A pinboard by
Geraldine Anukwu

I am a PhD Candidate in Geophysics at Universiti Sains Malaysia.


What comes to your mind when you hear of infrastructural failures such as building collapse, subsidence, differential settlements etc.? The first conclusion would be to probably attribute the cause of failure to poor design, construction or the use substandard materials. Often times, a thought is not given to the fact that the underlying geology and geomorphology of the subsurface materials could be the reason. It is no gain saying that the strength of any structure is as strong as what it rests on. Consequently, an accurate evaluation of the stiffness of near-surface materials (usually the top 30m) is thus critical in determining the integrity of overlying structures.

Over the years, geophysical techniques have proven to be effective in subsurface imaging due to its relatively low cost, better spatial coverage and non-invasiveness. For studies involving the evaluation of the soil stiffness, the surface waves techniques are particularly suited, as it utilizes the dispersive properties of seismic surface waves to provide the shear wave velocity, Vs, of subsurface materials. Vs, in turn, is closely related to the material stiffness.

The surface wave analysis usually involves field data acquisition, data processing to extract the dispersion curve and the inversion of the extracted dispersion curve to yield a Vs model for the subsurface. A very important step in the utilization of this technique is the extraction of the dispersion curve from transformed shot gather. An error in the extraction of the dispersion curve would result in an inaccurate Vs model for the subsurface. For geologically complex terrain, where there is an irregular variation in the stiffness of the subsurface, the presence of boulders, uneven topography and relatively shallow bedrock, this step is not straightforward.

As such, my research will focus on evaluating the effectiveness of the surface wave techniques for geologically complex terrain. The evaluation would be based on the three main steps: Field Procedure, Data Processing and Inversion. Synthetic models will be examined and compared with field records to determine the possible cause of the complexities in the transformed shot gather. The end game of this research is to be able to propose ways by which this technique can be effectively utilized in a geologically complex terrain.

For more exciting insight into the wonderful and awesome composition of the earth beneath us, stay tuned to my pinboard.


Shear-wave velocity structure of the Tongariro Volcanic Centre, New Zealand: Fast Rayleigh and slow Love waves indicate strong shallow anisotropy

Abstract: Models of the velocity structure of volcanoes can help define possible magma pathways and contribute to calculating more accurate earthquake locations, which can help with monitoring volcanic activity. However, shear-wave velocity of volcanoes is difficult to determine from traditional seismic techniques, such as local earthquake tomography (LET) or refraction/reflection surveys. Here we use the recently developed technique of noise cross correlation of continuous seismic data to investigate the subsurface shear-wave velocity structure of the Tongariro Volcanic Centre (TgVC) of New Zealand, focusing on the active Ruapehu and Tongariro Volcanoes. We observe both the fundamental and first higher-order modes of Rayleigh and Love waves within our noise dataset, made from stacks of 15 min cross-correlation functions. We manually pick group velocity dispersion curves from over 1900 correlation functions, of which we consider 1373 to be high quality. We subsequently invert a subset of the fundamental mode Rayleigh- and Love-wave dispersion curves both independently and jointly for one dimensional shear-wave velocity (Vs) profiles at Ruapehu and Tongariro Volcanoes. Vs increases very slowly at a rate of approximately 0.2 km/s per km depth beneath Ruapehu, suggesting that progressive hydrothermal alteration mitigates the effects of compaction driven velocity increases. At Tongariro, we observe larger Vs increases with depth, which we interpret as different layers within Tongariro's volcanic system above altered basement greywacke. Slow Vs, on the order of 1–2 km/s, are compatible with P-wave velocities (using a Vp/Vs ratio of 1.7) from existing velocity profiles of areas within the TgVC, and the observations of worldwide studies of shallow volcanic systems that used ambient noise cross-correlation methods.

Pub.: 08 Feb '17, Pinned: 24 Aug '17

Characteristics of the horizontal component of Rayleigh waves in multimode analysis of surface waves

Abstract: In surface-wave analysis, S-wave velocity estimations can be improved by the use of higher modes of the surface waves. The vertical component of P-SV waves is commonly used to estimate multimode Rayleigh waves, although Rayleigh waves are also included in horizontal components of P-SV waves. To demonstrate the advantages of using the horizontal components of multimode Rayleigh waves, we investigated the characteristics of the horizontal and vertical components of Rayleigh waves. We conducted numerical modeling and field data analyses rather than a theoretical study for both components of Rayleigh waves. As a result of a simulation study, we found that the estimated higher modes have larger relative amplitudes in the vertical and horizontal components as the source depth increases. In particular, higher-order modes were observed in the horizontal component data for an explosive source located at a greater depth. Similar phenomena were observed in the field data acquired by using a dynamite source at 15-m depth. Sensitivity analyses of dispersion curves to S-wave velocity changes revealed that dispersion curves additionally estimated from the horizontal components can potentially improve S-wave velocity estimations. These results revealed that when the explosive source was buried at a greater depth, the horizontal components can complement Rayleigh waves estimated from the vertical components. Therefore, the combined use of the horizontal component data with the vertical component data would contribute to improving S-wave velocity estimations, especially in the case of buried explosive source signal.

Pub.: 15 Dec '14, Pinned: 21 Aug '17

Investigation of site properties in Adapazarı, Turkey, using microtremors and surface waves

Abstract: Determination of suitable areas for new residential buildings is crucial to increase the resilience of buildings against earthquake hazards. After the 1999 Izmit earthquake Mw 7.4, the city of Sakarya has been expanding rapidly in terms of both the population and number of superstructures. Despite the fact that Sakarya suffered several large earthquakes in the last two decades, the geophysical properties of the region have not been adequately investigated. In this study, the site properties of Sakarya University, Esentepe Campus, and its surrounding Serdivan district, which is one of the important parts of the city, are determined using microtremor measurements and surface wave analysis. Nakamura’s spectral ratio method (spectral ratio between the horizontal and vertical components, H/V or HVSR) was used to determine the fundamental frequency and site amplification values. The shear wave velocity profiles of the studied sites were determined using the multichannel analysis of surface waves method. Seismic measurements were performed at 34 locations to record the surface waves in the area. The fundamental frequency and site amplification values are determined as 1.02–11.68 and 1.33–5.96 Hz, respectively, from the microtremor measurements. The fundamental frequency is between 4.0 and 11.5 Hz in the campus area and between 1.3 and 2.0 Hz near the D-100 intercity highway. The site amplification was determined to be 1–2.5 in the campus area. The greatest site amplification (6) is obtained at thick alluvium deposits in the valley between hills. The average shear wave velocity values are within the range of 300 and 1120 m/s. Some parts located on the hill area have better soil condition (B) categories, according to the National Earthquake Hazard Reduction Program, and have comparatively high shear wave velocities in the range of 740–1080 m/s, whereas low velocity values are found at the thick alluvium deposits (D). We found that the geological properties and topographies play very important roles on the shear wave velocity and the amplification factors in the investigated area.

Pub.: 14 Oct '16, Pinned: 01 Aug '17