SOEE3250/5675/5115 Inverse Theory Lecture 10; notes by G. Houseman

Transcription

1 SOEE3250/5675/5115 Inverse Theory Lecture 10; notes by G. Houseman Travel-time tomography Examples of regional lithospheric tomography CBP / SCP projects data acquisition: array / sources arrival time residuals model specification model validation, synthetic tests inversion solution

2 Seismic Tomography Seismic tomography has developed since its beginnings in about 1980 to an essential tool in regional geophysical mapping - and to a lesser extent in industry. Arrival-time tomography relies on a careful analysis of the differences in arrival time of a coherent wavefront impinging on an array of receivers. Advance or delay in the arrival of the signal (usually < 1 second) is interpreted in terms of the raypath having crossed fast or slow regions. An accurate common time base is required - GPS Raypaths must be known or calculated. In order to remove the ambiguity of where on the raypath the fast or slow regions are located, different raypaths from different sources are required. A velocity model parameterisation is defined, and the model parameters are defined by inversion. because the raypaths depend on the solution the inversion is non-linear.

3 South Carpathian Project SCP array ( ): (South Carpathian Project) CBP array ( ): (Carpathian Basins Project) [Dando et al., GJI, 2011] + 44 permanent stations image provided by Yong Ren

4 Events used in the tomographic inversion: May 2006 to Aug events used in P-wave inversions (all symbols), 124 events used in S-wave inversions (grey)

5 Perspective view of Ray-paths Perspective view of the raypaths to one station in the LF98 array. The non-random sampling of raypaths is produced by the non-random distribution of earthquake sources with good coverage to East and North and poor to South and West in this example.

6 Seismic Travel-time Tomography When a seismic wavefront passes through a medium of variable wave-speed, the rays are refracted and may be either slowed down or speeded up. Rays are refracted toward the fast regions (blue) and away from the slow regions (red). By analysing the small differences in arrival time relative to arrival time for the spherical Earth model, fast and slow regions can be mapped in 3D.

7 CMG-6TD sensor

8 CMG-6TD station in Austria

9 CMG-6TD station in Hungary

10 Example Data - Vanuatu event / LF98 Records from C line show systematic increase in arrival time of about 15 seconds from east to west, for an event to the northeast. Correlation of initial coherent P-arrivals gives accurate measure of relative arrival time.

11 Event Residuals (LF98) Relative to rays computed from source to array for a theoretical ellipsoidal Earth, the arrival times are scattered in a Gaussian bell about some mean. The arrival residuals for one event average to zero. The half-width of the Gaussian for the ellipsoidal model is about 400 msec (grey curve). By introducing 3D velocity variation, we aim to decrease the width of the residual scatter.

12 Maps of P-wave velocity variation in upper mantle of Carpathian-Pannonian Region 75 km: isolated slow anomalies related to Miocene volcanism and basin depocentres 200 km: East Alpine fast anomaly prominent, generally slow Pannonian upper mantle Tomographic Inversions by Ben Dando (Leeds PhD) published in Geophysical Journal International (2011)

13 Maps of P-wave velocity variation in upper mantle of Carpathian-Pannonian Region 300 km: Fast Alpine anomaly extends eastwards beneath the Pannonian Basin 400:km:Pannonian fast anomaly becomes more prominent with depth

14 Maps of P-wave velocity variation in upper mantle of Carpathian-Pannonian Region 500 km: Pannonian fast anomaly spreads out beneath the entire Pannonian Basin 600 km: Alpine fast anomaly diminished as Pannonian anomaly increases

15 NS vertical sections of P-wave velocity variation in upper mantle of Carpathian-Pannonian Region 14 E: East Alpine fast anomaly extends to 400 km 16 E: fast anomaly extends between 200 and 500 km Alpine fast anomaly

16 Vertical sections of P-wave velocity variation across the western part of the Pannonian fast anomaly Profile A on the west side suggests continuity with the Alpine anomaly Profile B, further east shows a generally fast transition zone

17 Vertical sections of P-wave velocity variation across the western part of the Pannonian fast anomaly Profile F (WSW-ENE) shows continuity with the Alpine anomaly Profile D (NW-SE),shows apparently slow core inside the fast anomaly

18 Interpretation of Seismic Velocity Anomalies Seismic wavespeed or velocity generally increases with depth due to effect of pressure. Large differences in velocity between different tectonic provinces (at a given depth) may be caused by differences in temperature, or presence of partial melt. A 1% increase in Vp or Vs could be caused by a decrease in temperature of about 160ºC. Low velocity zone at km may be due to partial melt, or temperature increasing rapidly with depth.

19 Inversion of data generated from synthetic model with depressed 660 km boundary Actual inversion of P-wave data

20 CBP Receiver Function images ( km) of upper mantle seismic discontinuities (Hetényi et al., GRL, 2009)