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Ground Vibrations Near Explosions.*

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Ground Vibrations Near Explosions.* GROUND VIBRATIONS NEAR EXPLOSIONS.* By BENJAMIN F. HOWELL, JR. INTRODUCTION ONE WOCLD expect that, since seismic waves from explosions are the basis of a whole industry (seismic surveying), their nature would have been throughly investigated and described. However, the exploration geophysicist is not pri- marily interested in the nature of the seismic pulses, but in their velocities and the paths they travel to his recording instruments. It is common practice in studying seismic exploration records to assume that only compressional pulses are clearly recorded, though occasionally the presence of some transverse wave energy is postulated to explain otherwise incomprehensible observations. Other types of wave motion are treated as part of the background noise, and wherever possible are excluded from the recorded spectrum by the use of ap- propriate filters in the amplifiers. However, proper interpretation of the records obtained requires some knowledge of the detailed character of the ground motion; and, therefore, any information on the nature of the seismic energy which is generated by an explosion is certain to have some value. The series of experiments described here was undertaken with the intention of increasing the knowledge of the basic seismic forms to be expected in the record of an explosion. PREVIOUS EXPERIMENTAL WORK Most of the work done in the past has been on large explosions recorded at near-by permanent seismic observatories. These investigations treat primarily data recorded many kilometers from the source of the energy. Because of the large distances, the shapes of the pulses are much altered from what they were near their source, and are not well suited to studies of their detailed nature. The form of a compressional pulse near an explosion was studied by J. A. Sharpe (1942). 1 He analyzes the pulse shape to be expected from theoretical considerations and compares it with those observed. Since the initiating forces in the ideal case are entirely radial, there should be no shear waves directly generated, a conclusion borne out by observation for explosions buried at sufficient depth beneath the surface. Shear waves are, however, to be expected from a superficial explosion where force is applied vertically to the surface (Lamb, 1904). In practice we do not encounter an ideal homogeneous , isotropic, elastic medium. Although the deeper layers of rock may be considered homogeneous, * This paper is a summary of a Ph.D. thesis in geophysics at the California Institute of Technology. The complete thesis may be consulted in the Institute library. Manuscript received for publication April 18, 1949. 1 All references are to the bibliography at the end of the paper. [ 2s5 ] 286 BULLETIlff OF THE SEISI~OLOGICAL SOCIETY OF A:MERICA the surface layer certainly is not, and can be satisfactorily approximated only by assuming that it varies at least in the vertical direction. Isotropy, also, is only a first approximation; foliated and bedded rocks have dif- erent properties perpendicular and parallel to the foliation and bedding. Study of the transmission of body waves through the earth indicates that at large depths the condition of elasticity is approximately held for seismic waves. The transmission of compressional waves through a medium of the type of the uppermost layer has been studied by Lampson and his co-workers (Lamp- set smwry ,p oB. Asov , MM. PEF .O0,,,MI,SEC. PEAKITO PEAK tO I _~ " I ,~~ .- , -0- i , , ,, , L2¢ 2 4 IO 3o ioo 2oo I ( I ,I I ,I FREQUENCY IN CYCLES PER SECOND ,I Fig, 1. Calibration curves of vertical component of seismograph. son, 1942, 1946; Rust and Mounce, 1942; Weatherby, 1943); bu~ a great deal more experimental work will have to be done before the wave mechanics in such a medium can be described quantitatively for all cases. Yet most of our observations of seismic waves are made at the upper boundary of a layer of this sort. A direct approach to the problem was made by L. D. Leer (1939, 1946; also Leer and Ewing, 1932). Leer recognized two types of waves in records from explosions besides the well-known compressional, transverse, Love, and Ray- leigh waves. The first of these, the "C" or "coupled" wave, is characterized by simultaneous in-phase motion on all three components. The second, the "H" or "hydrodynamic" wave, is similar to the Rayleigh wave, except that whereas the particle motion in a ltayleigh wave is retrograde with respect to the direc- tion of propagation, in the hydrodynamic wave the particle motion is direct, being like that of a gravity wave in a liquid. H has been reported until now GROUND VIBRATIONSNEAR EXPLOSIONS 287 only in the atomic bomb record made in the course of the Los Alamos test and in the record of one explosion in