Bulletin of the Seismological Society of America, Vol. 104, No. 3, pp. 1205–1211, June 2014, doi: 10.1785/0120130206
Cavity Decoupling of Small Explosions in Limestone by Anastasia Stroujkova, Robert Reinke, Jeff Duray, and Jessie Bonner
Abstract
A series of small chemical explosions were conducted in May 2012 at Kirtland Air Force Base, New Mexico, as a part of the HUMBLE REDWOOD III experiments. Two charges of 111 kg of ammonium nitrate and fuel oil were detonated at 7 m depth in limestone. The second explosion was detonated in a small asymmetric cavity produced by the first explosion at the same working point as the first explosion. Light Detection and Ranging (LiDAR) was used to image the cavity and to determine an equivalent spherical radius of 0.82 m. The seismic amplitudes for the cavity shot were reduced by a factor of 3–4 compared to the confined shot in limestone. In this article, we quantify the generation of the small cavity and the seismic decoupling it produced.
Introduction Detonation of a nuclear explosion in a cavity is an important evasion scenario for monitoring the Comprehensive Nuclear-Test-Ban Treaty (Glenn and Goldstein, 1994; Murphy et al., 1997). The National Research Council report (NRC, 2012) suggests that predicting the effectiveness of cavity decoupling is uncertain due to limited data on cavity decoupling in a variety of rocks. The report mentions that the dataset on cavity decoupling has not improved during the past decade. This article describes a unique experiment in New Mexico, in which the cavity formed by an 111 kg of ammonium nitrate and fuel oil (ANFO; or 90 kg trinitrotoluene [TNT] equivalent) fully confined chemical explosion in limestone was imaged using Light Detection and Ranging (LiDAR). After the imaging was complete, an equivalent yield explosion was detonated inside the cavity at the same working point as the first explosion. We quantify the differences in the waveforms produced by the confined and decoupled explosions, compare the observed cavity radius to predicted radii based on explosion source theory, and assess the extent of decoupling by the second explosion based on the compressive and tensile strengths of the limestone.
Experiment Overview As a part of the HUMBLE REDWOOD III (HR III) experiment, three 111 kg ANFO (90 kg TNT equivalent) charges were detonated above and below the surface in limestone. HR III-3 was detonated 2 m above the limestone and is not the focus of this article; it is studied in Bonner et al. (2013). In this article, we analyze the two buried explosions (Table 1). The first event (HR III-4) was detonated at the bottom of the 61 cm diameter borehole at a depth of 7 m. After lowering the ANFO charge, the borehole was grouted to prevent venting. After the explosion, the borehole was redrilled
in order to place a second 111 kg charge at the same depth in the rubble zone produced by the first shot. However, rather than a rubble zone being encountered at shot depth, the drill bit punched into a small, irregular-shaped cavity, which extended both above and below the preshot position of the HR III-4 charge. A pedestal of sand was created in the bottom of the cavity so that a second 111 kg ANFO charge could be placed at the exact same working point. More sand was added above the charge, partially filling the cavity with sand, but the charge was decoupled from the cavity walls. The borehole was regrouted, and the second blast (HR III-5) was detonated 11 days after the first explosion. The seismic signals were recorded on a network of nearsource (< 120 m) collocated accelerometer and overpressure sensors, circular rings of acoustic sensors at 0.3 km and 1 km, a semicircular ring of seismic stations at 1 km, and multiple seismic and infrasound sensors at local-to-regional distances. Figure 1 shows the locations of the explosions and the near-source seismic stations. These instruments recorded three components of motion at a sampling rate of 500 samples per second. Close-in (ranges between 20 and 120 m) accelerometers were also fielded at a depth of 1 m (Fig. 1b). These stations were recorded at a sampling rate of 5000 samples per second (20, 40, and 60 m) and 1000 samples per second (80, 100, and 120 m). The LiDAR image of the cavity is shown in Figure 2. The volume of the cavity is approximately V 2 2:33 m3 (Rob Cilke, Applied Research Associates, personal comm., 2013). The equivalent radius (the radius of a spherical cavity with volume V 2 ) is 0.82 m, which corresponds to a scaled radius of approximately R2 18 m=kt1=3. A pedestal of highporosity quartz sand was poured on the bottom of the cavity to the level of the first shot before placing the explosive container (V 1 0:14 m3 , corresponding to the equivalent spherical charge radius of R1 0:32 m). After the charge was placed
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A. Stroujkova, R. Reinke, J. Duray, and J. Bonner
(a)
in the cavity was approximately 1:24 m3, or 53% of the cavity volume. The remaining 6% of the cavity volume was occupied by the explosive container.
