis of the Louisiana. The samples were collected 

impact of sample size on the test from areas that had and had not experiresults. enced methane emissions during mining. 
flGURE 4.--Strlpchart recordings of relatlve acoustlc responses from three samples with dlfferent gas contents. 

A, No gas; B,4 cm3 per 100 g; C, 8 crna per 100 g. 

Each sample collected was large enough to 

allow for multiple testing. After the 

samples were returned to the laboratory, 

a split of each one was subjected to the 

dissolution test to determine the actual 

gas content. A remaining portion from 

each sample was then dissolved in water, 

and its average acoustic response was 

recorded. 

Results from the two test series are 

listed in table 1. Gas contents of the 

samples ranged from 0 to 8.6030 cm3 per 

100 g, while the acoustic responses 

ranged from 50 to 76 dB. Figure 5 shows 

the gas content versus acoustic response 

data for the samples collected and 

indicates the correlation coefficient, or 

r2 value (3, pp. 229-249) was 0.97. 

Using the established correlation shown 

in figure 5, gas content values for domal 

salt samples can be estimated from the 

following equation: 

where Y = estimated total gas content, 

cm3 per 100 g, 

and X = average acoustic response, dB. 
AVERAGE ACOUSTIC EMISSIONS, dB 

FIGURE 5.-Relatlonshlp between gas content and acoustlc responses of salt samples. 

TABLE 1. - Gas content and acoustic REPEATABILITY OF RESULTS 

response test results 

The repeatability of an individual gas 

content estimation using the acoustic 

method was investigated for two different 

domal salt samples: one sample, collected from an outburst area, was expected to contain a large quantity of 

gas, while the other sample was collected 

from a relatively gas-f ree area. Each 

sample was large enough to be divided 

into 20 equal splits, and each split was 

tested using the acoustic method. 

Table 2 lists the test results from the 

sample collected in the area that contained no gas. The acoustic responses 

when the 20 individual splits were tested 

ranged from 50 to 53 dB. The spread of 

the data can probably be attributed to 

differing gas contents within individual 

splits and differences in the results of 

the two tests. Table 2 also lists the 

test results from the sample collected 

from the outburst area. The average 

Av acoustic 

response, dB 

50 

50 

50 

50 

50 

50 

5 1 

52 

53 

53 

53 

53 

55 

56 

58 

65 

67 

67 

74 

76 

Test 

l........... 

2........... 

3 ........... 

4........... 

S........... 

6 ........... 

7........... 

8........... 

9........... 

lo.......... 

11.. ........ 12.......... 

13.......... 

14.......... 

15.......... 

16.......... 

17.......... 

18. ......... 19.......... 

20.......... 

Gas content, 

cm3 per 100 g 

0.0000 

.0013 

.OOOO 

.OOOO 

.0101 

.OOOO 

.I520 

.6010 

1.1536 

.9734 

2.3036 

.3224 

2.7570 

2.3690 

4.6878 

6.1427 

8.2040 

7.5615 

8.6030 

7.4248 
acoustic response from the 20 individual 

splits ranged from 69 to 75 dB. Overall, 

any individual test result is probably 

accurate to only '10 pct. 

TABLE 2. - Repeatability test results 

(Av acoustic response, decibel) 

Test I Sample with 1 Sample from outburst prone area 

7 2 

73 

73 

72 

73 

74 

75 

7 3 

73 

75 

70 

69 

75 

7 5 

7 3 

72 

74 

7 1 

74 

7 5 

1......... 

IMPACT OF SAMPLE SIZE 

no gas 

5 1 

An experiment was conducted to determine the impact of sample size on the 

acoustic test method. A large domal salt 

mine sample was split into 10 smaller 

samples that ranged in size from 10 to 

100 g, and each of the samples was subject to the acoustic test (table 3). 

Assuming each split contained like quantities of gas, the average acoustic 

responses were not affected by sample 

size. Therefore, the test method does 

not require careful selection of samples of equal size to ensure meaningful 

results. 

TABLE 3. - Sample size test results 

APPLICATION OF TEST RESULTS 

Various observable features in domal 

salt mines have been correlated with the 

occurrence of gas-enriched zones and 

outburst-prone areas. The features include increased crystal size, occluded 

gas bubbles, contorted cleavage surfaces, 

and disking of core samples (11). These 

features are not consistent from mine to 

mine, and often they are observed only 

after the gas-enriched zone is intercepted by mining. Since the gas content 

of outburst-prone salt samples is usually 

high, the acoustic test method can be 

used for advance identification of these 

Av acoustic 

response, dB 

72 

7 1 

72 

69 

70 

7 1 

69 

70 

69 

69 

Test 

l............... 

2............... 

3............... 

4............... 

5............... 

6............... 

7............... 

8............... 

9............... 

lo.............. 

Each active face within the mine can be 

sampled on a daily basis, and the results 

can be obtained before production begins. 

