Adam O'Neill

Dominant Higher Surface-wave Modes and Possible Inversion Pitfalls

Dominant higher modes of surface waves are known to be generated at sites with large stiffness contrasts and/or reversals between layers. At many engineering sites, quite often all modal energy is transferred to successively higher modes with increasing frequency. These ‘jumps’ may occur at low (20 Hz) to high (60 Hz) frequencies, and, more than once over the recorded bandwidth, but the common features are that the jumps are in amounts exceeding 50% of the phase velocity, and dispersion does not return to the fundamental mode at high frequency. A nonlinear geophone spacing helps to resolve these modes more uniquely. Full-wavefield synthetic seismogram modelling reinforces the effect to be due to a steep, nonlinear stiffness gradient in the uppermost 1–2 meters of soil. If a high-velocity substrate at depth is not present, the ‘effective’ phase velocity of the higher modes can exceed that of the maximum shear-wave velocity of the model, where leaky modes persist. When this happens, conventional modal dispersion modelling cannot be applied in the inversion. If an artificial stiff layer is added at depth, jumps across several modal boundaries can occur, which is also generated in a Gibson half-space, both at very low and high frequencies. This is a major pitfall, where mode-misidentification, especially at low frequency, can lead to errors in the estimated shear wave velocity models of over 50%. Guided P-waves also manifest as large dispersion discontinuities, but usually at higher frequency and phase velocities, with buried explosive sources. Although Poisson9s ratio has a strong influence of the generation of—and frequencies of transitions to—dominant higher modes, the higher-mode phase velocities themselves are relatively independent of shallow P-wave velocity.

Applications of Love Wave Dispersion for Improved Shear-wave Velocity Imaging

Love wave dispersion and associated analytic partial derivatives are theoretically derived using the reflection/transmission (R/T) method. The numerical implementation is then applied to linearised inversion of synthetic and field data. For simple cases dominated by the fundamental mode, Love wave sensitivity and inversion stability is higher than the Rayleigh wave dispersion. However, in general, Rayleigh wave inversion converges more rapidly than Love waves, although similar low misfits can be achieved. When assumed interfaces are used, the inversion of fundamental-mode Love wave dispersion of the normally dispersive profile provides a more accurate result, because the Love wave dispersion is independent of the Poissons ratio. In more realistic, irregularly dispersive profiles, deep structure is only interpreted using higher-mode Love and Rayleigh wave dispersion. A field test over a shallow geologic fault with coincident Love and Rayleigh wavefields shows the fundamental–mode Love wave dispersion above 20 Hz to have at least 10% higher phase velocities than Rayleigh waves. However, at lower frequencies, Love wave dispersion is at least 10% slower. The resulting inverted models show up to 25% difference in shear-wave velocity. This is attributed to transverse isotropy of V SH and V SV in shallow fluvial sediments and allows improved geological horizon interpretation and soil discrimination over conventional, single-component surface wave inversion. A second test at a seismograph station site shows the Love wavefield less scattered and mode identification is simple, unlike Rayleigh waves over the same line, and the inversion results correlate well to downhole shear-wave velocity logs.

Lateral resolution and lithological interpretation of surface-wave profiling

In civil engineering, geological hazards can be in the form of both soft and hard zones. Soft zones, notably sinkholes, can lead to collapse structures reaching the surface and expensive damages during or after construction. Hard zones might be an impediment to excavation or, if they are to be used as foundations, it is desirable to know the thickness and strength.

Genetic Algorithm Inversion of Rayleigh Wave Dispersion from CMPCC Gathers Over a Shallow Fault Model

Lateral shear-wave velocity imaging, based on 1D surface wave inversion, is becoming routine in commercial site characterisation. A Genetic Algorithm (GA) is applied to the inversion of fundamental mode Rayleigh wave dispersion over a 3-layered, shallow, vertical-fault model. The data are calculated using a 2D finite difference method, then sorted into CMP cross-correlated (CMPCC) gathers, from which the dispersion is measured. Dispersion is relatively smooth over flat portions of the model, as well as when the spread is equally centered over the sharp lateral discontinuity. When between one and two-fifths of the spread is “overhanging” the fault, low-frequency dispersion is corrupted by wavefield scattering. Each dispersion curve is inverted using a 1D forward model, constrained to four layers, with parameters allowed to vary within liberal limits except the layer 1 shear-wave velocity. A novel variation is that the optimisation is partially directed, by including the most successful shear-wave velocity ...

Seismic Surface Waves Special Issue Guest Editorial

Invasive surveys ( e.g., penetrometers, drilling, trenching, etc.) are probably still the mainstay of geotechnical investigative methods. Geophysics is generally used to interpolate between and/or support the information from boreholes. For geotechnical projects which are usually cost or equipment constrained, or where environmental factors prohibit drilling, geophysics alone has been used in some rare cases. However, as sites become less accessible and various factors prohibit drilling, geophysics could take on an increasingly dominant role. Examples include inside narrow tunnels, deepwater foundations or highly urban environments. In the not too-distant future, expeditions to other moons and planets will certainly rely on nondestructive testing. For shallow soil mapping on Mars, the power …