183 6. The use of passive seismic methods for Geothermal exploration and monitoring of seismic data from different locations. Yet, analog technology still limited the extent of what could be captured, often requiring events to “trigger” the recording mechanism. Triggered recordings meant that only seismic events exceeding a certain threshold were captured, resulting in the loss of data from smaller earthquakes and seismic tremors. The digital revolution in the 1970s transformed earthquake detection. Digital seismometers replaced analog systems, offering higher resolution and accuracy, as well as the capacity to store continuous, high-quality data. The introduction of continuous recording was a breakthrough: it enabled the capture of all seismic activity, from minor tremors to major earthquakes. Continuous digital recording removed the need for triggered mechanisms and allowed real-time monitoring, essential for identifying seismic events before larger quakes. Additionally, with the digital storage of data, seismologists could archive vast amounts of seismic information, facilitating data analysis and pattern recognition across regions and over time. Through these technological advancements, seismology has evolved from occasional triggered recordings to continuous, high-resolution digital monitoring, culminating in a networked, data-rich approach that enhances our ability to monitor, understand, and respond to earthquakes. The ambient seismic signal – One person’s trash is another person’s treasure Entering the age of continuous seismic recording, the field of seismology has expanded its scope beyond the study of large earthquakes to include the rest of the ambient seismic signal, which was until then often referred to as seismic “noise” and was usually disregarded as irrelevant or undesirable. This negative vision is due to the inherent difficulties of conducting active seismic surveys in such environments where this seismic “noise” shows high amplitude, in which case the picking of body waves arrival time becomes less accurate or impossible. Rather than filtering out this background motion, scientists now treat seismic noise as a valuable signal, one that can reveal critical details about the Earth’s structure and dynamics. Passive seismic methods designate all the methodologies and tools that have been developed to infer subsurface information from the analysis of those seismic recordings. Figure 6.1 shows a 250 seconds duration recording of the ambient seismic signal, represented in the temporal domain. This signal typically illustrates the dual structure of the ambient seismic signal, which can simplistically be described as two main components, coherent seismic events on one hand, and incoherent seismic signals on the other hand. Different passive seismic approaches exist to extract information from the coherent or incoherent components of the ambient seismic signal.
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