Downhole Seismic Survey Research Articles

Assessing the Accuracy of a Borehole-Controlled P-Wave Velocity Model of Yucca Flat, Nevada Using Large-N Seismic Data

ABSTRACT With geologic data from over 950 boreholes, Yucca Flat basin, residing on the Nevada National Security Site, has excellent borehole control on stratigraphy. These data were used to create a Geologic Framework Model (GFM) of the basin. Of these boreholes, 188 have corresponding downhole seismic survey data, which were used to determine average P-wave velocities of the geologic units and create a GFM seismostratigraphic model (GFM-SS). With the acquisition of six new active-source large-N datasets in Yucca Flat, we can now quantitatively assess the accuracy of the GFM-SS previously controlled only by borehole data. For each of the six datasets, we subset the GFM to the region of interest and create a forward model of P-wave travel times for the GFM-SS given the large-N source-receiver geometries. We first made trial-and-error adjustments to the unit velocities (while keeping the layer geometry intact) to improve the travel-time residuals. We then implemented a simulated annealing approach to find the optimal velocity model for each dataset. Our results indicate that the borehole-controlled model overestimates alluvium velocities across Yucca Flat. This result persists even when we make smaller GFM-SS models that are local to individual large-N experiments. We hypothesize that this result is a combination of shorter ray paths and the resulting lack of interaction with large-scale features (such as faults), as well as less attenuation of high frequencies in the borehole data. Both the current GFM-SS and the updated model based on median velocities that we present here overgeneralize local unit velocities, which can be quite heterogeneous in Yucca Flat.

Identification of Subsurface Structures and Dynamic Modulus Determination Based on Downhole Seismic Surveys (Case Study)

Seismic features of the site are studied in earthquake prone areas. In these studies, the seismic design parameters of the site are determined based on geological and geotechnical engineering features. In this regard, part of the building regulations (European Standard) is related to site classification. In order to classify the land, the bylaws provide various methods such as mean shear wave velocity up to 30 m depth. In the present study, the values of the dynamic modulus of layers are calculated using conventional relationships based on the values of wave velocities for different depths. In this study, data from 22 boreholes, in which Downhole Seismic surveys were performed, were identified and implemented to identify subsurface layers and to determine the dynamic modulus in the area. The results of these studies in land classification were used to evaluate the site effect based on European Standard. The results show that the area under study is classified in class B according to this standard.

Assessment of land subsidence as an environmental threat facing Dammam city, eastern Saudi Arabia based on soil geotechnical parameters using downhole seismic approach

To meet the increasing in population, urbanization and public transportation needs of Dammam big city a new transportation infrastructure network is planned and being developed. In the same time, environmental threats of land subsidence on network has been raised and needs to be addressed.Site characterization for designing this transportation lines is crucial to ensure reliable and economic substructure design, because weak site soil conditions may cause land subsidence problems. Downhole seismic (DS) testing is commonly used to determine compression wave (P) and shear wave (S) velocity profiles in geotechnical engineering, which are required in evaluating responses to shaking of geotechnical sites and structures. A DS survey was conducted through boreholes at the center of the Dammam metropolitan city, Saudi Arabia. A sledgehammer horizontally striking a wooden plate generated shear (S) waves polarized in the crossline and inline directions, whereas a vertical hammer hitting a metal plate generated the P wave. Data were obtained using Freedom Data PC, a downhole tool with a triaxle geophone package, and a seismograph. In the downhole technique, the geophones were equestrian on an internal rotating block, and a built-in fluxgate compass as well as servo motor system maintained the orientation of the geophones inside the borehole. The instrumentation and processes complied with the standard test methods for downhole seismic testing (Standard and D7400-08, 2008). This represents, a versatile platform for evaluating structures/infrastructures and geophysical seismic engineering surveys. The estimated seismic velocities were used in estimating the various elastic moduli and density variations within the mapped boreholes. The integrated approach of proper geotechnical characterization and 1D velocity profile proved to be a helpful tool for assessment of subsidence of Dammam city in the future which, in turn, will reduce the environmental threats facing the city.

Geological structure of the Khylly field according to 3D seismic data

Relevance of the work. The paper considers challenging problems related with outlining of intervals with oil and gas presence in the mature Khylly field by use of latest 3D seismic survey techniques in order to gain larger crude resources base. The purpose of this research is to discover the most promising intervals of target horizons with relatively high reservoir properties outlined by 3D seismic data. The subjects of research are 3D seismic survey data, downhole seismic survey – Vertical Seismic Profiling (VSP) and well logging diagrams. The object of research is the Khylly deposit. The paper describes in brief geological and geophysical characteristics, stratigraphic and lithological features of rocks making the section. It is noted that despite repeated surveys by use of various geological and geophysical techniques, the field setting is not thoroughly studied and it has been covered by 3D seismic survey in 2012. Research results. 3D seismic survey applied across Khylly area is resulted in drawing of 4 structural maps for III and I horizons of Productive Series (PS), Akchagyl and Lower Absheron suites. Taking into account the relevance of structural planes of various stratigraphic levels and III horizon of PS being one of the major reference horizons the paper gives description of structural map drawn for this horizon. The detailed velocity model is designed based on VSP data with wide use of velocity analysis data. It has been made clear that Khylly area has block structure and each block has been described in detail. Based on acquired data it has been recommended to drill exploratory well R-1. Conclusion. Processing and interpretation of seismic data are aimed at solving some geological problems; the main task was to obtain results that ensure the study of the geological structure in the seismic survey area, including tracing of seismic horizons, faults and outlining the areas and section intervals which may be of interest due to possible oil and gas presence. VSP data acquired in well 2012 and velocity analysis made it possible to design velocity model of the section under the study, with the use of which the temporary 3D cube was transformed into a depth cube. The quality of seismic data is good and made it possible to solve the tasks set for this research.