34° 58′ 12′′ WG20
34° 57′ 36′′
WG21
WG19 WG18 WG17 WG16 WG15 WG14 WG13 WG12
Data Analysis S5
WG11
34° 57′ 00′′
WG10 E4 WG09 WG08 WG07 WG06 WG05 WG04 WG03WG02 WG01
34° 56′ 24′′ -106° 29′ 24′′
-106° 28′ 48′′
FACT2
-106° 28′ 12′′
-106° 27′ 36′′ -106° 27′ 00′′
(b) 34° 57′ 29′′ NW80m NW60m NW40m NW20m
W80m
E20mE40m E60m
34° 57′ 25′′
S20m
SW80m
E80m E100m E120m
S40m S60m S80m S100m S120m
34° 57′ 22′′ -106° 28′ 34′′
-106° 28′ 30′′
-106° 28′ 26′′
-106° 28′ 23′′
Figure 1.
(a) Map of the experiment. The shot location is shown with a star; the triangles show the station locations. The distance between the sources and the semicircular array is 1 km. The area in the rectangle is enlarged in (b), which provides a map of the close-in three-component accelerometer array.
upon the sand pedestal, additional sand was placed into the cavity, partially filling it, as well as the borehole above, to a level of 0.73 m above the top of the cavity. The remainder of the borehole was then grouted to the surface. The total volume of sand in the cavity was approximately 1:48 m3. Although there was a significant amount of sand in the cavity, it had high porosity (φ ≈ 0:36), low density (1570 kg=m), and a moisture content of about 3%, and it was not coupled to the walls. Thus the total volume occupied by the quartz grains in the cavity was close to 41%. The volume of air
Decoupling is typically quantified by a decoupling factor (DF) that relates the reduction of the seismic amplitudes in the decoupled shot to a fully confined explosion. The DF varies with frequency. Although the cavity decoupling results in smaller DF at high frequencies (e.g., Murphy and Barker, 1995), the presence of the fracture zone surrounding the cavity may lead to high-frequency amplitude reduction (e.g., Stroujkova et al., 2013), thus producing the opposite effect compared to the cavity decoupling. To find the frequency-dependent DF for the HR III-5 experiment relative to HR III-4, we calculated spectral ratios for P-wave windows. DFs measured using the HR III-4 (fully confined) and HR III-5 (decoupled) data range between 2 and 4 at most frequencies. Figure 3a shows the waveform and the spectra comparison between HR III-4 and HR III-5 at station W80, located 80 m from the source. Notice there is an approximately 2.5 ms phase delay for the P wave and a slightly shorter pulse width for HR III-5 compared to HR III-4. The event spectra calculated using the Welch method (Welch, 1967) and the spectral ratios are shown in Figure 4b,c. The spectral ratios are close to 3 at low frequencies and decrease to 1 at high frequencies. Other near-source stations show similar behavior. Examples of the waveforms recorded by the semicircular array are shown in Figure 4a. The waveforms exhibit remarkable similarity between the two explosions; however, the amplitude of the second explosion is reduced by a factor of 3. Similar to the near-source station (W80), the spectral ratios are close to 3 below the corner frequency and become less than 3 above (Fig. 5c). Figure 5a shows the amplitudes of the first P-wave pulses for HR III-4 and HR III-5 for the seismic stations recording the explosion at distances less than 2 km. The absolute P-wave amplitudes exhibit significant variations (by factor of 2–3), with azimuth caused by spatial variability in the propagation path. Based on the irregular-shaped cavity produced by the explosion, we expect that asymmetric source radiation may also be a factor contributing to these azimuthal variations. Even with the significant site, path, and possible source effects, we note that the P-wave amplitude ratios (Fig. 5b) are more homogeneous than the amplitudes themselves and consistently vary between 3 and 4. This
Table 1 HR III Event Information Event
Yield (kg)
TNT Equivalency Yield (kg)
Depth (m)
Explanation
HR III-4 HR III-5
111 111
90 90
7.02 6.94
Virgin limestone/shale (charge near limestone/shale contact) Reshoot in cavity/fracture zone of HR III-4
Cavity Decoupling of Small Explosions in Limestone
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Figure 2. LiDAR image of the cavity created by HR III-4 where HR III-5 was conducted. The depth of the samples is color coded, ranging from 6.2 m (red) to 7.8 m (blue).
