Title: Seismic imaging using waveform inversion techniques
Researchers: Clara Gomez Garcia, Dr Sergei Lebedev
The amount of natural resources in Ireland is abundant, however, the capacity for mineral exploration, required to maintain and grow the raw materials industry in the country is, currently, insufficient. Seismic imaging is an effective tool for mineral exploration: images of seismic discontinuities and seismic-velocity distributions in the subsurface, interpreted together with structural-geology, geochemistry, well-log and other data, offer valuable information on the geological structure at depth. Active-source seismic surveys are established approach, which produces detailed resolution of cross-sections beneath survey lines (2D profiles or 3D models). However, it is difficult to do these surveys in all locations of interest due to their high cost (typically, millions of Euros). Passive seismic methods are an inexpensive alternative, able to contribute substantially to the mineral exploration in Ireland. The potential of passive seismic methods has been proven at different scales in space and time, including scales relevant to natural resource exploration (tens of meters to kilometres). However, these techniques require further development and fine-tuning for specific exploration locations and targets. At the present stage of their development, the methods offer images with resolution lower than active seismics, suitable for reconnaissance surveys, while also providing important complementary information, including absolute seismic velocities. Continuing development of the passive methods promises substantial increases in their resolution. Passive seismic methods, in particular ambient-noise interferometry, have been used successfully at low frequencies (<1 Hz), but at higher frequency (>1 Hz) ocean-generated ambient noise is scarce. Yet, the higher frequencies are required for imaging at 10’s-100’s-meter scales. This is a current challenge.
In the proposed project, we focus on resolving seismic discontinuities in the subsurface (akin to conventional reflection seismics). Seismic velocities will also be refined with discrete, seismic-event data. The new methods and their underlying hypotheses are: (i) High-resolution imaging using seismic events in Ireland. Most Irish events are quarry blasts, with the rest being tectonic earthquakes and mine blasts. Seismic waves propagated from quarry blasts offer high-frequency energy not available in the ambient noise field. Quarries in Ireland are numerous, well-distributed, and provide uniquely suitable signal, to be utilized in this project. Cross-correlation of the signals can constrain seismic-velocity distributions in the subsurface; auto-correlation of the scattered coda can reveal reflectors at depth. (ii) Waveform inversion of the auto-correlation functions. Autocorrelation of surface displacement at a seismometer yields information on the reflection response beneath it. With an array of seismometers or geophones, it may be possible to resolve vertical profiles similar to portions of reflection-seismics sections. Simple picking of the auto-correlation-function peaks, however, limits the applicability and resolution of the method. We will develop and apply waveform inversion of the auto-correlation function for the 1D reflectivity structure under the station. The signal may come either from the ambient-noise wavefield or from discrete-event coda.
- Develop, test and apply new passive-seismology methods for high-frequency imaging of seismic discontinuities (reflectors) and seismic velocities, at regional-to-deposit scales.
- Fine-tune for Ireland, using all available seismic sources, natural and man-made.
- Validate the methods and image the subsurface near the active mine. Prepare methods for exploration across Ireland.