Joint 3D relocation of earthquakes by surface and borehole data Aldo Vesnaver & Lara Lovisa Trieste, 7 October 2008 Annual Meeting of the Italian EAGE-SEG Section

Main topics Microseismicity caused by hydrocarbon production and storage, and by CO2 sequestration Wadati’s method in wells ... ... and joint 3D tomography by active and passive seismic, from well and surface receivers

Method Voxels in irregular grids Ray tracing for direct, reflected, refracted, diving and converted waves

Method 1. Time Zero by Wadati’s method (or Least Squares) 2. Initial model from 3D seismic, well data and geology 3. Hypocentre estimate by “Shrinking Grids” 4. 3D tomography 5. Iterations of steps 3 and 4, until convergence

Method 1. Time Zero by Wadati’s method (or Least Squares) 2. Initial model from 3D seismic, well data and geology 3. Hypocentre estimate by “Shrinking Grids” 4. 3D tomography 5. Iterations of steps 3 and 4, until convergence

Wadati in a well ?

Wadati in a well ?

Wadati in a well ?

Wadati in a well ? Rp, Rs = Recorded P or S time Tp, Ts = Actual P or S traveltimes Vp, Vs = Average P or S velocity To = Time Zero

Method 1. Time Zero by Wadati’s method (or Least Squares) 2. Initial model from 3D seismic, well data and geology 3. Hypocentre estimate by “Shrinking Grids” 4. 3D tomography 5. Iterations of steps 3 and 4, until convergence

Method - “Shrinking grids”

3D recording geometry 225 receivers at surface 28 receivers in 2 wells (Jervis and Dasgupta, 2006) (Dasgupta, 2006)

3D recording geometry Vertical arrays Area : 3 x 3 km Maximum depth : 2.015 km

Relocation – 1.5D model (“true” traveltimes, without Wadati) NonLinLoc Hypo3D

Relocation – 3D model Missing (“true” traveltimes, without Wadati) NonLinLoc Hypo3D

Method 1. Time Zero by Wadati’s method (or Least Squares) 2. Initial model from 3D seismic, well data and geology 3. Hypocentre estimate by “Shrinking Grids” 4. 3D tomography 5. Iterations of steps 3 and 4, until convergence

Synthetic model P velocity km km/s

Reflection tomography P velocity km km/s

Active + passive tomography P velocity km km/s

Synthetic model S velocity km km/s

Reflection tomography S velocity km km/s

Active + passive tomography S velocity km km/s

Conclusions Wadati’s Method does NOT work in wells and if the Vp/Vs ratio changes a lot along the ray-paths of recorded events Thus, surface and well data should be jointly processed for the Time Zero The joint 3D tomography of active + passive seismic can better image the reservoir Gualtiero (Walter) B