Constraints on the Permeability Structure of Alluvial Aquifers From the Poro-Elastic Inversion of Multifrequency P-Wave Sonic Velocity Logs

We evaluate the performance of traveltime tomography and full-wave inversion (FWI) for near-surface modeling using the data from a shallow seismic field experiment. Eight boreholes up to 20-m depth have been drilled along the seismic line traverse to verify the accuracy of the P-wave velocity-depth model estimated by seismic inversion. The velocity-depth model of the soil column estimated by traveltime tomography is in good agreement with the borehole data. We used the traveltime tomography model as an initial model and performed FWI. Full-wave acoustic and elastic inversions, however, have failed to converge to a velocity-depth model that desirably should be a high-resolution version of the model estimated by traveltime tomography. Moreover, there are significant discrepancies between the estimated models and the borehole data. It is understandable why full-wave acoustic inversion would fail — land seismic data inherently are elastic wavefields. The question is: Why does full-wave elastic inversion also fail? The strategy to prevent full-wave elastic inversion of vertical-component geophone data trapped in a local minimum that results in a physically implausible near-surface model may be cascaded inversion. Specifically, we perform traveltime tomography to estimate a P-wave velocity-depth model for the near-surface and Rayleigh-wave inversion to estimate an S-wave velocity-depth model for the near-surface, then use the resulting pairs of models as the initial models for the subsequent full-wave elastic inversion. Nonetheless, as demonstrated by the field data example here, the elastic-wave inversion yields a near-surface solution that still is not in agreement with the borehole data. Here, we investigate the limitations of FWI applied to land seismic data for near-surface modeling.

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Elastic inversion of near- and postcritical reflections using phase variation with angle

Geophysics ◽

10.1190/geo2011-0230.1 ◽

2012 ◽

Vol 77 (4) ◽

pp. R149-R159 ◽

Cited By ~ 29

Author(s):

Xinfa Zhu ◽

George A. McMechan

Keyword(s):

Wave Velocity ◽

Spherical Wave ◽

Phase Variation ◽

P Wave ◽

Wide Angle ◽

Reverse Time ◽

Reverse Time Migration ◽

S Wave ◽

Time Migration ◽

Elastic Inversion

Near- and postcritical (wide-angle) reflections provide the potential for velocity and density inversion because of their large amplitudes and phase-shifted waveforms. We tested using phase variation with angle (PVA) data in addition to, or instead of, amplitude variation with angle (AVA) data for elastic inversion. Accurate PVA test data were generated using the reflectivity method. Two other forward modeling methods were also investigated, including plane-wave and spherical-wave reflection coefficients. For a two half-space model, linearized least squares was used to invert PVA and AVA data for the P-wave velocity, S-wave velocity, and the density of the lower space and the S-wave velocity of the upper space. Inversion tests showed the feasibility and robustness of PVA inversion. A reverse-time migration test demonstrated better preservation of PVA information than AVA information during wavefield propagation through a layered overburden. Phases of deeper reflections were less affected than amplitudes by the transmission losses, which makes the results of PVA inversion more accurate than AVA inversion in multilayered media. PVA brings useful information to the elastic inversion of wide-angle reflections.

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Investigating the stratigraphy of an alluvial aquifer using crosswell seismic traveltime tomography

Geophysics ◽

10.1190/1.2195487 ◽

2006 ◽

Vol 71 (3) ◽

pp. B63-B73 ◽

Cited By ~ 23

Author(s):

Geoff J. Moret ◽

Michael D. Knoll ◽

Warren Barrash ◽

William P. Clement

Keyword(s):

Seismic Tomography ◽

Horizontal Resolution ◽

P Wave ◽

Grain Size Analysis ◽

Size Analysis ◽

Data Quality Control ◽

Alluvial Aquifers ◽

Velocity Models ◽

Sedimentary Architecture ◽

The Impact

In this study, we investigate the use of crosswell P-wave seismic tomography to obtain spatially extensive information about subsurface sedimentary architecture and heterogeneity in alluvial aquifers. Our field site was a research wellfield in an unconfined aquifer near Boise, Idaho. The aquifer consists of a ∼ 20-m-thick sequence of alluvial cobble-and-sand deposits, which have been subdivided into five stratigraphic units based on neutron porosity logs, grain-size analysis, and radar reflection data. We collected crosswell and borehole-to-surface seismic data in wells [Formula: see text] apart. We carefully considered the impact of well deviation, data quality control, and the choice of inversion parameters. Our linearized inverse routine had a curved-ray forward model and used different grids for forward modeling and inversion. An analysis of the model covariance and resolution matrices showed that the velocity models had an uncertainty of [Formula: see text], a vertical resolution of [Formula: see text], and a horizontal resolution of [Formula: see text]. The velocity in the saturated zone varied between [Formula: see text] and [Formula: see text]. Inclusion of the borehole-to-surface data eliminated the X- shaped pattern that is a common artifact in crosswell tomography, and the increased angular coverage also improved the accuracy of the model near the top of the tomogram. The final velocity model is consistent with previous stratigraphic analyses of the site, although the locations of some of the unit boundaries differ by as much as [Formula: see text] in places. The results of this study demonstrate that seismic tomography can be used to image the sedimentary architecture of unconsolidated alluvial aquifers, even when the lithologic contrasts between units are subtle.

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Subduction zone fluids and arc magmas conducted by lithospheric deformed regions beneath the central Andes

Scientific Reports ◽

10.1038/s41598-021-02430-9 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

E. Contreras-Reyes ◽

D. Díaz ◽

J. P. Bello-González ◽

K. Slezak ◽

B. Potin ◽

...

Keyword(s):

Electrical Resistivity ◽

Subduction Zone ◽

Continental Crust ◽

Subduction Zones ◽

Hydrothermal Systems ◽

P Wave ◽

Complex Permeability ◽

Nazca Plate ◽

Lithospheric Deformation ◽

Permeability Structure

AbstractDehydration of the oceanic subducting slab promotes the formation of magmatic arcs, intra-slab intermediate-depth seismicity, and hydration of the overlying mantle wedge. However, the complex permeability structure of the overriding plate controls the magma and fluid migration and their accumulation at shallower depths. In this regard, mapping the inner structure of the overriding crust and mantle is crucial to understand the magmatic and hydrological processes in subduction zones. We integrate 3-D P-wave, $$V_p/V_s$$ V p / V s , and electrical resistivity tomographic models of the northern Chilean subduction zone to map the magmatic and fluids derived from the subducting oceanic Nazca plate. Results show a continental crust relatively thick (50–65 km) characterized by a lower zone of high $$V_p$$ V p values (7.2–7.6 km/s), which is interpreted as the presence of plutonic rocks. The mantle lithospheric wedge is weakly hydrated ($$V_p/V_s$$ V p / V s = 1.75–1.8) while the forearc continental crust is traversed by regions of reduced electrical resistivity values ($$< 10^2$$ < 10 2 $$\Omega m$$ Ω m ) interpreted as zones of relatively high permeability/fracturing and fluid content. These regions spatially correlate with upper plate trans-lithospheric deformation zones. Ascending melts accumulate preferentially in the back-arc, whereas hydrothermal systems form trenchward of the volcanic arc. The results highlight the complex permeability structure of the upper South American plate.

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