Modelling Local Seismograms of French Nuclear Tests In Taourirt Tan Afella Massif, Hoggar, Algeria Stéphane Gaffet, B. Massinon, J.-L. Plantet, Y. Cansi

To cite this version:

Stéphane Gaffet, B. Massinon, J.-L. Plantet, Y. Cansi. Modelling Local Seismograms ofFrench Nuclear Tests In Taourirt Tan Afella Massif, Hoggar, Algeria. Geophysical Journal International, Oxford University Press (OUP), 1994, 199 (3), pp.964 - 974. ￿10.1111/j.1365-246X.1994.tb04028.x￿. ￿hal-00497734￿

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HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Geophys. J. Int. (1994) 119,964-974

Modelling local seismograms of French nuclear tests in Taourirt tan Afella massif, Hoggar, Algeria

S. Gaffet,' B. Massinon: J.-L. Plantet2 and Y. Cansi2 ' Institut de GCodynamique, 250 rue Albert Einstein, Sophia-Antipolis 1, 06560 Valbonne, France Laboratoire de DPtection et de GPophysique, BP 12, 91680 Bruye'res-le Chritel, France

Accepted 1994 June 10. Received 1994 June 10; in original form 1993 October 18 Downloaded from https://academic.oup.com/gji/article/119/3/964/629091 by guest on 03 February 2021

SUMMARY The influence of topography on the shape and amplitude of seismograms recorded at short distances is investigated for a set of nuclear explosions detonated in the Taourirt tan Afella mountain, Hoggar, Algeria, which was the French nuclear test site during the 1960s. The wavefield observed on one side of the mountain compared to that on the other shows a large-amplitude phase generated by reflection of the direct field inside the mountain. This phase is back-diffracted and can be seen in one azimuth and not in the opposite. The existence of such a phase depends on the source location inside the mountain. The amplitudes of standard phases are also modified with a factor of 2.5 by the topography between observations made in one azimuth compared to the others. Key words: nuclear explosion, numerical modelling, site effect.

1 INTRODUCTION 2 DATA COLLECTED The French nuclear test site used in the 1960s is located in Thirteen underground explosions were detonated in the the Hoggar massif (Fig. l), 4 km west from In Eker, and massif between November 1961 and February 1966 (see 150 km north from Tamanrasset in the south of Algeria. The Table 2). Three sets of the recorded ground velocities are topography has been processed with GMT software (Wessel displayed in Fig. 3, which correspond to the explosions Jade, & Smith 1991). The test site is located in a granitic massif, Opale and Rubis respectively. Only Rubis has been the Taourirt tan Afella (5'2' N; 24'3' N), which is intruded recorded at short distance (1.73 km) from the source at BRI. in two gneiss series (Faur6 1972; Boullier & Bertrand 1981). All explosions have been recorded at distances of around The massif is intruded to the west side of a 1 km thick 15 km at BRIII and 30 km at INA. mylonitic zone (Fig. 2, from FaurC 1972). This zone, The shape of the ground-velocity waveform differs greatly approximately oriented N-S separates the In Eker gneiss between these three explosions. The recordings at BRIII series (west side) and the Tefedest gneiss series (east side). and INA for Rubis show pure P wave and Rayleigh wave. Both series present a N-S schistosity. The area around the The recording of Opale at INA shows a longer surface wave massif is covered by a thin layer of sand. The massif is 8 km train on the horizontal component. The greatest difference is long in the N-S direction and 5.6 km wide in the E-W found for recordings of explosion Jade which shows a long direction and has a maximum height of 2000m in a region and strong coda associated with surface waves at BRIII and with 1000 m mean elevation. INA. These differences may be explained by different The study is an analysis of the relation between causes, i.e. explosion working (spalling, coupling), water topography and waveform of the French explosions saturation in the source region (and its evolution between detonated in the Taourirt tan Afella massif. This study each experiment), or surrounding geological structures and concerns the influence of the topography on the ground topography which induce multipathing and amplification motions at local distances (i.e. from 1 km to 30 km) with the effects. aim of understanding the waveforms observed in the N-S and E-W azimuths at BRI (approximately 2 km south-west 3 STUDY from the explosions studied), BRIII (15 km west), INA (i.e. In Amguel, 30km south). Teleseismic propagation is not Teleseismic influence of the tan Afella topography shape has accounted for here. been detailed by Rocard (1964) and McLaughlin & Jih

964 Seismograms of nuclear tests in Algeria 965

5 5' 02'E 5' 04E 24' 06" 24' 06" J 0' 2WE

50"

40" 24' 04"

30"

20" Downloaded from https://academic.oup.com/gji/article/119/3/964/629091 by guest on 03 February 2021

24' 02"

24' Wlr; 24' OO" 'E 5' 02'E 5' ME

Figure 1. Location of 1960s French nuclear test site. Explosions are depicted with black circles and stars. Stars are used for Jade, Opale and Rubis, the explosions examined in this study.

