Ranajit Ghose

Quantifying Biomediated Ground Improvement by Ureolysis: Large-Scale Biogrout Experiment

Biogrouting is a biological ground improvement method, in which microorganisms are used to induce carbonate precipitation in the subsurface in order to increase the strength and stiffness of granular soils. In this paper the results of a large-scale experiment ( 100  m3 ) are presented, in which the feasibility of biogrouting as a ground improvement method is investigated using techniques and equipment similar to those used in potential applications. In situ geophysical measurements were used to monitor the biogrouting process during treatment and indicated that the stiffness had increased significantly after one day of treatment. The results of unconfined compressive strength tests on samples which were excavated after treatment were used to assess the distribution of mechanical properties throughout the cemented sand body, which correlated quite well with the results of the in situ geophysical measurements. The stiffness increase could be quantified as a function of the injected volume of grouting agent...

Shallow to very shallow, high‐resolution reflection seismic using a portable vibrator system

In shallow engineering‐geophysical applications, there is a lack of controlled, nondestructive, high‐resolution mapping tools, particularly for the target depth that ground‐penetrating radar cannot reach but which is too shallow for other conventional geophysical methods. For soft soil, this corresponds to a depth of 2 to 30 m. We have developed a portable, high‐frequency P-wave vibrator system that is capable of bridging this gap. As far as the important contribution of the seismic source is concerned, penetration and resolution can be individually controlled through easy modulation of the sweep signal generated by this electromagnetic vibrator. The feasibility of this system has been tested in shallow (10–50 m) to very shallow (0–10 m) applications. Seven field data sets representing varying geology, site conditions, and exploration targets are presented to illustrate the applicability. The first three examples show the potential of this portable vibrator source in shallow applications. Under favorable ...

Integrating S‐wave seismic‐reflection data and cone penetration test data using a multiangle multiscale approach

A cone penetration test (CPT) is the most common geotechnical testing method used to estimate in situ the strength properties of soil. Although CPT provides valuable information, this information is restricted to the location of the measurement. We propose a new concept to integrate shallow S‐wave reflection seismic data with CPT data in order to obtain laterally continuous subsoil information. In this vein, a valid quantitative means to relate seismic reflections to CPT data is a primary requirement. The approach proposed here is based on the characterization of the scaling behavior of the local fine‐scale S‐wave velocity information extracted from the seismic reflection data and the same behavior of the CPT cone resistance. The local velocity contrast information is extracted by linearized Zoeppritz inversion of the amplitude‐preserved prestack reflection data. We have formulated a multiscale analysis approach employing the continuous wavelet transform in order to quantitatively characterize the nature ...

Quantitative integration of seismic and GPR reflections to derive unique estimates for water saturation and porosity in subsoil

For shallow subsoil, the estimates of in-situ porosity and water saturation are important, but until now it has been difficult to estimate them reliably. We relate seismic and GPR reflection coefficients to porosity and water saturation using a shared earth model. Using this model, we propose a method to integrate quantitatively seismic and GPR angle-dependent reflection coefficients. The new approach has been tested through numerical simulations. The results clearly show that from either seismic or GPR data alone it is impossible to obtain unique estimates for porosity and water saturation; however, a correct integration of those two data types leads to unique and stable estimates at a subsoil layer boundary. Potential applications of this approach exist in hydrogeology and environmental, agricultural and geotechnical engineering.

High-frequency shear wave reflections from shallow subsoil layers using a vibrator source: sweep cross-correlation versus deconvolution with groundforce derivative

Summary Shear waves are important in geotechnical investigations of soil. We have evaluated on shallow shear-wave reflection data the influence of removing accurately the effective source function on the separation and the amplitude of the reflection events. A small vibrator can produce high frequencies and the generated groundforce can be estimated quite accurately. Instead of conventional cross-correlation to compress the raw vibrograms, for each sweep we have performed deconvolution using the estimated groundforce derivative. Our results show that deconvolution of the source response improves the separation of the reflections, correlating better with the actual soil layer boundaries. The side-lobe energy of the effective source wavelet is smaller after deconvolution compared to cross-correlation, and this improves the separation. As the source function is removed separately at each shot location, the true-amplitude reflection section offers information of the actual lateral variation of shear-wave reflectivity, which is important to derive the variation of soil strength at a subsoil boundary.