Core samples from horizontal holes could 

also be tested to provide gas content 

information in advance of mining. Where 

mining is to advance in a gas-enriched 

area, additional precautions can be implemented to reduce the hazards associated with methane emissions. Supplementary ventilation capabilities and 

increased awareness of the potential 

problem can prevent production delays and 

lessen the hazards associated with mining 

Sample 

weight, g 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

zones and areas. in gas-enriched and outburst-prone areas. 
CONCLUSIONS 

The Bureau of Mines has developed a 

fieldworthy method for determining gas 

contents of domal rock salt samples. The 

method is based on the relationship between occluded gas contents and acoustic 

responses as the samples are dissolved in 

water. The relationship was determined 

by performing a power curve regression on 

collected samples from three domal salt 

mines in Louisiana. The actual gas content of the samples was determined by the 

dissolution test method and ranged from 

no gas to 8.6030 cm3 per 100 3 NaC1o The correlation coefficient (r ) of the 

established relationship is 0.97. 

Multiple testing of large domal rock 

salt samples was conducted to determine 

the repeatability of the acoustic results. Two separate samples were tested 

to evaluate repeatability at both low and 

high gas content values. The acoustic 

responses from the low and high gas content samples ranged from 50 to 53 dB and 

69 to 75 dBy respectively. 

The impact of sample size on the acoustic test was determined by splitting 

a large salt block into 10 individual 

splits that ranged in size from 10 to 

100 g. The average acoustic emissions 

from the 10 splits ranged from 69 to 

72 dB. 

The results of the acoustic test method 

can be used by mine operators to aid in 

identifying gas-enriched zones prior to 

their interception by mining if coreholes 

are used. Using grab samples, the method 

can also be used to verify areas of high 

gas content. Since areas with higher 

occluded gas contents can be expected to 

experience higher methane emissions during mining, the test can aid in determining additional ventilation needs. Data 

from the test can be obtained in 5 min 

or less, and a laboratory environment 

is not necessary to ensure meaningful 

results. 

REFERENCES 

1. Skow, Me L., A. Go Kim, and Me Ma 

Deul. Creating a Safer Environment in 

U.S. Coal Mines. The Bureau of Mines 

Methane Control Program, 1964-79. A Bureau of Mines Impact Report. BuMines 

Spec. Publ., 1981, 50 pp. 

2. Grau, Ro He 111, and Jo C. LaScola. 

Methane Emissions From U.S. Coal Mines in 

1980. BuMines IC 8987, 1984, 13 pp. 

3. Jeran, P. W., Do Ho Lawhead, and 

Mo C. Irani. Methane Emissions From an 

Advancing Coal Mine Section in the Pittsburgh Coalbed. BuMines RI 8132, 1976, 

10 PP* 

4 . . Methane Emissions From Four 

Working Places in the Beckley Mine, 

Raleigh County, W.Va. BuMines RI 8212, 

1977, 16 pp. 

5. Cervik, Jo Behavior of Coal-Gas 

Reservoirs. BuMines TPR 10, 1969, 10 pp. 

6. Hadden, Jo Do, and A. Sainato. 

Gas Migration Characteristics of Coalbeds. BuMines TPR 12, 1969, 10 pp. 

7. Krickovic, S., C. Findlay, and 

W. M. Merritts. Methane Emission Rate 

Studies in a Northern West Virginia 

Mine. BuMines TPR 28, 1970, 11 pp. 

80 Bruce, W. Em, and W. H. Donley. 

Methane Occurrence in U.S. ~etall~onmetal 

Mines: Emanation Mechanisms and Gas 

Volumes. Paper in Proceedings of the 2nd 

U.S. Mine Ventilation Symposium (Reno, 

NV, Sept. 23-25, 1985). A. A. Balkema, 

1985, pp. 333-343. 

9. Hyman, Do Me Methodology for Determining Occluded Gas Contents in Domal 

Rock Salt. BuMines RI 8700, 1982, 11 pp. 

10. Iannacchione, A. To, G. L. Finfinger, To Me Kohler, and Do M. Hyman. 

Investigation of Methane Emissions From 
an Advancing Face in the Belle Isle Domal 

Salt Mine, Louisiana. BuMines RI 8723, 

1982, 24 pp. 

11. Iannacchione, A. T., R. H. Grau 

111, A. Sainato, T. M. Kohler, and S. J. 

Schatzel. Assessment of Methane Hazards 

in an Anomalous Zone of a Gulf Coast Salt 

Dome. BuMines RI 8861, 1984, 26 pp. 

12. Schatzel, S. J., and D. M. Hyman. 

Methane Content of Gulf Coast Domal Rock 

Salt. BuMines RI 8889, 1984, 18 pp. 

U.S. GOVERNMENT PRINTING OFFICE 1987 - 605.01 7 60 078 

13. Iannacchione, A. T., and S. J. 

Schatzel. The Characterization of the 

Occurrence of Methane in Gulf Coast Domal 

Salt Mines. Paper in Proceedings of the 

2nd U.S. Mine Ventilation Symposium 

(Reno, NV, Sept. 23-25, 1985). A. A. 

Kalkema, 1985, pp. 353-360. 

14. Naiman, A., R. Rosenfeld, and 

G. Zirkel. Understanding Statistics. 

McGraw-Hill 3d ed., 1983, pp. 229-249.