Resposta em Microtremor de um Movemento de Massa no Distrito Federal, Brasil

Abstract

The present study provides a brief description of the ambient noise recorded at a slow moving mass movement in Ribeirão Contagem Basin. The area is an interesting natural laboratory as river detachment processes in a number of different stages can be identified and are easily accessible. We investigate the site dynamic characteristics of the study area by recording ambient noise time-series at nine points, using portable nine three-component short period seismometers. The time-series are processed to give both horizontal to vertical spectral ratio (HVSR) curves as well as time-frequency plots of noise power spectral density (SPD). The HVSR curves illustrate and quantify aspects of site resonance effects due to the local geological setting. Probability density function (PDF) shows that noise level lies well between new high noise model (NHNM) and new lower noise model (NLNM) and their probabilities are higher above 2 Hz. HVSR curves present a uniform lithologically controlled peak at 2 Hz. Directional properties of the wavefield are determined by beamforming method. The f-k analysis results in the E-W component show that at 5 Hz phase velocities are close to 1700 m/s while at 10 Hz dropped to 250 m/s. We observed that between 5 and 16 Hz the incoming wavefield arrive from 260 degrees. Further studies will apply a detailed noise analysis for relating the dynamics of the landslide (which can be retriggered by river erosion as well as rainfalls and seismic shaking) to possible changes in detectable physical properties. Keywords: Seismic noise; Power Spectral Density; Probability Density Function; HVSR; f-k analysis ResumoO estudo apresentado fornece uma ampla descrição do ruído ambiental registrado de um deslizamento de encosta lento localizado na bacia do Ribeirão Contagem no Distrito Federal. A área de estudo é um laboratório natural na medida que os processos erosivos do rio podem ser analisados em diferentes estágios de evolução dentro de um setor de fácil acesso. A pesquisa abrange o estudo das características dinâmicas da área por meio do registro das séries temporais do ruído ambiental em nove pontos mediante o uso de sismómetros de período curto e três componentes. As series temporais são processadas para obter as curvas de relação espectral horizontal-vertical (HVSR) e também os gráficos tempo:frequência dos espectros de potência (SPD). As curvas HVSR ilustram e quantificam aspectos dos efeitos de ressonância local devidos ao ambiente geológico. A função de desnsidade de probabilidade (PDF) mostra que os níveis de ruído registrados estão enquadrados entre os limites conhecidos como NHNM (new high noise model) e NLNM (new lower noise model) e que suas probabilidades são maiores acima de 2 Hz. As curvas HVSR apresentam um pico de resposta uniforme litológicamente controlado a 2 Hz. As propriedades direcionais do campo de ondas são determinadas pelo método da viga (beamforming method). A análise dos resultados na componente EW indica que na fase de 5 Hz a velocidade está próxima de 1700 m/s enquanto qu...

Ambient seismic noise

“…The slope chosen for this work is located in the cow and horse farm in a small vicinity wind, effects of wind on trees or buildings, industrial machinery, cars, and trains, or human footsteps is called noise. Based on the frequency contents, there are two sources of noise as natural and cultural commonly referred to as microseisms (< 1Hz) and microtremors (> 1Hz), respectively (Hussain et al 2017). During last decades or so, the common applicabilities of these techniques have increased exponentially.…”

“…The climate in the area is semi-humid tropical with a rainy summer and dry winter. The mean annual precipitation in the area is 1,442.5 mm (Hussain et al 2017 and reference therein).…”

“…This ellipticity is prominent due to high impedance contrast between soil and bedrock. The impedance contrast appears because of vanishing of spectral energy at the vertical component and inversion of rotation of fundamental mode (Hussain et al 2017). This information about the resonance frequency of sediments is provided without any previous knowledge of the geological and S-wave velocity structure of the subsurface (Gasper, 2017).…”

“…are observed (Figure 3). The ubiquitous frequency (2Hz) is the frequency of Saprolite layer over the bedrock and is not affected by the changes of the degree of saturation due to rainfall events (Hussain et al 2017;2018). The second peak is related to landslide and variations in this peak are observed because of the change in soil properties.…”