Seismic velocity and attenuation structures of the Queen Charlotte Basin from full-waveform tomography of seismic reflection data

We applied viscoacoustic waveform tomography to four seismic reflection lines from the central and northern part of the Queen Charlotte sedimentary basin and, using frequencies of 7–12 Hz, we estimated the compressional velocity and attenuation above a depth of approximately 1.2 km. We refined our previously published inversion strategy by alternating between phase-only and amplitude-plus-phase velocity inversion for the first two pairs of frequencies used, and added a second step, in which we inverted for attenuation from the lowest frequency using the final recovered velocity model and an initial homogeneous Qp-model. Our recovered velocity and attenuation models demonstrated an overall good correlation with the available sonic and gamma-ray logs. Modeled seismic data matches the field data well and 1D velocity and attenuation profiles extracted at line intersections show a good correlation, thus demonstrating the robust nature of the results.Recovered velocities aid in interpreting shallow structures not readily identifiable on the conventional migration such as Quaternary strata and Pliocene faulting. Recovered attenuation values in the sedimentary rocks are generally consistent with saturated sandstones and consistent with the geology interpreted from well logs. Localized regions of elevated attenuation and associated low velocities correlate with siltstones and shales, the presence of hydrocarbons, or inferred increases in porosity due to fracturing. Seafloor pockmarks, where venting of gas occurs, are underlain by low velocities and an anomalous attenuation variation, and pipe-like gas chimneys are interpreted in two other areas of Hecate Strait. Igneous basement is associated with high velocity and high attenuation in its uppermost part, suggesting the presence of volcanic rocks, but the elevated attenuation may also be due to scattering and elastic mode conversions not included in the viscoacoustic inversion

Frugal full-waveform inversion: From theory to a practical algorithm

As conventional oil and gas fields are maturing, our profession is challenged to come up with the next-generation of more and more sophisticated exploration tools. In exploration seismology this trend has let to the emergence of wave-equation-based inversion technologies such as reverse time migration and full-waveform inversion. While significant progress has been made in wave-equation-based inversion, major challenges remain in the development of robust and computationally feasible workflows that give reliable results in geophysically challenging areas that may include ultralow shear-velocity zones or high-velocity salt. Moreover, subsalt production carries risks that need mitigation, which raises the bar from creating subsalt images to inverting for subsalt overpressure

Two-dimensional waveform tomography of the Queen Charlotte Basin of Western Canada and the Seattle fault zone

Two-dimensional frequency domain visco-acoustic waveform tomography is applied to limited-offset marine seismic reflection data from the Queen Charlotte sedimentary Basin of western Canada, and from the Seattle Fault Zone in Puget Sound, Washington. It was possible to obtain high resolution P-wave velocity and attenuation images of the subsurface, and to practically evaluate the effectiveness of the visco-acoustic waveform tomography method. A specific data preconditioning and inversion strategy is developed to recover models to a depth of 1.2 to 1.3 km. The preconditioning of the data converts the field data to a form similar to that predicted by the acoustic waveform modelling algorithm. A multiscale inversion strategy was designed to mitigate non-linearity issues and to improve the estimation of attenuation. The starting velocity model is derived from first arrival traveltime tomography and the starting attenuation model is a homogeneous Q p -value. Four seismic lines in the Queen Charlotte Basin are imaged, and the recovered velocity models aid in interpreting shallow structures such as Quaternary strata and Pliocene faulting. The joint interpretation of the velocity and attenuation models enables the identification of siltstone, shales, the presence of hydrocarbons and seafloor pockmarks. The shallowmost basement rocks are interpreted to be volcanic. Using a section of the seismic data across the Seattle Fault Zone, synthetic visco-acoustic and visco-elastic modelling was used to verify the effectiveness of applying visco-acoustic waveform tomography to visco-elastic data. The results show that visco-acoustic waveform tomography of marine seismic reflection data is reliable when high velocity gradients are absent from the model. Finally, an interpretation is provided for the inversion results across the Seattle Fault Zone. The inverted velocity and attenuation models enable the identification of glacial and post-glacial Pleistocene, Tertiary sedimentary rocks, and Eocene volcanic rocks. Several north-dipping shallow thrust faults, anticlines and a syncline are identified across the Seattle uplift and the Seattle Fault Zone. The orientation of the faults are consistent with the interpretations of the Seattle Fault Zone as either a fault propagating fold with a forelimb breakthrough, or as the leading edge of a triangle zone within a passive roof duplex

Seismic waveform tomography across the Seattle fault zone in Puget Sound: Resolution analysis and effectiveness of visco-acoustic inversion of viscoelastic data

