Optimisation of Earthquake Monitoring for CCS Applications on Local and Microearthquake Scales

OPTIMISATION OF EARTHQUAKE MONITORING FOR CCS APPLICATIONS ON LOCAL AND MICROEARTHQUAKE SCALES
Gary Gibson, Abraham Jones, Januka Attanayake, Mike Sandiford

This project focuses on earthquake monitoring in CO2 storage sites in the Australian context. Seismic monitoring is relatively difficult for proposed CarbonNet storage sites in the Gippsland Basin because of soft near-surface rocks and unconsolidated sediments, together with the offshore location.

The issues include:

• the need to install seismographs on unconsolidated near-surface sediments resulting in high transitory noise from anthropogenic sources.
• deep sediments and soft rock, leading to strong surface wave noise from waves and wind.
• Site response, where near surface soft rock and/or unconsolidated sediments will significantly alter the seismic wave motion measured at the surface compared with that at bedrock. Low seismic wave velocities delay arrival times at the surface, affecting earthquake location accuracy if a time correction is not used. The effect on wave amplitude involves both amplification and attenuation. Amplification effects include impedance amplification and site resonance. Attenuation effects include absorption of energy, and reflection and refraction effects at layer interfaces.
• Very deep sediments and soft rock leading to rapid attenuation of signal amplitudes with distance, especially high frequency vibrations from smaller earthquakes.
• Ocean bottom seismographs at the seafloor interface between water and soft sediments will experience strong ongoing surface waves along this surface.

Measures to mitigate these problems include:

• using a high-density network in the target area to reduce the distance from earthquakes to the nearest seismographs, thus increasing signal to noise ratio.
• installing seismometers in postholes (~2 to 4 metres deep) or shallow boreholes (10 to 100 metres) to reduce both transitory and ongoing surface wave amplitudes.
• using new high-frequency seismometers and accelerometers with flat response to 100 or 200 Hz, which will record motion from small nearby earthquakes at frequencies above most of the ongoing surface wave noise, while still recording the long period motion from larger earthquakes required for focal mechanisms.
• improving epicentral location accuracy by installing some instruments on hard rock at greater distances that will not suffer from strong high-frequency attenuation and will give much clearer and more precise seismic wave arrival times,but giving little constraint on earthquake depth estimates.
• we can do little regarding attenuation or sea-floor surface wave noise, other than quantifying local attenuation using amplitude measurements, then using site-specific attenuation functions. These measurements will be complicated by the focal mechanism radiation pattern from each event.