충북 증평 지질학습장 시험부지에 대한 물리탐사 및 지질공학 자료의 해석

지하 매질의 물리적인 성질을 정확히 평가하기 위한 가장 일반적인 방법은 해당 지점에서 시추하여 채취한 암편을 대상으로 해석한 지질공학적인 실내시험 결과와 시추공 안에 삽입한 탐사 기구를 통해 얻은 지구물리학적인 자료를 함께 복합적으로 해석하는 것이다. 이 연구에서는 충북 증평의 지질학습장으로 예정된 시험부지 중의 한 곳에서 시추공 탐사 대신 수집한 지표 탄성파탐사(굴절법 및 표면파법) 자료로부터 얻은 P파 및 S파 속도구조와 이를 토대로 작성된 포아송 비 단면을 통해 노두에 좁은 폭으로 드러난 암석 환경(적색퇴적암, 회색화산암, 풍화 화산암) 및 접촉선에서 관찰된 단열, 파쇄대가 확인되는지 알아보았다. 또한 탐사 지역에 주로 분포하는 회색 화산암과 적색 퇴적암을 대상으로 지표지질 조사와 실내 지질공학 실험을 수행하였다. 탐사자료에서 평가된 동적 탄성계수 영률은 지질공학 자료의 정적 탄성계수보다 높았다. 실내 시험에서 회색화산암에 비해 공극률이 작고 함수비가 작은 것으로 평가된 적색퇴적암은 추가적으로 건기와 우기에 실시한 전기탐사(비저항 및 자연전위) 결과에서 건우기와 관계없이 상대적으로 전기비저항이 높고 그 변화 폭이 매우 작은 영역으로 확인되었다. 특히 높은 함수비와 공극률을 갖는 풍화된 회색 화산암과 함께 접촉선에서 좁게 발달한 단열 파쇄대가 우기 때의 충전율을 높이는 열린 통로의 역할을 한 것으로 해석된다. The best way of investigating the physical and mechanical properties of subsurface materials is the combined interpretation of data from borehole geophysical surveys and geotechnical experiments with rock samples. In this study two surface seismic surveys with refraction and surface-wave method are alternatively conducted for downhole seismic surveys in test site for geological field trip in Jeungpyung, Chungbuk. P- and S-wave velocity structures are delineated by refraction and MASW (multichannel analysis of shear waves) methods, respectively. Possion's ratio section, reconstructed from P- and S-wave velocities, is correlated to the outcrop geological features consisting of reddish sedimentary rock, gray volcanic rock, and joints/fractures. In addition, rock samples representative for reddish sedimentary and gray volcanic features are geotechnically analyzed to provide physical, mechanical properties, and elastic modulus. Dynamic elastic moduli estimated from geophysical data is found to be higher than the one from geotechnical data. Reddish sedimentary rock characterized with low porosity and moisture content corresponds to the zone of low electrical resistivities and their small variations in the resistivity sections between the rainy and dry days. This trend suggests that the weathered gray volcanic rock and the nearby fractures with higher low porosity and moisture content are interpreted to be good carrier especially in rainy season.

EMPIRICAL CORRELATION OF SHEAR WAVE VELOCITY AND N-SPT VALUE FOR JAKARTA

This paper proposes an equation representing shear wave velocity (Vs) as a function of SPT blow count (N-SPT). The equation is generated by statistical regression of site investigation data at many building project in Jakarta. The N-SPT values and Vs values were obtained from the same boreholes between 2005 and 2012. The Vs values were obtained by the downhole seismic survey. A total of 22 building and 35 borings provided 234 pairs of N-SPT and Vs values were used to get a regression equation. The new and previously suggested formulae have been compared and evaluated by using the same dataset. Wave propagation analysis require Vs as an input parameter, and the empirical equation may be useful for estimating Vs at site where only N-SPT data available. The information of empirical correlation can and perhaps should be considered in developing microzonation map of Jakarta as inputs in a continuous process of risk assessment and disaster mitigation risk reduction.

