Resolution Of Dispersion Image Research Articles

Phase-velocity and intercept-time scanning (PIS) technique for extracting experimental dispersion characteristics of surface waves

In active surface wave methods, the ability to extract the true dispersion trends from surface wave field data is critical for inferring accurate shear-wave velocity profiles. However, dispersion images such as those generated by the conventional wavefield transformation method are usually accompanied by numerical artifacts including side-lobes and aliasing, both of which decrease the dispersion image resolution and may lead to misinterpretation of dispersion modes. For example, the side lobes can be misinterpreted as higher or lower modes, which would introduce additional errors into the inverted stiffness profiles. A new experimental dispersion analysis method is presented to improve the resolution and sharpness of dispersion images and minimize the side lobes. The proposed method employs scanning of the phase-velocities and intercept-times of a series of harmonic signals obtained by Fourier transformation of raw multichannel data in the space-time (x-t) domain. Results obtained from synthetic and real field data demonstrate that the improved method can yield high-quality dispersion images compared to those of the conventional dispersion-imaging method. In addition, the improved method do not rely on the assumption that the impact point is the generation point of the Rayleigh waves, which is needed for the conventional method.

The Optimum Field Configuration for Active MASW Survey on Peat Soil

The application of the MASW method on engineering investigation required optimization of the field configuration to ensure high quality dispersion image for reasonable shear wave velocity profile estimation. The limited investigation with respect to peat soil condition has motivated the study to determine the optimum field configuration for peat soil. The challenging characteristics of peat soil including high void ratio, compressibility, water content and low shear strength further complicates the determination of optimum field configurations. The study focused on the determination of optimum field configurations for active MASW method which includes the receiver spacing, source offset, sensor frequency and sampling interval. The results obtained shows that, the optimum receiver spacing to obtain high signal to noise ratio dispersion image was 1 meter. Smaller receiver spacing causes domination of higher modes and wide bandwidth, while longer receiver spacing causes significant drop in signal to noise ratio governed by rapid energy dissipation with distance. For the source offset, the distance of half the total spread length (X1 = L/2) provides the best resolution and minimised near-field and far-field effect. While, 4.5 Hz sensor frequency and sampling interval between 100 to 250 s provides sufficient low frequencies for deeper depth investigation and denser data. Overall, the influence of receiver spacing, source offset and sensor frequency on the dispersion image resolution was significant.

A critical insight into the influence of data acquisition parameters on the dispersion imaging in passive roadside MASW survey

Passive roadside method of multichannel analysis of surface wave (MASW) is a simple passive technique of subsurface exploration that can provide deeper subsoil investigation compared to conventional active MASW technique. However, attributed to the lack of thorough experimental validation with other techniques and establishment of various acquisition parameters, the number of real world applications of the method is scanty. The paper presents the results of extensive experimentation to highlight the importance of two primary acquisition parameters, namely the length of receiver array and acquisition time, on the dispersion imaging of roadside survey at sites comprising different traffic volume. The length of receiver array directly affects the resolution of dispersion image and the estimated depth of shear-wave velocity profile. Thicker energy band and poor resolution in the lower frequency band of dispersion image are found to be the outcome of utilizing smaller arrays. The span of acquisition time is a site dependent parameter and is governed by the volume of the traffic and number of existing sources on the road surface nearby the test site. Comparatively smaller recording time is found effective in producing dispersion images with sufficient resolution in a heavier traffic volume case. The paper also illustrates a detailed comparison of results obtained from roadside and active survey in terms of the dispersion imaging and shear wave velocity profiles at the specific sites.

Comparison of Dispersion Image Resolution Acquired Using Multichannel Analysis of Surface Waves with Different Source Energy and Stacking

Active MASW method is a promising method to determine the stiffness characteristics of the subsurface strata. The accumulation energy to obtain high-resolution dispersion image depends largely on the energy generated from the active source. From the results obtained, the influence of different source energy, source plate and number of stacking on the dispersion image resolution were significant. Heavier source weight (e.g.7 kg sledgehammer) produced highamplitude energy band and provides even lower frequencies. However, if lighter source weight (e.g. small steel hammer) was used, it needs to be coupled with rubber plate to increase the low frequency content, thus generate deeper depth of investigation. Finally, the use of rubber plate and stacking increased the resolution of the dispersion image especially at lower frequencies by significantly suppressing the ambient noise.

