Passive Seismics Outlook

Passive Seismics Outlook

May 2015

A new era is blooming on geo-exploration, structural and seismology projects: the passive seismics approach. Passive seismics means seismicity not induced by active artificial means (i.e. surface blasting) referring to both naturally induced seismicity and ambient noise. Naturally induced seismicity relays an event source not man-triggered, but rather a natural event: most of the times this source is of a deep origin rather than surface blasting, which has important implications on tracing and processing. Ambient noise has an added value: overcoming the limitation of depending on a seismic event that covers the exploration area. Ambient noise exists everywhere, but it can be divided into two main types: a) high frequency noise, which focuses on local noise created by frequent events (i.e. traffic noise) and b) low frequency noise, which arises mostly by Earth phenomena like large scale ocean waves and wind currents.
Passive seismics implies recording without a controlled source; although this might sound as a drawback, several advantages have been recently discovered, especially in terms of the features of the required recording equipment/deployment: 1) passive arrays are relatively easy to set up in comparison with extensive 2D/3D surveys and explosion permits; 2) passive seismic survey can cover a larger extent, with less sensors needed for longer distances; 3) passive surveys can provide an excellent technique solution from local scale studies to reservoir and crustal research; 4) array management logistics benefits from the most robust and autonomous instrumentation; 5) passive seismics projects develop reliably and with low impact. On the other hand, challenges faced by passive seismics are the extension of the operations, difficult terrain and irregular layouts.
Project enhancement through passive seismics is now based on new processing techniques and flexibility of the available technology. Instrumentation with increased quality and flexibility has overcome previous poor reliable data and dependence on uncertainty. This instrumentation quality refers to high signal to noise ratio, system robustness, and low power, whereas new tendencies drive technologies towards fast wireless communication capabilities.
The new high-quality instruments open a thrilling new era of passive seismics jointly with the new geophysicist expertise. Passive seismics has emerged again to the stage of seismological scientific community to show all of its potential in addition to the classical earthquake seismology. Many applications in different sectors have successfully been carried out: monitoring volcanic activity, tracing and monitoring active faults, reservoir monitoring, CO2 sequestration, oil and gas exploration, earthquake site effect, mining and geotechnical application, geothermal hazards, microseismics, etc.
The data coming from this high-quality instrumentation, being 24 bits or higher, is also of higher quality than ever before. The dynamic range tells the broad range of signal applications the instrument can address given the signal to noise ratio. Broadband capabilities have an important role since they are strongly linked, yet with flexibility of the acquisition system. The capacity of the recording units (seismic dataloggers) to connect either passive sensors as geophones or actives like seismometers/accelerometers is also a very important feature. The frequency range will depend enormously on project objectives. Broadband instrumentation can range from 120 to 5 s in crustal and structural studies to 4.5 Hz, 10 and even 14 Hz on studies close to the source.
State-of-the-art standalone wireless systems are being released as autonomous equipment, expanding their usability on passive seismics projects. The main advantage is the independence of the seismic deployment where an individual failure does not to tow the entire system. Acquisition units deployed on the field will be placed in remote locations so that they can withstand roughness to operations and all weather conditions. To ensure reliability, robustness and compactness of the system, system feedback must be present, for example a quality control/state of health as proof of good operating recording, or data streaming.
Also, technical steps forward on storage capacity allow recording longer periods of unattended stations. Internal removable memory is desirable for minimizing data disturbing times and data safety. Alternatively, on critical projects/infrastructures and earthquake early warning systems, the data can be sent in real time through data streaming. The real time transmission drawback is power consumption: this highlights the importance of low power requirements on system operation. Battery requirements should be kept minimal to reduce maintenance procedures. Balancing real time availability of critical data and battery management is the main challenge faced by the passive seismics recording systems.
Geoscientists are increasingly using passive seismics to answer questions about the Earth’s mantle-crust properties. Deep regions, tens or hundreds of kilometres down, require arrays of sensors spaced similar distances apart. Much denser arrays, however, can sample higher frequencies to answer questions about the upper 10 kilometres of crust. Survey design and instrumentation are keys on the objective completion of each specific project. Passive seismic techniques have proven  to successfully reveal structure of the earth, describe dynamic processes and interpretable seismic attributes. Capabilities of the new era of passive seismics includes enhancing and complementing seismic active surveys or provide useful earthquake engineering data.
Passive seismics studies are based on different depths of research with diverse objectives: a) Seismic risk assessment of earthquake site effects b) Structural monitoring of civil engineering infrastructures: bridges, dams, nuclear power plants c) Preliminary studies to determine active seismic survey d) Geophysical exploration when active survey models are not applicable or too expensive e) Exploration surveys to help define drilling position f) Natural background seismicity prior to fracking or production g) Reservoir monitoring and microseismics.

About Worldsensing

Worldsensing is a global IoT pioneer. Founded in 2008, the infrastructure monitoring expert serves customers in more than 70 countries, with a network of global partners to jointly drive safety in mining, construction, rail and structural health.

Worldsensing is headquartered in Barcelona and has a local presence in the UK, North and South America, Singapore, Australia and Poland. Investors include Cisco Systems, McRock Capital, ETF, Kibo Ventures, JME Ventures and Bentley Systems.


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