Visco-acoustic waveform tomography was applied to marine seismic reflection data across the Seattle fault zone in Puget Sound in the northwestern USA. Using the recovered velocity and attenuation models, we performed a set of synthetic visco-acoustic and viscoelastic checkerboard tests, and compared the results to verify the effectiveness of applying visco-acoustic waveform tomography to viscoelastic field data. Visco-acoustic waveform tomography produces higher resolution velocity and attenuation models than ray-based tomography, but artefacts due to elastic effects such as mode conversion are present at layer interfaces where the velocity contrast is high. Elastic effects also affect attenuation values, which can be too high or too low in places because visco-acoustic inversion compensates the loss of amplitude due to mode conversion by inadequately estimating the attenuation.A comparison of the attenuation models inverted from viscoelastic and visco-acoustic synthetic data suggests that inverted attenuation values can be reliable when the velocity gradient is low, and the quality of the inversion improves in a highly attenuating medium or in a medium with high attenuation contrasts. Joint interpretation of the derived velocity and attenuation models enables us to identify Quaternary (glacial and postglacial Pleistocene) sedimentary, Tertiary sedimentary and Eocene volcanic rocks. Several shallow faults, anticlines and a syncline are identified across the Seattle uplift and the Seattle fault zone. Our interpretation of faults using the velocity model, attenuation model and migrated seismic section is consistent with two possible published models of the Seattle Fault Zone: either a thrust fault that accommodates north–south shortening by forming a fault-propagation fold with a forelimb breakthrough, or part of a passive roof duplex in which the Seattle Fault Zone is located at the leading edge of a triangle zone that is propagating into the Seattle basin

Cooperative inversion of seismic and magnetotelluric data in complex areas: A workflow

Joint inversion or cooperative inversion of different geophysical data has the potential of providing more accurate estimate of subsurface rock properties. We present a cooperative inversion approach for acoustic impedance using seismic and magnetotelluric data. In this approach, the magnetotelluric data, sensitive to the resistivity of rocks are used to get the large scale background spatial trends of the acoustic impedance model. The magnetotelluric data also provide complementary information to seismic in complex areas such as subbasalt, subbsalt and overthrust belts where the propagation of seismic wave is difficult. On the other hand, seismic data are used to get the small-scale features in the recovered model. The connections between resistivity, density and velocity are obtained from petrophysical relationships derived from bore-hole data. Additional information such as structural coupling obtained from geological information can also be added during the inversion process. We present an application of this technique to synthetic data. The synthetic example demonstrates how an improved result is achieved at step of of our cooperative inversion process. In particular we show how artifacts are progressively eliminated with new cycles of the cooperative inversion workflow

A workflow for cooperative inversion of seismic and magnetotelluric data

We present a cooperative inversion approach for acoustic impedance using seismic and magnetotelluric data. In this approach, the magnetotelluric data, sensitive to the resistivity of rocks are used to get the large scale background spatial trends of the acoustic impedance model, while the seismic data are used to get the small-scale features. The connections between resistivity and elastic properties of rocks are obtained from petrophysical relationships derived from borehole data. Structural constraints derived from seismic are used to improve the magnetotelluric inversion. We present an application of this technique to synthetic data derived from previous interpretation of seismic and magnetotelluric models in a mineral province. The synthetic example shows how an improved result is obtained using our cooperative inversion workflow

Cooperative joint inversion of 3D seismic and magnetotelluric data: With application in a mineral province

The integration of different geophysical data has the potential to provide more accurate estimate of subsurface rock properties. Several methodologies and attempts have been developed over the years with the objective of reducing exploration risk. We have developed a cooperative joint-inversion approach intended to facilitate recovery of acoustic impedance (AI) using seismic and magnetotelluric (MT) data. In this approach, the MT data provided a pathway for iteratively building large-scale low-frequency information content not directly recoverable from the seismic data themselves. The MT data provided complementary information to seismic, especially in seismically complex terrains such as overthrust belts, subbasalt and subsalt, carbonate reefs or for targets below deep cover containing limestone, concretionary layers, or basalt. On the other hand, the seismic data provided structural information necessary to derive accurate resistivity models from MT inversion and small-scale features during seismic impedance inversion. The connections between resistivity and the elastic property of rocks are obtained from petrophysical relationships derived from available borehole data, or if not available, from empirical relationships. We tested our technique on synthetic and field data. The application of cooperative joint inversion to 3D seismic and MT data sets acquired in a mineral province made it possible to recover AI distribution across a wide range of geologic environments. The resulting rock property images provided a direct link to geology that is exceedingly difficult, if not impossible, to extract from the individual data sets

Frugal full-waveform inversion: From theory to a practical algorithm

As conventional oil and gas fields are maturing, our profession is challenged to come up with the next-generation of more and more sophisticated exploration tools. In exploration seismology this trend has let to the emergence of wave-equation-based inversion technologies such as reverse time migration and full-waveform inversion. While significant progress has been made in wave-equation-based inversion, major challenges remain in the development of robust and computationally feasible workflows that give reliable results in geophysically challenging areas that may include ultralow shear-velocity zones or high-velocity salt. Moreover, subsalt production carries risks that need mitigation, which raises the bar from creating subsalt images to inverting for subsalt overpressure