Microseisms and an attempted application to exploration

A study of microseisms is made to determine some of their statistical properties and to investigate the feasibility of their use in determining the structures in the upper several kilometers of the earth's crust by the phase‐velocity method. It is found that the microseisms in the period range of 1 to 6 sec arrive from several directions with comparable strength and at the same time. There are occasional short intervals of 10 to 40 sec during which microseisms are mostly unidirectional. It is also found that these relatively short‐period microseisms are not stationary in the wide sense over time intervals longer than 5 or 10 minutes. The phase velocities of microseisms recorced with an array of eight instruments are measured in four different locations. The velocities, although scattered, are found to be in agreement with the computed theoretical dispersion curve for the fundamental Rayleigh mode, based on the available seismic velocity information. An error analysis is made and the confidence limits are placed within ±20 percent of the measured velocities. It is concluded that in determining shallow crustal structures the microseism‐phase‐velocity method potentially is at least as effective as the gravity method, although it is not as suitable for routine work as the gravity method.