(1988). Our main objective here is to understand the ground south-eastern explosions, respectively called explosions I displacement waveforms at local distances (less than 30 km) and 11. Fig. 6 shows the ground velocity computed with a in both N-S and E-W azimuths, in relation to the location Ricker source pulse central frequency of 3.5 Hz, at stations of the shot inside the massif and to the azimuth of recording. distributed from 31 km to the north-west to 31 km to the To realize this objective, the global response of the Taourirt south-east of both explosions. The time duration is 13 s. tan Afella structure (Duclaux & Michaud 1970; Munier The first arrival is a P wave (P=;"and Psi) that propagates 1982) is considered to be made up of both topography and alone in both azimuths for explosion I, while it consists of crustal structure responses. The total displacement field two pulses (Pi' and P:;) delayed by 0.8 s in the north-west generated by topography and crustal structure is made of and one pulse in the south-east (Pz)for explosion 11. The multiple diffractions between topography and crustal inverted polarity and amplitude variation of P:: compared structure. This implies that it is impossible to linearly to P:; may indicate that the second pulse has been separate the topographic effect from the underground- generated by reflection of the direct P wavefield inside the structure effect. The site effects presented below are related mountain. to the topography alone without taking into account the The surface waves show a more complex shape and underground structure below it. amplitude behaviour for explosion I than for explosion 11. The results presented below are relative to the topographical effects of Taourirt tan Afella. They show the (1) The relative amplitude in the north-west azimuth is 25 ground displacement modifications induced by (i) the topo- per cent and 40 per cent lower than the one obtained in the graphy shape and (ii) the detonation location inside the south-east for explosions I and I1 respectively. mountain. The numerical method used for computation is (2) Waveforms of R=;" and R;YAA,phases have a higher the discrete wavenumber-boundary integral equation frequency content for explosion I than has the Riw phase for method (Bouchon & Aki 1977; Gaffet & Bouchon 1989). explosion 11. The wave-propagation velocities are (Y = 6 km s1 and (3) In the case of explosion 1, the main surface Rayleigh p = a/fi,and the source-time function is a Ricker pulse. wave train, R=;", is followed by a coda, RSYAA,,which may Figure 4 shows, as reference, the ground velocities consist of a back-diffracted surface wave inside the obtained for an explosion buried at 300 m depth under a flat south-eastern side of the massif. The symmetric observation surface. The only waves generated are the direct P wave of such a coda cannot be made for explosion 11, i.e. this followed by the Rayleigh wave of lower amplitude. observation implies that the hillside steepness strongly Preliminarily, an observational analysis is made with two shapes the back-diffracted field. fictitious explosions located in both sides of the massif along a NW-SE section as shown Fig. 5. The explosions are A detailed comparison of the ground velocities at 21 km is buried at 290m and 100m depth for the north-western and displayed in Fig. 7 for both explosions. Besides the 966 S. Gaffet et al.

10

laourirt Tan Afelia In Eker

240 00 Downloaded from https://academic.oup.com/gji/article/119/3/964/629091 by guest on 03 February 2021

Ti medi idaouine Petit Taourirt

23" 50

Figure 2. This map, from FaurC (1972), illustrates the geological formation located around the Taourirt tan Afella test site.

Table 1. Location of the ground velocity waveform differences observed between north and south, recorders. the relative amplitude given in Fig. 7 shows that the surface ellipticity (called E, and defined here by the ratio Code Lat N Lon E HmaX/Zmax)of the Rayleigh wave remains constant, whatever the azimuth is, for a given explosion location. On BRI 24 02 07.7 05 01 17.8 the other hand, ellipticity varies when source location changes, i.e. = 0.60 and E~~ = 0.80 while E = 0.6812 BRIII 24 02 00.8 04 55 01.9 (Ben-Menahem & Singh 1981) should be obtained without mountain topography and a Poisson ratio of 0.25 as used in this paper. Thus, the phase velocity of the main Rayleigh INA 23 47 30.0 05 07 30.0 wave train is modified by source site condition. Seismograms of nuclear tests in Algeria 967

RUBIS

t 123 4 5 6 7 81 JADE OPALE l1,.8 , 1 I 1

BRlll Z w,

t Downloaded from https://academic.oup.com/gji/article/119/3/964/629091 by guest on 03 February 2021

n 4'' 05 05 1 BRZll E W n

. -95 I

Figure 3. Ground velocity recorded for Jade, Opale and Rubis at locations BRI for Rubis, and BRIII and INA for Jade, Opale and Rubis. Locations of receivers are shown in Table 1.