A New Inversion Procedure for Spectral Analysis of Surface Waves Using a Genetic Algorithm

A new inversion procedure for spectral analysis of surface waves (SASW) using a genetic algorithm (GA) is presented. The inversion process proposed in this study starts by running a forward solution for the Rayleigh dispersion equation, with sets of random inputs, to find the theoretical phase velocities. Then, it continues by finding new and better sets of inputs through processes that mimic natural mating, selection, and mutation in each generation. The goal of the GA is to find the best match between the theoretical and the experimental dispersion curves. Therefore, with each new generation there is a better agreement between the calculated output theoretical dispersion curve and the input experimental dispersion curve. To start the procedure, two options are available, either requesting the GA-based optimization process to obtain shear-wave velocities and thicknesses for each layer, or providing the thicknesses and requesting the optimization process to obtain the best set of shear- wave velocities. The GA part of the procedure is fast, stable, and accurate, with several advantages compared to the traditional methods. The strength and accuracy of the proposed procedure are presented through two example problems. We show that (1) the inversion process using a GA results in a good agreement between the theoretical and experimental dispersion curves, and (2) the shear-wave velocity pro- files obtained from the approach presented in this study and a downhole seismic survey show a good level of agreement.

Elastic shear modulus of soft clays from shear wave velocity measurement

Case records comprising the results of down-hole seismic surveys collected at nine sites worldwide, together with comparative results of laboratory bender element tests on reconstituted clay samples, were examined in an attempt to quantify the shear modulus of normally consolidated clays at very small strains of the order of 0·0001%. The shear modulus Gmax under the current state of stresses is given in a formula which includes a newly proposed void ratio function. An empirical expression incorporating the new void ratio function is also proposed for practical use in estimating Gmax profiles with depth in natural soil deposits from routinely available borehole data. In addition, effects of soil structure and ageing on Gmax are discussed on the basis of a case record. On a examiné les dossiers de cas comprenant les resultats de relevés sismiques à foration descendante, obtenus dans dix points du globe, ainsi que les résultats comparatifs d'essais d'éléments de flexion en laboratoire sur des échantillons d'argile reconstitutés, en vue d'essayer de quantifier le module de cisaillement d'argiles normalement consolidées à de très petites contraintes de l'ordre de 0,0001 p. 100. Le module de cisaillement Gmax, à l'état de contrainte existant est donné dans une formule qui comprend une nouvelle fonction proposée d'indice des vides. Une expression empirique comprenant la nouvelle fonction d'indice des vides est également proposée pour l'application pratique a l'estimation des proflls Gmax en fonction de la profondeur dans le dépôts de sol naturel a partir de données de foration descendante couramment disponibles. De plus, les effets de la structure du sol et du vieillissement sur Gmax sont examinés dans le contexte d'un cas d'espèce.

Crosshole and downhole seismic surveys in the UK Trias and Lias

Abstract Crosshole and downhole seismic surveying methods are being used increasingly in geotechnical investigations to evaluate dynamic properties of soils and rocks when earthquake effects on structures have to be considered. The paper describes two case histories for which crosshole and downhole seismic surveying has been carried out. It was found that the compressional and shear wave velocities are related to depth and have a close correlation with lithology. For both sites, crosshole measurements had higher values than those obtained from downhole methods. This might be construed as evidence for anisotropy. Both sites are interbedded, one with mudstones and limestones and the other with mudstones and siltstones. It is therefore suggested that the crosshole velocities are exaggerated in the vicinity of stronger beds in horizontally layered ground. The derivation of dynamic soil properties from the results of seismic surveys is discussed using the velocity data obtained from the two sites. The sensitivity of dynamic Poisson’s ratio to low V p / V s ratios is examined and the sensitivity of dynamic shear modulus to velocity variations is demonstrated.

Crosshole and downhole seismic surveys in the UK Trias and Lias : Pinches, G M; Thompson, R P Proc 24th Annual Conference of the Engineering Group of the Geological Society, Field Testing in Engineering Geology, Sunderland, 4–8 Sept 1988P475–496. Publ Sunderland: Sunderland Polytechnic, 1988

Crosshole and downhole seismic surveys in the UK Trias and Lias : Pinches, G M; Thompson, R P Proc 24th Annual Conference of the Engineering Group of the Geological Society, Field Testing in Engineering Geology, Sunderland, 4–8 Sept 1988P475–496. Publ Sunderland: Sunderland Polytechnic, 1988

Comparison of modulus values obtained from dilatometer testing with downhole seismic surveys and unconfined compressive tests at McKays Point Dam Site, California: Kaneshiro, J Y; Harding, R C; Johannesson, P; Korbin, G E Proc 28th US Symposium on Rock Mechanics, Tucson, 29 June–1 July 1987P211–221. Publ Rotterdam: A A Balkema, 1987

Comparison of modulus values obtained from dilatometer testing with downhole seismic surveys and unconfined compressive tests at McKays Point Dam Site, California: Kaneshiro, J Y; Harding, R C; Johannesson, P; Korbin, G E Proc 28th US Symposium on Rock Mechanics, Tucson, 29 June–1 July 1987P211–221. Publ Rotterdam: A A Balkema, 1987