ABSTRACT
We consider using a small infrasonic array to detect a surface explosion that may originate from mining activities or from a nuclear test, see for example Simons, (1996), Blandford (1996). lnfrasound in the atmosphere, i.e. sound waves whose frequencies are less than the lowest audible frequency, propagate great distances without substantial loss of energy. Sounds at these frequencies can be caused by volcanic explosions, earthquakes and nuclear or mining explosions. Experience has shown that infrasonic waves are approximately plane. The problem of optimal configuring of such arrays for purposes of monitoring a comprehensive nuclear test ban treaty is presently under consideration. For example, Fig. I shows time series from three microbarographic instruments in a small triangular array, called Small Fry, with approximately one kilometer sides on Palmyra Island in the Pacific that recorded an event called Tanana which was detonated about 25 km South of Kiritimati Island (Christmas Island at the time of the explosion). For a description of infrasonic recording instruments, see Cook and Bedard (1971 ). Although the coordinates of the array generating the three series shown in Fig. 1 were not available, it appears that the strongest part of the signal begins shortly after point 1410, first at series B, followed by A, followed by C as can be seen from the negative pulse 4230. We can imagine the sensors being located at various coordinates, (.\j,Y;) in the plane, as in Fig. 2, where we show a single location (x 1 ,yJ) and a plane wave arriving from the southwest (225°) with a frequency proportional to the distance between the lines. Problems of interest for this infrasonic array are (1) detecting the plane wave signal and (2) estimating the unknown parameters, mainly the velocity and azimuth of the propagating signal, which map directly into an estimator for the location of the explosion. An additional problem is determining the optimum geometry, i.e. the number and locations of the recording instruments, for a new array, for monitoring purposes that will tend to produce the smallest toea tion error.