(a)
(a)400
–4
x 10
5
Station WG21 P window
u (cm)
a (cm/s/s)
Station W80m 200 0
0
HR III-4 HR III-5
–200 –400
–5 0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0
0.2
0.4
(c)
(b) 1
10 10
–2
1.2
1.4
1.6
1.8
10
10 –6 10 –7
1 10
100
f (Hz)
10
100
f (Hz)
Figure 3.
(a) Acceleration traces (Z components) recorded by near-source station SW80 for HR III-4 (gray line) and HR III-5 (black line); (b) spectral amplitudes calculated for the time window marked in (a); (c) spectral ratios between HR III-4 and HR III-5.
suggests that if source effects are important to the radiation pattern, then both shots were influenced by similar phenomena. Finally samples of records from the close-in accelerometer array are shown in Figure 6. Because of the recording system’s 16-bit dynamic range and overly conservative ground-motion predictions, the data quality was less than ideal at some locations. However, the records in Figure 6 clearly display the amplitude reduction for HR III-5 relative to HR III-4, as well as the first arrival delay of 2.5 ms. The high-frequency early arrival on station E20m record for HR
Ratio
10 –5
10 0 –1
1
10 –4
u (cm)
10
0.8
(c)
(b) 10
Ratio
a (cm/s/s)
10 2
0.6
Time (s)
Time (s)
10 –8
HR III-4 HR III-5 10
HR III-4 / HR III-5 1 100
10
100
f (Hz)
f (Hz)
Figure 4. (a) Displacement seismograms (Z components) recorded by a station from the semicircular array (WG21) for HR III-4 (gray line) and HR III-5 (black line); (b) spectral amplitudes for the traces in (a); (c) spectral ratios between HR III-4 and HR III-5. III-4 is unexplained. For both HR III-4 and HR III-5, the first arrivals on the close-in array are best fit by an apparent velocity of approximately 3000 m=s.
Discussion Cavity Size Denny and Johnson (1991) proposed the following expression to calculate the radius of the explosive cavity created by detonation of a tamped explosion:
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A. Stroujkova, R. Reinke, J. Duray, and J. Bonner
(a)
0
0
(b)
30
330
30
330
–4
1.5x10 –4
300
1x10
300
60
60
–5
5x10
90
270
4
3
90
120
210
150
210
270
2
240
120
240
1
150 180
180
HR III-4 HR III-5
HR III-4 / HR III-5
Figure 5. (a) Azimuthal variations of the P-wave amplitudes for HR III-4 (gray circles) and HR III-5 (black circles); (b) ratios of the amplitudes between HR III-4 and HR III-5 as a function of azimuth. 80
Station E20m
a (m/s/s)
60
HR III-4 HR III-5
40 20 0 –20 –40 80 0
0.01
0.02
0.03
0.04
0.05
0.06
0.08
0.09
0.1
Station S20m
60
a (m/s/s)
0.07
Cavity Effects on Seismic Radiation
40 20 0 –20 –40 0 80
0.01
0.02
0.03
0.04
0.05
0.06
a (m/s/s)
0.07
0.08
0.09
0.1
Station NW20m
60 40 20 0 –20 –40
personal comm., 2012). The estimated cavity radius for a 90 kg TNT equivalent explosion is 0.98 m (lab estimates) or 1.69 m (in situ). Both of these estimates are larger than the observed cavity radius, although the estimate based on the laboratory measurements is within measurement error for the cavity. We note possible uncertainty on the true cavity dimensions for several reasons, as detailed in the next section.
