Microtremor

"It is a well established fact that the degree of damage due to  an  earthquake  depends on the dynamic properties of soil and on the amplification of seismic waves. Also geologic deposits can amplify strong ground motions depending on the physical conditions. Amplification can be identified by using written
records in different thickness of soils for one or several earthquakes, but the major restraint is the resource limitation i.e., availability of these records is impossible at all times. This limitation can be overcome by recording the microvibrations of the ground. The city of Vijayawada falling in seismic Zone III, according to Indian Seismic code 1893:2002, is one of the important urban centre and a major business centre in the
state of the Andhra Pradesh, India. It is located on the banks of the holy river Krishna which occupies an area of about 73 km2
  and is located 16°31’N latitude and 80° 39’E longitude. To estimate the peak frequency of ground and to study on  the  loose  alluvial  strata,  extensive microtremor testing has been
carried out  in  Vijayawada at 66 locations  to understand the site  specific ground response  during earthquake shaking. Microtremor  technique  is one of the recent advances in the geophysical survey techniques but is quite different from the  other methods. Analysis has been done at all test sites using
microtremor records  and  predominant frequency  map is  generated which  reflects the fundamental characteristics of possible site effects. "

We constrain the depth and seismic structure of stiff sediment cover overlying a prospective basement terrane using a passive seismic technique which uses surface wave energy from micro-tremor (also known as ambient seismic energy or seismic noise). This may be applied to mineral exploration under cover to decrease the inherent ambiguity in modeling potential field data for exploration targeting. We use data from arrays of portable broadband seismometers, processed using both the multimode spatially averaged coherency (MMSPAC) method and the horizontal to vertical spectral ratio (HVSR) method, to produce profiles of seismic velocity structure along a 12-km transect. We have developed field protocols to ensure consistent acquisition of high-quality data in near-mine and remote locations and a variety of ground conditions. A wavefield approaching the theoretical ideal for MMSPAC processing is created by combining the energy content of an off-road vehicle, driven around the seismometer array, and ambient sources. We found that this combination results in significantly higher-quality MMSPAC waveforms in comparison with that obtained using ambient energy alone. Under ideal conditions, a theoretical maximum depth of investigation of 600 m can be achieved with a hexagonal sensor array with 50-m radius and MMSPAC and HVSR. The modeling procedure we employ is sensitive to layer thicknesses of AE5%. A high-velocity layer in the sediment package reduces the sensitivity to deeper structure. This can limit the modeling of underlying layers but may be addressed by detailed analysis of the HVSR peaks. Microtremor recordings including off-road vehicle noise, combined with the MMSPAC and HVSR processing techniques, may therefore be used to constrain sediment structure and depth to basement in a cost-effective and efficient method that could contribute greatly to future mineral exploration under cover.

An irregular geometry of the track can influence the safe passage of railway traffic. Therefore, maintenance activities on railways are mostly directed towards the correction of track geometry. It has been shown that unevenly supported and unsupported sleepers make major contributions to the effect of track geometry deterioration. In addition, irregular sleeper support conditions can contribute to ballast fouling, thus reducing its service life. Therefore, assessing the support conditions of the sleepers in a railway track is essential. This paper reviews methods that can be used for this purpose. Then, we propose a method for the assessment of sleeper support conditions and present results obtained in field investigations. The proposed method is based on sampling of the micro-tremor on sleepers and interpretation of obtained data. The obtained results prove that the proposed method is reliable and therefore usable in practice.

The microtremor method is very useful in urban areas to estimate local site effects for microzonation
purposes. As seismic waves propagate from bedrock to the ground, their characteristics are site
specific due to the heterogeneity of substrata. Source, path and site effects are prime parameters
that affect ground response. Many destructive earthquakes that occurred in the past (e.g. Bhuj 2001,
Bingol 2003, Kashmir 2005, Haiti 2010, Tohoku 2011, Van 2011 and Yunnan 2012) clearly illustrate
the effects of local soils on damage severity and pattern. The Indian sub-continent has more than
400 major faults that influence seismic activity and India has experienced several devastating
earthquakes (Assam 1897 (M58?7), Kangra 1905 (M58?6), Bihar-Nepal 1934 (M58?4), AssamTibet 1950 (M58?7), Latur 1993 (M56?4), Chamoli 1999 (M56?8), Bhuj 2001 (M57?6) and Sikkim
2011 (M56?9)). The state of Andhra Pradesh is located in the central part of peninsular India (zones II and III according to Indian seismic code) and has a record of earthquakes along the coast. This study considered Vijayawada (zone III), the third largest city in the state. Microtremor surveys were carried
out at 75 different locations in the Vijayawada urban area and analysis was carried out using the Nakamura technique. Dynamic characterisation was carried out by considering the shape of the response curve, horizontal to vertical (H/V) amplitude, predominant frequency and the characteristic
soil profile at all the test sites. Based on this detailed analysis, a classification is proposed and a predominant frequency map of the study area was developed. It was found that areas in the north eastern and south eastern parts of the city, with silty and clayey sand formations, have comparatively high predominant frequencies (>4 Hz). The northern and western parts of the city, with high silty clay and silty sand, are characterised by moderate frequency values (2–4 Hz). Low (,2 Hz) values of predominant frequencies were observed at a few locations. The H/V amplitudes are high (2–3) in the eastern and western regions.