Figure 8 displays the realistic geographical configurations Though R;Prth has a longer duration of large-amplitude that correspond to explosions Jade, Opale and Rubis. The oscillations for Rubis, i.e. approximately one cycle more, sections and relative explosion locations obtained for Jade compared to Jade/Opale, the relative amplitude of the and Opale are so similar at the wavelengths studied, i.e. surface wave train RngOrth of Jade/Opale is 1.5 times the A, = 1km (see right side of Fig. 7), that they will now be relative amplitude for Rubis at epicentral distances greater considered as one detonation point. This means that the than 15 km. The relative amplitudes obtained in the N-S Rayleigh waveform differences observed at BRIII (Fig. 3) azimuth are very different between both sites. The relative cannot now be associated with the topographical effects amplitude for Jade/Opale is 2.5 times the one for Rubis; this studied but should be related to the explosion source itself. result reproduces well the observation of P and Rayleigh Besides the N-S section for Rubis and the two W-E wave-amplitude ratio at station INA (Fig. 3). Rubis shows a sections, the N-S section related to Jade/Opale does not longer Rayleigh wave train RTuth, followed by a coda respect any cylindric aspect, i.e. in this case, results obtained RsouthC(A), than RYthfor explosion Jade/Opale. by the corresponding 2-D simulation will be solely The W-E section ground velocities are compared in Fig. descriptive and should be cautiously related to real data 10 for both sites. The topographic heterogeneity dimensions such as those mentioned hereafter, at INA for instance. of the two configurations are 5 km long X 1 km height for Taking into account the previous remark, Figs 9 and 10 Jade/Opale and approximately 3 km long X 0.5 km height display the ground velocities calculated in N-S and E-W for Rubis. This implies that coupling wavelengths for Rubis azimuths for explosions Jade/Opale and Rubis. The source should be reduced by half compared to Jade/Opale. Such an pulse is a ,Ricker function centred at 3.5Hz. The effect is clear in the eastern azimuth with RTt frequency time-window duration is 13 s. Explosion Jade/Opale is behaviour between both sites, while it does not appear in buried at 370m vertical and 310m normal depths, the the western azimuth. Two consecutive P waves, PXst large explosion Rubis is buried at 550m vertical and 450m and P:yt low amplitudes, delayed by 0.8 s, are generated at normal depths. Explosion Jade/Opale is located inside the the explosion site corresponding to Jade/Opale. This minor mountain i.e. approximately 100 m above the 1000 m mean second phase may be a diffracted P surface wave, its time elevation of the surrounding tableland, while explosion delay and small amplitude being induced by the propagation Rubis is buried at a level approximately 100m under this along the mountain topography. The Rayleigh wave train, 1000 m elevated plateau. RTst generated at the Jade/Opale site has a longer time The phases noted PngOrthand R>Orth (Fig. 9) show similar duration, 1.7 s, and a smaller relative amplitude than that waveforms for both explosion sites Jade/Opale and Rubis. observed for Rubis. It also shows a low-amplitude Rayleigh 968 S. Gaffer et al.

Velocity at 3.5 HZ- Time window: 7.80 s

-16 h

-IS h

-I4 km

-13 km

-12 km

-11 km

0 -10 h cl- V - Downloaded from https://academic.oup.com/gji/article/119/3/964/629091 by guest on 03 February 2021 .. -9 h