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
Time (s)
Figure 6. Comparison between HR III-4 and HR III-5 vertical accelerometer records at the 20 m range (stations E20 m, top plot; S20 m, middle plot; and NW20 m, bottom plot). 1:47 × 104 W 3 Rc 0:3848 0:2625 0:0025GP ; VS P0 10 1
1
in which V S is the shear-wave velocity, P0 is the overburden pressure (which is related to shot depth), W is the yield of the explosion, and GP is the gas porosity. Table 2 provides both in situ (obtained by seismic-while-drilling cross hole surveys in the immediate ground-zero area) and laboratory measurements for the parameters required to estimate Rc in equation (1) (Casey T’Kindt, Applied Research Associates,
According to our seismic data analysis, the presence of the cavity reduced the seismic amplitudes on average by a factor of 3.41. In addition, there is an approximately 2.5 ms time delay in P-wave arrivals. This time delay is particularly noticeable at the close-in stations due to high sampling rate and shorter signal duration. The amplitude reduction for HR III-5 shot agrees with previous experiments, and the decoupling factor of 3–4 indicates partial decoupling. In this section, we discuss the effects of the cavity filled with sand on the amplitude reduction and the time delay between the explosions. Atchison et al. (1964) studied decoupled explosions in limestone and granite and determined that the ratio between the stress amplitudes can be related to the ratio of the cavity radius to the explosion charge radius using the distance exponents. The decoupling ratio between the explosions with the initial cavity radii R1 and R2 is given by 3y−mn σ1 R2 m ; σ2 R1
2
in which m is the distance exponent (amplitude decay with distance) in the transition zone (between the final cavity radius and the elastic radius), and n is the distance exponent in the seismic zone. The approximate values of m and n according to Atchison et al. (1964) are n 1:90 and m 2:00 for
Cavity Decoupling of Small Explosions in Limestone
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–3
Accelerometer at 20 m
–4 Cavity (sand/air mix) Rc = 0.82 m
log(A)
–5 Intact material Vp = 3000 m/s
d
–6
n = 1.88
Rc Damage zone d ~ 3.1 m Vp ~ 1265 m/s
–7
–8
Figure 8. Idealized diagram showing the spherical cavity (partially filled with sand) and the damage zone.
–9 –10
–3
–2.5
–2
–1.5
–1
–0.5
0
log(r)
Figure 7.
Amplitude decay with range for HR III-4.