Influence of Source Energy and Stacking on Active MASW Method Dispersion Image

The study investigates the influence of source weight, type of source plate and number of stacking on the resolution of the dispersion image obtained from the active Multichannel Analysis of Surface Waves (MASW) method at peat soil condition. Fundamental-mode dispersion curve determination is governed by the optimum configuration of the active MASW method. Optimum configuration will ensure reliable inverted shear wave velocity profile. The heterogeneity and problematic characteristics of peat soil provide challenges on the MASW survey. Based on the field tests conducted, the effect of the source weight was significant. Heavier source weight relatively increases the dispersion image resolution especially at lower frequencies but sacrificing the resolution at higher frequencies. Therefore, heavier source weight (i.e. 7 kg sledgehammer) was recommended for deeper depth of investigation with little concern on the shallow depth. The use of rubber plate also contributed to the increases of energy accumulation at lower frequencies. Increasing number of stacking increases the overall energy band as the stacking allows significant suppression of the ambient noise. 5 to 10 stacks were recommended for peat soil condition. Finally, overall comparison between the dispersion images obtained at both locations shows that thicker peat soil layer results in lower resolution and low signal-to-noise ratio dispersion image.

Quantification of the resolution of dispersion image in active MASW survey and automated extraction of dispersion curve

Multichannel analysis of surface waves is a non-invasive method in the field of geophysics which is used for identifying shear wave velocity profile of the subsurface along its depth. The reliability and accuracy of the obtained profile strictly depends on the resolution of the dispersion image obtained from an active survey. The dispersion image is a representation of the distribution of the energy of propagating waves on a frequency-phase velocity space. Based on the relative energy accumulation in the dispersion image, the fundamental (M0) and higher modes of dispersion can be identified. As there are several parameters that can affect the resolution of dispersion images, it is important to determine the optimum magnitude of each parameter that would contribute in generating dispersion images of higher resolution. Until date, the resolution of dispersion image is described only qualitatively, wherein fundamental mode dispersion curve was extracted based on the best possible visual discretion of the user. Erroneous selections of dispersion points often led to the misidentification of the dispersion mode, leading to incorrect shear wave velocity profile. This paper reports an image-processing based methodology to quantify the resolution of dispersion images and automated extraction of M0 dispersion curve through threshold energy filtering of the dispersion image. Such methodology removes the user discretion-induced subjectivity associated with the manual extraction of dispersion curve, thereby resulting in a robust extraction technique.

Effect of Source Characteristics on the Resolution of Dispersion Image from Active MASW Survey

In order to obtain a reliable shear wave velocity profile from an active MASW survey, it is important to develop a good-resolution dispersion image. In this regard, this article describes the effect of source characteristics on the quality of the recorded wavefield, and hence, on the resolution of the obtained dispersion image. Active MASW survey is conducted at three locations at IIT Guwahati campus, having different substrata characteristics based on the variation of stiffness. Based on the dispersion image, averaged power spectrum and the shear wave velocity profiles, it is understood that owing to substrata heterogeneities, proposing a universal guideline for conducting active MASW survey is extremely difficult. To obtain a good-resolution image, it is suggested to conduct the surveys with heavier sources to obtain good-resolution dispersion images yielding deeper strata information. Lighter sources, if used, should be accompanied by appropriate stacking of the dispersion images to enhance or strengthen the resolution of the dispersion images, even at the low frequencies. It is also suggested that proper material should be used as base plate/striker plate for active MASW survey depending upon the target depth and feasibility of the investigation.

