Comparison of 2-D and 3-D full waveform inversion imaging using wide-angle seismic data from the Deep Galicia Margin

Full waveform inversion (FWI) is a data-fitting technique capable of generating high-resolution velocity models with a resolution down to half the seismic wavelength. FWI is applied typically to densely sampled seismic data. In this study, we applied FWI to 3-D wide-angle seismic data acquired using sparsely spaced ocean bottom seismometers (OBSs) from the Deep Galicia Margin west of Iberia. Our data set samples the S-reflector, a low-angle detachment present in this area. Here we highlight differences between 2-D, 2.5-D and 3-D-FWI performances using a real sparsely spaced data set. We performed 3-D FWI in the time domain and compared the results with 2-D and 2.5-D FWI results from a profile through the 3-D model. When overlaid on multichannel seismic images, the 3-D FWI results constrain better the complex faulting within the pre- and syn-rift sediments and crystalline crust compared to the 2-D result. Furthermore, we estimate variable serpentinization of the upper mantle below the S-reflector along the profile using 3-D FWI, reaching a maximum of 45 per cent. Differences in the data residuals of the 2-D, 2.5-D and 3-D inversions suggest that 2-D inversion can be prone to overfitting when using a sparse data set. To validate our results, we performed tests to recover the anomalies introduced by the inversions in the final models using synthetic data sets. Based on our comparison of the velocity models, we conclude that the use of 3-D data can partially mitigate the problem of receiver sparsity in FWI

Imaging of the Deep Galicia Margin using Ocean Bottom Seismic Data

The Galicia margin, west of Iberia, is one of the most studied magma-poor rifted margins to understand the rifting process leading to continental breakup. Seismic imaging has been instrumental in understanding rifting in the Galicia margin. In this work, I derived a high-resolution P-wave velocity model of the Deep Galicia margin (DGM) where the final breakup of the continental crust happened. The velocity model is derived employing a 3D acoustic full waveform inversion(FWI) technique in the time domain using sparsely acquired wide-angle ocean bottom seismometer(OBS) data. The model is validated by tracking phase changes of the first arrivals during the inversion, and by comparing the predicted waveforms with the observed for all the instruments. In addition, the anomalies introduced by FWI were validated by performing synthetic inversion runs by recovering the anomalies using a synthetic dataset predicted using the final FWI model as observed dataset. The final model shows an improved alignment with the structures observed on 3D prestack time migrated multichannel seismic images compared to the starting model. Comparison of the 3D FWI model result with 2D result derived along a profile through the 3Dseismic volume highlighted the differences between the imaging methods in a real world setting .The 3D FWI result constraints better the complex faulting within the pre- and syn-rift sediments, crystalline crust and below a detachment fault, known as the S-reflector, compared to the 2D result. Below the S-reflector, 3D FWI has enhanced the pattern of serpentinisation compared to the starting model with local low velocity zones occurring around fault intersections. Based on my comparison, I conclude that the use of 3D data can partially mitigate the problem of receiver sparsity in FWI. Using the high-resolution 3D model, I attempted to understand the nature of the crystalline crust by comparing the velocity range of the crystalline crust in the DGM with other similar tectonic settings. The velocity limits of the crystalline crust in the DGM include velocities of both the upper and lower crust observed in other similar settings, indicating that it is comprised of both the upper and lower crust. Unlike in many other settings, there is no clear evidence in the P-wave velocity profiles for a separate upper and lower crust within the crystalline crust. The high-resolution model also shows evidence for exhumation of the lower crust under the footwall of the fault blocks to accommodate the extension. I generated a serpentinisation map of the DGM at a depth of 100 ms below the S-reflector and compared the map with a map generated by training a machine learning algorithm using velocities from 2D FWI. A mean serpentinisation of ~33 % is estimated below the S-reflector using the 3D FWI model. Seismic images of the DGM are developed in time and depth domains using the first-order multiples from the OBS data and a technique called mirror imaging. In this technique, the seafloor along with the OBS is mirror imaged with respect to the sea-surface and placed at a depth of twice the water column depth. Such an adjustment allows incorporation of the multiples in to migration algorithms just like primary reflections. I observe that mirror imaged sections show a good match within the sediment column with the multichannel images, but the quality of the images deteriorates below the top of the basement partially due to weak signal strength of the multiples. Primary reflections from the OBS illuminate very narrow sections of the subsurface, hence the quality of the image using them is poor compared to the mirror images. Mirror imaging can become a standard processing step in studies where no multichannel data are available

Insights into exhumation and mantle hydration processes at the Deep Galicia margin from a 3D high-resolution seismic velocity model

High-resolution velocity models developed using full-waveform inversion (FWI) can image fine details of the nature and structure of the subsurface. Using a 3D FWI velocity model of hyper-thinned crust at the Deep Galicia Margin (DGM) west of Iberia, we constrain the nature of the crust at this margin by comparing its velocity structure with those in other similar tectonic settings. Velocities representative of both the upper and lower continental crust are present, but there is no clear evidence for distinct upper and lower crustal layers within the hyper-thinned crust. Our velocity model supports exhumation of the lower crust under the footwalls of fault blocks to accommodate the extension. We used our model to generate a serpentinization map for the uppermost mantle at the DGM, at a depth of 100 ms (~340m) below the S-reflector, a low-angle detachment that marks the base of the crust at this margin. We find a good alignment between serpentinized areas and the overlying major block bounding faults on our map, suggesting that those faults played an important role in transporting water to the upper mantle. Further, we observe a weak correlation between fault heaves and serpentinization beneath the hanging-wall blocks, indicating that serpentinization was controlled by a complex faulting during rifting. A good match between topographic highs of the S and local highly serpentinized areas of the mantle suggests that the morphology of the S was affected by the volume-increasing process of serpentinization and deformation of the overlying crust