Lithonia granite and n 2:10–2:65 and m 2:15–2:73 for limestone. The average values for the solid rocks are n 2:00 and m 2:06. We estimated the distance exponent for HR III-4 data using the seismic station located between 80 and 400 m. Figure 7 shows the plot of the displacement amplitude decay with the distance. The linear fit is n 1:88. As was also noted by Atchinson et al. (1964), we observe that m is typically larger than n by 3%–5%; we therefore select a value of 1.93 for m. For our explosions, the HR III-4 charge radius was 0.32 m, but the cavity radius for HR III-4 was approximately 0.82 m (R1 =R2 2:56). Using n 1:88, m 1:93, and γ 1:2, the predicted decoupling ratio is approximately 4.59, which is higher than observed. However, this estimate is valid for the air-filled cavity without sand, whereas in reality the cavity was partially filled with sand. If we take into account the volume of sand in the cavity and its porosity, we can estimate an equivalent spherical cavity radius based on the total void space in the cavity, including the sand porosity but neglecting the water content. This gives an equivalent radius of approximately 0.71 m. This approach yields a decoupling factor of ∼3:6, close to the average observed value of 3.41, but this may well be an oversimplification of the behavior of the sand–air mixture within the cavity. We observe a consistent 2.5 ms delay in the initial P-wave arrival at the nearest accelerometer stations (e.g., Figs. 3 and 6). This delay can be explained by the stress-wave propagation through the air–sand mix in the cavity and through a region
of damaged material outside the cavity produced by HR III4. Figure 8 contains an idealized diagram of the possible in situ condition for the HR III-5 cavity shot. For both HR III-4 and HR III-5, the shock wave travels to the edge of the charge (r 0:32 m) at the same speed. In the case of HR III-4, the shock wave should travel 0.5 m through the region from the charge boundary (at 0.32 m) to the edge of the eventual cavity (at 0.82 m) at the in situ V P speed of approximately 3 km=s, yielding a travel time of approximately 0.17 ms. In the case of HR III-5, the shock wave likely travels through the air–sand mixture and the damage zone outside the cavity at a much slower speed. Development of an elaborate physical model for shock propagation through the sand- and air-filled cavity is beyond the scope of this study; however, White and Byrne (1994) discuss a large-scale laboratory study of stress-wave propagation through sand with very similar properties (wet density 1580 to −1700 kg=m3 and moisture content of 2.8%–4%) to the HR III-5 cavity sand. At shock-wave stress levels of 20–40 MPa (induced by a high-velocity plate impact in a sand column), they found propagation velocities of 300–400 m=s. Assuming a shock-wave propagation velocity of 400 m=s implies a transit time through the 0.5 m sand–air region of 1.25 ms. This value likely represents an upper bound to the transit time because the velocity is likely much higher in a small region very near the charge. Because the propagation time through the charge is the same for both HR III-4 and HR III-5, we estimate that 1:25–0:17 ms 1:08 ms of the observed 2.5 ms delay comes from propagation through the sand–air mixture in the cavity. The remaining 1.42 ms delay can be accounted for by a V P decrease in the damage annulus outside the cavity. After
Table 2 Limestone Physical Properties V P (m=s)
Density (kg=m3 )
Depth (m)
Ultrasonic P wave
In situ P wave
Poisson Ratio
Young Modulus E (GPa)
Grain
Bulk (dry)
Porosity (%)
Unconfined Compressive Strength (MPa)
5.21
4990–5200
1000–3000
0.23
57.8
2681
2604
2.9
176.21
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A. Stroujkova, R. Reinke, J. Duray, and J. Bonner
Orlenko (2002), we estimate the radius of the damage (rubble) zone (Rr ) as Rr Rc
E 3σ c
1 3
;
3
in which E is the rock’s Young modulus, Rc is the cavity radius after the explosion, and σ c is the compressional strength of the emplacement rocks. Using the value σ c 176 MPa, the Young modulus measured in the lab (E 57:8 GPa, from Table 2), and Rc 0:82 m, we obtain the radius Rr 3:92 m for the rubble zone produced by HR III-4. We note that the in situ strength of the limestone is likely reduced by the presence of pre-existing fractures; therefore, the measured values can be used as the upper bound on the strength estimate. The apparent in situ V P observed on the close-in accelerometer array (at 20, 40, and 60 m) is 3 km=s. If we assume that a value of V P in the damage annulus for HR III-4 is equal to 3 km=s and that the remaining 1.42 ms delay for HR III-5 is produced in this region, then the average post-HR III-4 V P in this region is approximately 1265 m=s for the damage zone extending between Rc and Rr . To account for the total 2.5 ms travel-time delay, an average velocity of 878 m=s would be required. According to Santi (2006), the minimum V P for geologic material to stand unsupported in a vertical face is 1000 m=s. This value is below the interpreted average value of 1265 m=s. In addition, a damage radius of approximately 3.9 m is roughly consistent with the extent of observable surface deformation based on pre- and post-test digital photogrammetric surveys of the test bed (Lenox and Farley, 2012). This is an approximate estimate for the time delay and not a rigorous analysis of the shock-wave propagation. In addition to the decoupling produced by the sand- and air-filled cavity, the broken rock surrounding the explosive cavity is likely to further reduce the signal generated by HR III-5. To unambiguously determine the cavity decoupling effects for HR III-5 would require a reference shot in the same damaged material as that immediately surrounding the cavity, which is not possible. We note that the spectral ratios shown in Figures 3 and 4 exhibit the classic cavity decoupled signature with reduced DF at the higher frequencies (e.g., Murphy and Barker, 1995). In addition the observed HR III-5 DF of ∼3–4 at the low frequencies corresponds to the factor of 3.6–4.7 amplitude reduction predicted by Atchison et al. (1964) for the cavity, depending on whether or not the sand is considered when evaluating the effective cavity radius.
