Rayleigh Wave Dispersion Curve Inversion with the Artificial Bee Colony Algorithm

Rayleigh wave dispersion curve inversion is a non-linear iterative optimization process with multi-parameter and multi-extrema. It is difficult to carry out inversion and reconstruction of stratigraphic parameters quickly and accurately with a single linear or non-linear inversion for the data processing of Rayleigh waves with complex seismic geological conditions. We proposed a new method that combines artificial bee colony algorithm (ABC) and damped least squares algorithm (DLS) to invert Rayleigh wave dispersion curve. First, food sources are initialized in a large scale of the model based on the prior geological information. Then, after three kinds of bee operators (employed bees, onlooker bees and scout bees) transform each other and perform search optimization with several iterations, the targets are converged near the optimal solution to obtain an initial S-wave velocity model. Finally, the final S-wave velocity model is obtained by local optimization of DLS inversion with fast convergence and strong stability. The correctness of the method has been verified by one high-velocity interlayer model, and it was further applied to a real Rayleigh wave dataset. The results show that our method not only absorbs the advantages of ABC global search optimization and strong adaptability, but also makes full use of the advantages of DLS inversion, such as high accuracy and fast convergence speed. The inversion strategy can effectively suppress the inversion falling into local extrema, get rid of the dependence on an initial model, enhance the inversion stability, further improve the convergence speed and inversion accuracy, while has good anti-noise ability.

Thermochemical structure of the lithosphere and upper mantle beneath Superior craton: Results from multi-observable probabilistic inversion

<p>We present new thermochemical models of the lithosphere and upper mantle beneath the Superior craton and surrounding regions. The study area is dominated by the Archean Superior Province, surrounded by Proterozoic orogenic belts such as the Trans-Hudson Orogen (THO) to the north and the Grenville Orogen to the southeast. Portions of the Rae and Hearne cratons north of the THO are also studied, as is the Mid-continent Rift to the south. Over a period of ∼3 Ga, the region has seen assembly and modification by accretionary and orogenic events, periods of rifting, and the influence of a number of mantle hotspots. Here, we use a probabilistic inverse method to jointly invert Rayleigh wave dispersion data, Vp data, geoid anomalies, surface heat flow, and absolute elevation. The output is a 3D model of the seismic, temperature, bulk density, and compositional structure of the whole lithosphere beneath the Superior craton.</p><p>The resulting model will provide new opportunities for joint studies of the structure of the upper mantle and will shed light on the thermal and compositional variations beneath the region. In this presentation, we will discuss the results from our model and several robust features that carry important geological and geodynamical implications for this region.</p>

A shallow Vs model from full wave inversion for hydraulic fracturing site

<p>In recent years, small dense arrays with inter-station distances ~1-km are widely deployed for various geotechnical proposes, including exploring the subsurface geothermal reservoirs, monitoring the hydraulic fracturing, and estimate the local seismic hazards. Those small-dense arrays record several hours or days microtremor datasets and can be used to extract noise H/V ratio and short-period Rayleigh wave dispersion curve. To fulfill those geotechnical purposes, the extracted H/V ratio and Rayleigh wave dispersive curves are used to invert for 1-D layered velocity structures based on traditional ray theory, ignoring the 3-D Rayleigh wave propagating effects. In the present-day, with advances in computational power, 3-D numerical seismic wave propagation in realistic Earth models has become feasible and this has led to efforts to invert full waveforms. However, as H/V ratios from microtremors are produced by multiple random sources close to the surface with complex scattering effects, they are still unrealistic to be simulated by 3-D numerical seismic wave simulations. In this study, we use a strategy to incorporate the theories of H/V ratios from microtremor and the 3-D waveform simulations to invert for shallow sub-surface speed model using a small array. The details of the strategy are that we first use the traditional method to build an initial speed model by jointly inverting the Rayleigh wave dispersion curves and H/V ratios, and then, we further update the initial model with full-wave inversion of the Rayleigh waveforms calculated from ambient noise cross-correlations. The small array used here was composed of 21 broadband seismic stations, aimed to monitor the underground hydraulic fracturing. Therefore, the newly build model has important geotechnical usages of locating stimulated micro-seismic events and diagnose the hydraulic fracturing effects.</p>