Comparison of two- and three-dimensional full waveform inversion imaging using wide-angle seismic data from the Deep Galicia Margin

Full waveform inversion (FWI) is a data-fitting technique capable of generating high-resolution velocity models with a resolution down to half the seismic wavelength. FWI is applied typically to densely sampled seismic data. In this study, we applied FWI to 3D wide-angle seismic data acquired using sparsely spaced ocean bottom seismometers (OBSs) from the Deep Galicia Margin west of Iberia. Our dataset samples the S-reflector, a low-angle detachment present in this area. Here we highlight differences between 2D, 2.5D and 3D-FWI performances using a real sparsely spaced dataset. We performed 3D FWI in the time domain and compared the results with 2D and 2.5D FWI results from a profile through the 3D model. When overlaid on multichannel seismic images, the 3D FWI results constrain better the complex faulting within the pre- and syn-rift sediments and crystalline crust compared to the 2D result. Furthermore, we estimate variable serpentinisation of the upper mantle below the S-reflector along the profile using 3D FWI, reaching a maximum of 45 per cent. Differences in the data residuals of the 2D, 2.5D and 3D inversions suggest that 2D inversion can be prone to overfitting when using a sparse dataset. To validate our results, we performed tests to recover the anomalies introduced by the inversions in the final models using synthetic datasets. Based on our comparison of the velocity models, we conclude that the use of 3D data can partially mitigate the problem of receiver sparsity in FWI

Insights into exhumation and mantle hydration processes at the Deep Galicia margin from a 3D high-resolution seismic velocity model

High-resolution velocity models developed using full waveform inversion (FWI) are capable of imaging fine details of the nature and structure of the subsurface. Using a 3D FWI velocity model of hyper-thinned crust at the Deep Galicia Margin (DGM), we constrain the nature of the crust at this margin by comparing its velocity structure with those in other similar tectonic settings. Velocities representative of both the upper and lower continental crust are present in this hyper-thinned crust. However, unlike in many other rifted margin settings, there is no clear evidence for distinct upper and lower crustal layers within the hyperextended crust. Our velocity model also shows evidence for exhumation of the lower crust under the footwalls of fault blocks to accommodate the extension. We used our model to generate a serpentinization map for the uppermost mantle at the DGM, at a depth of 100 ms (~340m) below the S-reflector, a low-angle detachment that marks the base of the crust at this margin. Based on this map, we propose that serpentinization began during rifting and continued into a post-rift phase until the faults were sealed. We find a weak correlation between the fault heaves and the degree of serpentinization beneath the hanging-wall blocks, indicating that serpentinization was controlled by a complex crosscutting and unrecognized faulting during and after rifting. A good match between topographic highs of S and local highly serpentinized areas of mantle suggests that the serpentinization process resulted in variable uplift of the S-surface

Ocean bottom seismic data from the continent-ocean transition in the Deep Galicia Margin, offshore west Iberia: active source data

Seismic data was acquired to study the transition from rifted continental crust to oceanic crust at the Deep Galicia Margin from June to August 2013. 3D Multichannel reflection and coincident wide-angle seismic data were acquired simultaneously as part of a seismic experiment over an area of 80 km long and 25 km wide in the Deep Galicia margin. The multichannel reflection seismic volume was acquired by the R/V Marcus G. Langseth, which provided a source for the ocean bottom seismic data. A total of 86 ocean bottom hydrophones/seismometer deployments were carried out by F/S Poseidon. Two airgun arrays with total gun volumes of 3,300 cu.in. were deployed as seismic sources. Shots were fired alternately using two source arrays every 37.5 m (shot interval of ~ 16 s with ship speed of 4.5 knots). Data were converted into SEGY format. Further details are available at https://doi.org/10.1038/NGEO2671

Ocean bottom seismic data from the continent-ocean transition in the Deep Galicia Margin, offshore west Iberia

Seismic data was acquired to study the transition from rifted continental crust to oceanic crust at the Deep Galicia Margin from June to August 2013. 3D Multichannel reflection and coincident wide-angle seismic data were acquired simultaneously as part of a seismic experiment over an area of 80 km long and 25 km wide in the Deep Galicia margin. The multichannel reflection seismic volume was acquired by the R/V Marcus G. Langseth, which provided a source for the ocean bottom seismic data. A total of 86 ocean bottom hydrophones/seismometer deployments were carried out by F/S Poseidon. Two airgun arrays with total gun volumes of 3,300 cu.in. were deployed as seismic sources. Shots were fired alternately using two source arrays every 37.5 m (shot interval of ~ 16 s with ship speed of 4.5 knots). Data were converted into SEGY format. Further details are available at Bayrakci et al., 201

Ocean bottom seismic data from the continent-ocean transition in the Deep Galicia Margin, offshore west Iberia

Seismic data was acquired to study the transition from rifted continental crust to oceanic crust at the Deep Galicia Margin from June to August 2013. 3D Multichannel reflection and coincident wide-angle seismic data were acquired simultaneously as part of a seismic experiment over an area of 80 km long and 25 km wide in the Deep Galicia margin. The multichannel reflection seismic volume was acquired by the R/V Marcus G. Langseth, which provided a source for the ocean bottom seismic data. A total of 86 ocean bottom hydrophones/seismometer deployments were carried out by F/S Poseidon. Two airgun arrays with total gun volumes of 3,300 cu.in. were deployed as seismic sources. Shots were fired alternately using two source arrays every 37.5 m (shot interval of ~ 16 s with ship speed of 4.5 knots). Data were converted into SEGY format. Further details are available at https://doi.org/10.1038/NGEO2671