Conclusions We have studied two 111 kg ANFO explosions (90 kg TNT equivalent) in limestone, with the second explosion being conducted in the cavity created by the first shot. The cavity produced by HR III-5 shot has a nonspherical shape and an equivalent spherical radius of 0.82 m, or 18 m=kt1=3.
The cavity radius agrees well with the radius predicted by the Denny and Johnson (1991) model (0.98 m) based on laboratory estimates for the emplacement rock. The cavity was partially filled with sand after the charge for shot HR III-5 was placed. The ratio of the radius of the HR III-5 cavity to the initial explosive charge radius is 2.6, whereas the seismic amplitudes were reduced on average by a factor of 3.41. Based on work done by the rock mechanics/blasting community, the ratio between the emplacement cavities should result in a decoupling factor of about 3.6–4.7. Thus, the observed decoupling factor is in good agreement with the predicted value range. For comparison, an air-filled cavity of this size in salt produced a decoupling factor of 26 for nuclear explosions and about 8 for chemical charges (Glenn and Goldstein, 1994). Another study by Murphy et al. (1997) has shown that small chemical explosions in limestone are fully decoupled if the scaled cavity radii are greater than 27 m=kt1=3 . We note, however, that these explosions were conducted at significantly greater depths, so the results cannot be directly related to the shallow explosions described in this paper. In addition to the amplitude reduction, a 2.5 ms delay was observed for HR III-5 first arrivals compared to those for HR III-4. Based on our approximate estimate of the extent of a postshot damage region, as well as average material properties in this region, we found that the 2.5 ms delay can be caused by slower pressure-wave propagation through the sand-filled cavity and the damage annulus produced by HR III-4.
Data and Resources Seismoacoustic data from the HR III experiments were collected by many groups, including the Defense Threat Reduction Agency (DTRA) Test Division, Lawrence Livermore National Laboratory, Los Alamos National Laboratory, National Securities Technologies, the University of Mississippi National Center for Physical Acoustics, Incorporated Research Institutions for Seismology–Program for the Array Seismic Studies of the Continental Lithosphere (IRISPASSCAL), and Weston Geophysical Corporation. Inquiries about the HR III dataset should be addressed to the Program Manager and coauthor Robert Reinke or the co-HR III Program Manager, Elizabeth Lenox (
[email protected]). Some of the data will become available after a two-year proprietary period through the IRIS Data Management Center under the experiment name HR III.
