Monitoring and Studying Audible Sounds Inside Different Types of Soil and Great Expectations for its Future Applications
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Pure and Applied Geophysics
Monitoring and Studying Audible Sounds Inside Different Types of Soil and Great Expectations for its Future Applications MOHAMED A. GAMAL,1 MOHAMED H. KHALIL,1 and GEORGE MAHER1 Abstract—A majority of seismological studies are concerned with soil properties in low frequencies (1–10 Hz), but little is known about these properties in the audible sound domain (20–20,000 Hz). This is probably due to the high attenuation of the high frequencies within the soil, resulting in a minimal effect on buildings. For this study, 172 stations were recorded over different types of soils using variable types of P-wave and S-wave geophones to examine the variation of soil properties in the range of audible sound (20–3000 Hz). High resolution 32 bit spectrograms for the sounds recorded within every soil sample were analyzed. Moreover, a model for empty room was built in the subsurface to study changes in sound caused by the existence of large voids or cavities in the subsurface. The sound wave was able to differentiate between rigid, hard soil and softer, weaker soil. While highstrength rocks or soils tend to show sharp sound pitches (300–3000 Hz), weaker soils show lower sound pitches (20–100 Hz). The existence of subsurface voids or cavities tend to make sound pitches more regular, higher and sharper than those in the surrounding soils. This is most probably due to resonance of sound in closed places (e.g. a violin). Soil energy levels and how they change due to the soil’s excitation were studied. Soil research in the field of audible sounds is considered an emerging field with several applications (e.g. geological hazards, water exploration, and oil exploration and so on). There is a need for special highresolution equipment to be developed for the same. This equipment should be capable of recording wide range of sound frequencies preserved in the soil and directly producing high-resolution spectrograms. Keywords: Spectrograms, audible sound, soil frequency, pitches, resonance of sound, microtremors, seismic frequencies, sound frequencies, soil voiceprint.
1
Geophysics Department, Faculty of Science, Cairo University, Giza, Egypt. E-mail: [email protected]; moh. [email protected]; [email protected]; Georgemaher [email protected]
1. Introduction Early studies employed seismic noises or the socalled ‘‘microtremors’’ in the frequency range of 1–10 Hz to study soil properties. In 1957, Kanai became the first scientist to introduce the technique of using microtremors or ambient seismic noises to determine soil response (soil amplification) to earthquake energy. After him, there were a lot of scientists who continued Kanai’s work (Aki 1957, Kanai and Tanaka 1961; Kanai 1962; Nogoshi and Igarashi 1971; Kagami et al. 1982, 1986; Rogers et al. 1984; Celebi et al. 1987; Lermo et al. 1988; Nakamura 1989, 1997, 2000; Bour et al. 1998; Dimitrios et al. 2001; Mahajan et al. 2012; Harutoonian et al. 2012; Mahajan et al. 2012; Evangelia Bouranta et al. 2013; Adib et al. 2015; Ismail Akkaya 2015; Mohamad Rid
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