The improvement of geotechnical subsurface models through the application of S-wave reflection seismic exploration

Due to its relatively high sensitivity to the distribution of elastic parameters, the seismic technique is quite suitable for geotechnical subsurface investigations. However, practical implementation of the P-wave exploration technique for soft-soil and shallow-groundwater conditions, as often encountered in the Netherlands, may often prove problematic; the highimpedance contrast at the groundwater table may induce strong reverberations and guided waves, resulting in a distortion of the seismic record at shallow depth. The use of buried sources may be of some advantage, but at the cost of a considerable decrease in the speed of operation.

In-Situ Permeability from Integrated Poroelastic Reflection Coefficients

A reliable estimate of the in?situ permeability of a porous layer in the subsurface is extremely difficult to obtain. We have observed that at the field seismic frequency band the poroelastic behavior for different seismic wavetypes can differ in such a way that their combination gives unique estimates of in?situ permeability and porosity simultaneously. This is utilized in the integration of angle? and frequency?dependent poroelastic reflection coefficients in a cost function. Realistic numerical simulations show that the estimated values of permeability and porosity are robust against uncertainties in the employed poroelastic mechanism and in the data. Potential applications of this approach exist in hydrocarbon exploration, hydrogeology, and geotechnical engineering.

Contrasting behavior between dispersive seismic velocity and attenuation: Advantages in subsoil characterization

A careful look into the pertinent models of poroelasticity reveals that in water-saturated sediments or soils, the seismic (P and S wave) velocity dispersion and attenuation in the low field-seismic frequency band (20-200 Hz) have a contrasting behavior in the porosity-permeability domain. Taking advantage of this nearly orthogonal behavior, a new approach has been proposed, which leads to unique estimates of both porosity and permeability simultaneously. Through realistic numerical tests, the effect of maximum frequency content in data and the integration of P and S waves on the accuracy and robustness of the estimates are demonstrated.

Converted waves in a shallow marine environment: Experimental and modeling studies

Seismic waves converted from compressional to shear mode in the shallow subsurface can be useful not only for obtaining shear-wave velocity information but also for improved processing of deeper reflection data. These waves generated at deep seas have been used successfully in hydrocarbon exploration; however, acquisition of good-quality converted-wave data in shallow marine environments remains challenging. We have looked into this problem through field experiments and synthetic modeling. A high-resolution seismic survey was conducted in a shallow-water canal using different types of seismic sources; data were recorded with a four-component water-bottom cable. Observed events in the field data were validated through modeling studies. Compressional waves converted to shear waves at the water bot-tom and at shallow reflectors were identified. The shear waves showed distinct linear polarization in the horizontal plane and low velocities in the marine sediments. Modeling results indicated the presence of a nongeometric shear-wave arrival excited only when the dominant wavelength exceeded the height of the source with respect to the water/sediment interface, as observed in air-gun data. This type of shear wave has a traveltime that corresponds to the raypath originating not at the source but at the interface directly below the source. Thus, these shear waves, excited by the source/water-bottom coupled system, kinematically behave as if they were generated by an S-wave source placed at the water bottom. In a shallow-water environment, the condition appears to be favorable for exciting such shear waves with nongeometric arrivals. These waves can provide useful information of shear-wave velocity in the sediments.

Characterization of a Concealed Fault Zone Using P and S-wave Seismic Reflection Data

The Vila Franca de Xira fault zone is the central sector of the OVLS fault zone, one of the major geological structures of the Lower Tagus Valley area in Portugal. Evidences previously gathered from geological and geophysical data suggest that it is an active fault zone and is responsible for the 1531 earthquake that caused extensive damages in Lisbon and the surrounding areas. However, no clear evidence of Quaternary seismic activity has been proven until now. The characterization of the fault zone in Holocenic terrains is therefore important for seismic hazard assessment. A P-wave and an S-wave seismic reflection profiles were acquired over an existing oil-industry and high-resolution P-wave seismic lines. The processing and preliminary interpretation of our data shows that S-wave data provides a superior resolution compared to the P-wave data and is the preferred geophysical method to characterize shallow faults zones in the study area. We have confirmed that faulting affects the Holocene alluvium and a vertical offset of 1-2 m was found in the S-wave data. This finding will allow a better definition of the return periods and the maximum expected earthquake magnitude of the OVLS fault zone.