Acknowledgments There are many people who helped in the planning, execution, and data collection stages of the HR III experiments. The HR III explosions were funded by the Defense Threat Reduction Agency (DTRA) Test Division. Phillip Cole of DTRA supported some of the data collection and analyses. Additional programmatic support was provided by Mike Giltrud of DTRA. Elizabeth Lenox of DTRA was the co-HR III Program Manager. We also thank the following people for support: Brian Allen, Stephen Arrowsmith, Steve Azevedo, Diane Baker, Lonnie Bamert, Phillip Cole, Greg Dutro, Jessica Egli, Aaron Ferris, Cathy Pfiefer, Jill Franks, Derrick Hess, Claus Hetzer, Cheng Ho, Michael Howard, Dan Kleinert, Mark Leidig, Al
Cavity Decoupling of Small Explosions in Limestone Leverette, Bridget O'Neil, Delaine Reiter, Dave Thomas, and Roger Waxler. We especially thank William Harrell and Bill Mason for help in deploying the semicircular network of seismic stations. We thank Jim Lewkowicz for his field support and his faith and trust in all our endeavors.
References Atchison, T. C., W. I. Duvall, and J. M. Pugliese (1964). Effect of decoupling on explosion-generated strain pulses in rock, U.S. Department of the Interior, Bureau of Mines, RI 6333, 49 pp. Bonner, J. L., D. Russell, and R. Reinke (2013). Modeling surface waves from aboveground and underground explosions in alluvium and limestone, Bull. Seismol. Soc. Am. 103, no. 6, 2953–2970, doi: 10.1785/ 0120130069. Denny, M. D., and L. R. Johnson (1991). The explosion seismic source function: Models and scaling laws reviewed, in Explosion Source Phenomenology, American Geophysical Monograph, 65, 1–24. Glenn, L. A., and P. Goldstein (1994). Seismic decoupling with chemical and nuclear explosions in salt, J. Geophys. Res. 99, no. B6, 11,723. Lenox, E., and K. Farley (2012). Interim Digital Photogrammetric Report for the HUMBLE REDWOOD III Experiments (DTRA/J9CXTT), 47 pp. Murphy, J., and B. Barker (1995). A Comparative Analysis of the Seismic Characteristics of Cavity Decoupled Nuclear and Chemical Explosions, Pl-TR-95-2117, Phillips Laboratory, 96 pp. Murphy, J., I. Kitov, N. Rimer, V. Adushkin, and B. Barker (1997). Seismic characteristics of cavity decoupled explosions in limestone: An analysis of Soviet high explosive test data, J. Geophys. Res. 102, no. B12, 27,393–27,405. National Research Council (NRC) (2012). The Comprehensive Nuclear Test Ban Treaty-Technical Issues for the United States, National Academies Press, Washington, D.C. Orlenko, L. P. (2002). Fizika Vzryva (Physics of the Explosions, in Russian), 3rd edition, Fizmatlit, Moscow, Russia, Vol. 1, 656 pp, ISBN: 5-92210219-2. Santi, P. M. (2006). Field methods for characterizing weak rock for engineering, Environ. Geotech. Eng. XII, 1–11. Stroujkova, A., J. Bonner, and T. Rath (2013). Effect of fractures on seismic amplitudes from explosions, Bull. Seismol. Soc. Am. 103, 580–587, doi: 10.1785/0120120082.
1211 Welch, P. D. (1967). The use of fast Fourier transform for the estimation of power spectra: A method based on time averaging over short, modified periodograms, IEEE Trans. Audio Electroacoust. AU-15, 70–73. White, H. G., and J. T. Byrne (1994). DNA/WES ground motion test facility —Results and analysis of impact tests against masonry and socorro plaster sand testbeds, U.S. Army Corps of Engineers Technical Report SL-94-2, 155 pp.
Weston Geophysical Corporation 181 Bedford St. Suite 1 Lexington, Massachusetts 02420
[email protected] (A.S.)
Defense Threat Reduction Agency 1680 Texas St. SE Kirtland AFB, New Mexico 87117
[email protected] (R.R.)
Applied Research Associates Kirtland AFB, New Mexico 87117
[email protected] (J.D.)
Weston Geophysical Corporation 2603 Copeland St. Lufkin, Texas 75904
[email protected] (J.B.)
Manuscript received 31 July 2013; Published Online 15 April 2014