5h

4br

+4h

+5 h

&in

+I h +ah

+9 h

+I0 h

Figure 4. Horizontal (H) and vertical (Z) components of the ground velocity computed for an exposion buried at 300m depth under a flat surface in a homogeneous space. The source is a Ricker pulse centred at 3.5 Hz. The time window is 7.8 s. Receivers are identified by their epicentral distances (from -18 km to +18 km). wave train that arrives before the main surface wave train, complex wave diffraction scheme by topography than the and this precursor does not appear for the Rubis site. one for the W-E section. A general observation of stacks presented in Figs 9 and 10 A detailed comparison between ground-velocity com- is that seismograms obtained in the N-S azimuth show more ponents calculated at 21 km epicentral distance from each complex time duration, waveform and frequency content side, i.e. azimuths, for both Jade/Opale and Rubis sites behaviours than those calculated for the W-E section. This is displayed in Fig. 11. The different phases discussed observation may be related to a more complex N-S previously are labelled above each component. A new time, topography shape (see Fig. 8), therefore inducing a more surface ellipticity, based on the relative amplitudes found for Seismograms of nuclear tests in Algeria 969

24' 06h 24-06"

24' 04"

-5 0 5 Downloaded from https://academic.oup.com/gji/article/119/3/964/629091 by guest on 03 February 2021

24' 02" 24' 02" -5 0 5 offset in km

24' 00" 24' OO" 5 'E 5' 02'E 5' WE

Figure 5. Geometrical configuration used to compute the seismograms presented in Figs 6 and 7. A NW-SE section crosses the tan Afella massif with two fictitious explosions called I and I1 in the text. Locations in 1-D section are given on the right side, with elevation in metres and offset from explosion in kilometres. each component, remains constant whatever the azimuth is Rayleigh wave train for explosion Jade/Opale in the west and appears to be sensitive to both topographic hetero- and for explosion Rubis in the north. These precursors are geneity shape and explosion location. E = 0.68 for explosion generated with a similar and symmetric diffraction scheme Jade/Opale in the N-S azimuth, E-0.62 in the W-E (see the W-E section for Jade/Opale and the N-S section azimuth while ~e0.72for explosion Rubis in the N-S for Rubis in Fig. 8). These precursors are generated by the azimuth and E = 0.64 in the W-E azimuth. direct P wavefield diffracted into Rayleigh wave on the The different components stacked in Fig. 11 align both P opposite side of the mountain while the main Rayleigh wave arrivals and the main Rayleigh wave trains, i.e. train is generated by the direct P wavefield diffracted into R;o;:hgor south. Precursors are depicted before the main Rayleigh wave on the nearby topography. Except for explosion Rubis in the N-S section, the main frequency Table 2. French explosions and their characteristics. content varies from one side to the other for each section and explosion. It appears that the more the topography Explosion Date Time UT Lat N Lon E rnb steepens, the lower the frequency of the main Rayleigh Agate 1961/11/07 llh 29m 59.931s 24' 3' 25.5" 5O 3' 7.6" wave train becomes. This phenomenon appears simul- taneously on both components. Such behaviour is not seen Beryl 1962/05/01 10h OOm 00.458s 24' 3' 46.8" 5' 2' 30.8" for Rubis at the N-S section, which it may be explained by Emeraude 1963/03/18 10h 02m 00.351s 24' 2' 28.9'' 50 3' 7.9" identical steepness on both sides of the mountain. Amethyste 1963/03/30 09h 59m 00.328s 24' 2' 36.0" 5" 3' 25.2" 4 CONCLUSION Rubia 1963/10/20 13h OOm 00.011s 24' 2' 7.8" 5O 2' 19.0" 5.6 ( USGS ) The results show the great influence of topography on the OOm 24' 3' 13.1" 5O 3' 8.6" opale 1964/02/14 Ilh 00.347s ground velocity recorded in the frequency band 1-4Hz, at Topaze 1964/06/15 13h 40m 00.367s 24' 3' 59.8" 5O 2' 4.4" epicentral distances from 3 to 31 km for three explosions Turquoise 1964/11/28 10h 30m 00.035s 24' 2' 30.7" s4 2' 30.1" detonated inside the granitic massif of the Taourirt tan Afella. The general result is that the topography alone Saphip 1965/02/27 llh 30m 00.039s 24' 3' 31.4" 5O 1' 52.3" 5.8 ( USGS) strongly shapes the ground velocities, their amplitudes and Jade 1965/05/30 llh OOm 00.037s 24' 3' 18.0'' 5O 3' 3.1" their frequency contents. Corindon 1965/10/01 10h OOm 00.043s 24" 3' 53.7" 5O 2' 2.6" The main results are as follows. Tourmaline 1965/12/01 10h 30m 00.088s 24" 2' 37.4" 5" 2' 48.9" 5.1 ( USGS ) (1) The back-scattered wave strongly shapes the seismo- Grenat 1966/02/16 Ilh OOm 00.035s 24" 2' 39.0" 5" 2' 28.4" grams. The surface wave appears to be generated after 970 S. Gaffet et al.

