Component analysis of very near surface seismic wave propagation and an examination of resonant frequencies in three soils
Kuhls-Gilcrist, Laura Elizabeth
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Exploring mechanical wave propagation through various soil types is important for detecting buried objects, such as anti-personnel landmines. Such detection is problematic due to the complex nature of shallow Earth materials, including its mineralogical and matrix composition, and grain shape, size, packing and distribution. Additionally, many anomalies are difficult to detect and distinguish due to their small size and similarity to objects such as roots and rocks. In order to detect landmines using seismic wave propagation, a component analysis of mechanical waves in three soils and during three seasons is conducted. A newly developed controlled-frequency acoustic seismic source is used to modulate applied frequency, amplitude, and load time. Data is also collected using buried model-landmines, and by suspending geophones above the Earth's surface to determine if landmines can be detected in such a manner. Results show that certain frequencies, preferred frequencies , travel through specific soils more efficiently, and are dependent upon soil type and season, but are geometry independent. This implies that using preferred frequencies increases signal amplitude, thereby increasing the signal-to-noise ratio and making detection of near-surface anomalies more efficient. Results due to increasing applied amplitude show an increase in the resultant amplitude and a change in signal velocity, indicating nonlinearity at the near-surface. Implying that data processing based upon linear assumptions may be improved upon for near-surface processing by taking into account nonlinearity. Additionally, buried landmines may be detected visually on raw seismic data and by using shifts in preferred frequency values and magnitudes. Lastly, when the geophones are suspended less than one inch, data are similar to results with normal geophone coupling, indicating that landmine detection may be possible.