New England. C was also observed in these recoMs, and in other records from blasts in New England. Pulses which resemble C and H were observed in this investigation. THE APPARATUS The apparatus used in this research was generously lent by the United Geo- physical Company. It consisted of three electromagnetic induction seismom- eters, three amplifiers, and a recording oscillograph designed for refraction- type geophysical prospecting. The three seismometers were originally intended to respond only to the vertical component of ground motion. Two of them were modified to respond, instead, to horizontal components, one toward and away from the shot, the other perpendicular to this. Figure 1 is the calibration curve of the vertical component for sinusoidal motion. The curves for the other two components are similar. The low-frequency limit was determined by the sensitivity of the seismometers, which decreased rapidly below their natural frequency of 3~ cycles per second (henceforth abbreviated c.p.s.). The high- frequency limit could be adjusted by means of a filter. The sensitivity of the horizontal components to vertical motion was about 1/10 that to horizontal motion. The sensitivity of the vertical component to horizontal motion was about 1/50 that to vertical motion. The records taken in the field were naturally not sinusoidal (figs. 3-5), and hence the amplitudes of the ground motions could not be determined precisely from the calibration curves. In addition, much of the measured motion had periods of more than ~ second. Below 5 c.p.s, the gain varies with frequency, and an appreciable amount of error is introduced by computing the amplitude of motion for an incorrect frequency. LOCATION OF THE SURVEY AND FIELD PROCEDURE The records were made about 42 kilometers southeast of Los Angeles (see map, fig. 2). Measurements were made at 14 locations along an east-west line 97 to 3,284 meters from the shot point. The seismometers were set on the surface, or half buried in the ground at the side of a road, or in a dry irrigation ditch about a meter deep. The recording apparatus was mounted in a truck. The charges, consisting of 21/~ or 5 pounds of Hercules 60 per cent petrogel, were set off at depths ranging from 6.7 to 12.5 meters. Little detailed information is available on the geology of the district. The surface material is everywhere a fine-grained clayey soil. The surface rocks are thought to be Recent alluvium, probably overlying Pleistocene marine ter- races and alluvial fan deposits of considerable thickness. The water table is about three meters beneath the surface. Study of the refracted compressional waves indicates that there is a "weath- 288 BULLETII~ OF THE SEISI~cIOLOGICAL SOCIETY OF A]k~ERICA ered" or "low-velocity" layer between 1.2 and 8.4 meters thick the compres- sional wave velocity of which is between 120 and 735 m/s., probably increasing rapidly with depth, a second layer between 29 and 106 meters thick whose comprcssional wave velocity is 1,565 m/see., and a third layer of unknown thickness the compressional wave velocity of which is 1,950 m/see. ~LOS ANGELES~ CITY HALL//~ N /_~ 16KM" /~@o:- N I I KM. I TELLA AVE. RECORDING LOCATION ~X~ ~J I/I ! I l | I : : : : SHOT POINT' | Fig. 2. Location where measurements were made. THE RECORDS Figures 3-5 are tracings of a selected group of the records. In each case the three traces represent, from top to bottom, vertical, longitudinal, and trans- verse motion. The sensitivities of the three channels are not identical; the relative amplifications of the horizontal channels with respect to the ver- tical in decibels at 10 c.p.s, are given beside each horizontal trace. Positive values correspond to greater amplification in the horizontal channel, negative to lesser. On the charts there is also shown the amplification of the vertical channel at 10 e.p.s, in decibels above I ram. peak-to-peak spot deflection on the paper per millimeter per second peak ground velocity; also, the nominal cut-off frequency of the filter, the distance from the shot to the recording station, the size of the charge, and the depth at which it was detonated. Timing marks are given representing half-second intervals following the shot. GROUND VIBI~ATIONS NEAR EXPLOSIONS 289 / (. ) ~o~ q / ./ \ \ ;( (- .~, ~ ~ . m g ,.-4 ~o O¢~¢) ~'~ o "~ ° 0.~ Q 290 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMEI~ICA ¢4 H +z GROUND VIBRATIONS NEAR EXPLOSIONS 291 , \ (" "~-'x1 )- i~ ~5 > < <' "~i _ <~-J ) .)~- 5O \ \, ~..7~ 0 (( _ "s /( < I.k \\ 0 -) ? .'/ .f \x, -- \<..., \- ! / !:! / C. I } . ] //J" oO (I% i 0 ) f (,i( Ii 0 J 3 m () (( ~,~ I" ~ ) \, p-~<-- (') \\ (.. I ~ ~- i (,~I \.\ d. ~ g I . G 292 BULLETIN OF TI=IE SEISMOLOGICAL SOCIETY OF A~EEICA TIlE IN SECOND: ,f Y 2( Y Y Y 18 ,, I 12 S ~.. o~I, ~ ~-~ .••.•~ __:XI. 0 500 I000 t500 2000 2500 5000 :5501 DISTANCE IN METERS Fig.
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