Explosion I (NW)- N147 section Explosion II (SE) - N147 section nw nw nw P- R-rR P nw-lrP nw I--R nw Z C(Al Z -31 - NW -31 -NW u H.. -29 0.34 -29

-27 0.34 -27

-25 0.34 -25 Downloaded from https://academic.oup.com/gji/article/119/3/964/629091 by guest on 03 February 2021 -23 0.34 -23

-21 0.34 -21

-19 -19

-1 7 -17

-15 -15

-13 -13

-11 -11

-9 -9

-7 -7

-5 -5

-3 km -3 km +3 km +3 h +5 +s +7 +7

+9 +9

+I1 +11

+13 0.66 +13

+15 0.66 +15 n, n, +I +I 7 7 "mP 0.66 n. +I9 "5, 0.65 +19 +21 0.65 +21

+23 +23

+25 +25

+27 +27

R9 +29

+31- SE +31- SE P

Figure 6. Comparison of ground velocity computed at epicentral distances of 3-31 km from each side of the mountain, for explosions I (left side) and I1 (right side). The first column in the right side of each stack gives the relative amplitude for each explosion, the second column gives the epicentral distances. See text for explanation. Time duration is 13 s and source time function is a Ricker pulse centred at 3.5 Hz. Expbswn I (NW) - N147 section Expbswn II (SE) - NI47 section

R "lY RA"W

P"" A7 z- 053 -2lkm-NW

zT&-- 1 +I1 km - SE I +ZIkm-SE

B R " R *' 0 4s 0 4s

-J Downloaded from https://academic.oup.com/gji/article/119/3/964/629091 by guest on 03 February 2021 R "" -

P"" P"" R ""

0.46 -21 bn-Nw 0.42 .21htm-NW

H V 0.61 +21bn-S.E P"" PSe I I R se B

Figure 7. Comparison of the NW and SE horizontal and vertical ground-velocity components recorded for explosions I and I1 at 21 km epicentral distance. The names of different phases are given with their significances explained in the text. Relative amplitudes are given for each

24' 06" 24' 06" 5 :: loo0 -5 0 5 Jade

24' 04" :: :: lMxf

24' 02" 24' 02" -5 0 5 Opale

-5 0 5 I, I I 24' OO" 24' OO" I'E 5' WE 5' WE W fi -*LtlOO0 I' I I -5 0 5 Rubis Figure 8. Realistic configuration for Jade, Opale and Rubis. Two 2-D sections (N-S and W-E) are reported on the right side for each detonation point. Elevation and offset from shot are given in metres and in kilometres, respectively. 972 S. Gaflet et al.

Jade/Opale - NS section Rubis - NS section SOl4th soutl, SOUlli SOUlll mI 1 RA 7f-RC(.A) P.-A 2 0 89 -31 - Solitli 0.35 -31 - South H 0 89 -29 0.35 -29

0 89 -27 0 35 -27 0 0 89 -25 0 35 -25 Downloaded from https://academic.oup.com/gji/article/119/3/964/629091 by guest on 03 February 2021 09 -23 0.35 -23

09 -21 0 35 -21 09 -I 9 -19 0 91 -17 0.36 -17

0.91 -15 0.36 -15

0.92 -13 0.36 -13

0.93 -11 0.36 -11

0.92 -9 -9 0.91 -7 -7

0.83 -5 -5

0.91 -3 kin -3 km I +3 km +3 km +5 +5

+7 +7 0.47 +9 +9

0.38 +I1 +11

0.34 +I3 +13

0.31 +15 +15

+I7 +I 7

+I 9 +19

+21 +21

+23 +23 +25 +25 +27 +27 +29 +29

+31- North +31- Nodh

Figure 9. Stacks of ground velocities for explosions Jade/Opale and Rubis and receivers in the N-S section. The time duration is 13 s and the source function is a Ricker pulse centred at 3.5 Hz. Relative amplitude for each explosion and negative, i.e. southern azimuth, and positive, i.e. northern azimuth, offsets from explosions are reported in the right side first and second column respectively of each stack. Different phases are labelled and discussed in the text. Seismograms of nuclear tests in Algeria 973

Jade/Opale WE section - west Rubis - WE section west wesI west P-1 A Z A 0.21 -31 - West 0.31 -31 - West