Influence of data acquisition and signal preprocessing parameters on the resolution of dispersion image from active MASW survey

Influence of data acquisition and signal preprocessing parameters on the resolution of dispersion image from active MASW survey

Recommendations for generating dispersion images of optimal resolution from active MASW survey

Multichannel Analysis of Surface Waves (MASW) is proving to be the most recent and popular non-invasive method which can characterize the subsurface in quick and better ways. This article reports a detailed study of active MASW survey conducted in a soil site characterized by the presence of heterogeneous soil stratification with crushed debris. In this regard, the effect of receiver layout and key data acquisition parameters (offset distance, far-field and near-field phenomena, receiver spacing, total receiver spread length and numbers of deployed receivers) on the resolution of the obtained dispersion image is elucidated. The influence of the signal pre-processing parameters such as sampling frequency, sampling length, filtering and muting, is also highlighted. The effect of source characteristics on the quality of the recorded wavefield has been elaborated, and in this context, the enhancement of the resolution of dispersion image by stacking has also been discussed. Based on the outcomes, it is recommended that the sampling frequency and sampling time should be optimal so that complete propagation of wave phases through the geophone array is achieved, while at the same time, the time-stamp suffers minimum noise adulteration. Combined application of band-pass filtering and optimal temporal muting is required to obtain best resolution dispersion images. For sites having predominantly softer soils at the shallower depths (Vs,30 < 100 m/s), an optimal offset of 4–6 m with an inter-receiver spacing of 1 m produces the best resolution dispersion images. The resolution of the dispersion image at the lower frequencies can be increased either by using a heavier source, or adopting multiple stacking of dispersion images generated from low energy impacts.

Reason and Condition for Mode Kissing in MASW Method

Identifying correct modes of surface waves and picking accurate phase velocities are critical for obtaining an accurate S-wave velocity in MASW method. In most cases, inversion is easily conducted by picking the dispersion curves corresponding to different surface-wave modes individually. Neighboring surface-wave modes, however, will nearly meet (kiss) at some frequencies for some models. Around the frequencies, they have very close roots and energy peak shifts from one mode to another. At current dispersion image resolution, it is difficult to distinguish different modes when mode-kissing occurs, which is commonly seen in near-surface earth models. It will cause mode misidentification, and as a result, lead to a larger overestimation of S-wave velocity and error on depth. We newly defined two mode types based on the characteristics of the vertical eigendisplacements calculated by generalized reflection and transmission coefficient method. Rayleigh-wave mode near the kissing points (osculation points) change its type, that is to say, one Rayleigh-wave mode will contain different mode types. This mode type conversion will cause the mode-kissing phenomenon in dispersion images. Numerical tests indicate that the mode-kissing phenomenon is model dependent and that the existence of strong S-wave velocity contrasts increases the possibility of mode-kissing. The real-world data shows mode misidentification caused by mode-kissing phenomenon will result in higher S-wave velocity of bedrock. It reminds us to pay attention to this phenomenon when some of the underground information is known.

Offset and linear spread geometry in the MASW method

This paper evaluates the effect of near offset and geophone configuration on dispersion image for linear spreads in the multichannel analysis of surface waves (MASW) method. In order to determine the effect of near offset, MASW records were collected for a variety of offsets and the corresponding calculated dispersion images were compared for a fixed layout. To reveal the spectral resolution of dispersion images, MASW records were collected using nine different types of linear spread configuration, then dispersion images for each MASW record were calculated using the phase velocity analysis technique and were compared to the present preferable linear spread type for a MASW survey. The results show that at least one pair of near offset shots is required for distinguishing the fundamental mode from the higher modes without ambiguity, and for picking a dispersion curve in a wide frequency range. The first shot distance should not be longer than four times the geophone interval, and the second shot distance should be equal to or longer than the one-third spread length, depending on the power of the seismic source. A better dispersion image resolution and dispersion curve in the wide frequency range can be obtained by using the SL-type geophone configuration. Besides this, in a survey in which higher frequencies are more important, the SII-type configuration may be preferable. With the SII-type spread configuration, the fundamental mode can easily be distinguished from the higher modes.