-29 0.31 -29

-27 -27

-25 -25 Downloaded from https://academic.oup.com/gji/article/119/3/964/629091 by guest on 03 February 2021

0.2 -23 -23

-21 -21

1. 1. 0.2 -19 -19

-1 7 0.3 -17

0.2 -15 0.3 -15

-13 0.3 -13

0.24 -11 0.35 -11 1 0 29 -9 -9

r-0 37 -7 -7

0.47 -5 0.65 -5

J -3 km -0.92 -3 km

0.54 +3 km +3 km

0.37 +5 +5

0.3 +7 +7

0.3 +9 +9

0.3 +I1 +I I 0.3 +I3 +13

0.3 +15 +15

+I 7 +I 7

+19 +19

+21 +21

4 0.29 +23 +23

i 0.29 +25 +25

A 0.29 +27 +27

+29 1 0.38 +29

+31 -East A 0.38 +31 -East east 1 Y ens1 4 '6

Figure 10. Same as Fig. 9 but for the W-E section displayed in Fig. 8. 974 S. Gaffet et al.

Explosion Jade/Opale - NS and WE sections Explosion Rubis - NS and WE sections

north

0.31 +2f km .Nwlh 0.54 +2f km .Nonh

I 21km -Sou:h

0.4 -21 km .We$: 0.73 -21 km. We$:

0.57 +Zf km .Em1 Downloaded from https://academic.oup.com/gji/article/119/3/964/629091 by guest on 03 February 2021 0 4s 0 4s I - 0.23 +2f bn. Nonh 0.39 +Ifkm .Nonh

0.69 -21 km - South 0.72 -21 km .South

0.25 -21 km .Wesl 0.47 -21 km . West

Figure 11. Comparison of the N, S, W and E horizontal and vertical ground velocity components calculated for explosions Jade/Opale and Rubis at 21 km epicentral distance. The relative amplitudes are given for each explosion and component. Source-time function and window duration are those defined in Figs 9 and 10. Phases labelled in Figs 9 and 10 are reported and discussed in the text.

diffraction of the direct field onto both the opposite and the REFERENCES nearby sides of the massif. (2) The back-scattered field is made up of both a P Ben-Menahem, A. & Singh, S.J., 1981. Seismic waves and sources, surface wave and a Rayleigh wave train. Springer-Verlag, New York. (3) We show that for a receiver located at the same Bouchon, M. & Aki, K., 1977. Discrete wavenumber representation epicentral distance, the amplitude of the vertical ground of seismic source wave fields, Bull. seism. SOC. Am., 67, velocity may vary with a factor of 3, from one side of the 259-277. massif to the other. Boullier, A.-M. & Bertrand, J.-M., 1981. Tectonique tangentielle profonde et couloirs mylonitiques dans le Hoggar central We show that the duration of the surface wave train (4) polycyclique (AlgCrie), Bull. SOC. ge‘ol. France, XXXIII, 17-22. may be strongly enlarged depending on the azimuth of Duclaux, F. & Michaud, L., 1970. Conditions expkrimentales des observation. tirs nuclCaires souterrains franpis, 1961-1966, C. R. Acad. Sci. (5) Precursors to the main Rayleigh wave trains may Paris, 270, 189-192. exist, depending on diffraction conditions of the direct P FaurC, J., 1972. Recherche sur les effets gCologiques d’explosions wavefield. nuclCaires souteraines dans un massif de granite Saharien, (6) The steepness of the topography determines the Report CEA-R-4257. frequency content. Gaffet, S. & Bouchon, M., 1989. Effects of two-dimensional topographies using the discrete wavenumber-boundary integral equation method in P-SV cases, J. acoust. SOC. Am., 85,2277-2283. McLaughlin, K.L. & Jih, R.-S., 1988. Scattering from near source topography: teleseismic observations and numerical simula- ACKNOWLEDGMENTS tions, Bull. seism. SOC. Am., 78, 1399-1414. Munier, G., 1982. Construction d’un modble de croate sous le Hoggar Central, Report CEAILDG 53-82. Rocard, Y., 1964. Formation du signal seismique lors d-une This work was supported by the Advanced Research explosion souterraine, C. R. Acad. Sci. Paris, 258,2373-2375. Projects Agency and was monitored by the Air Force Office Wessel, P. & Smith, W.H.F., 1991. Free software helps map and of Scientific Research under Grant 90-0356. display data, EOS Trans. Am. geophys. Un.,72,441.