Downhole Array Research Articles

Comment on "Comparison of Time Series and Random-Vibration Theory Site-Response Methods" by Albert R. Kottke and Ellen M. Rathje

Methods of site‐response analysis require theoretical and empirical testing and validation before they can be used in seismic‐hazard assessment. Comparison of site amplification functions (SAFs) calculated using earthquake data recorded by seismic arrays with SAF obtained using analytical approaches represent the most important tests of reliability of those methods. High‐quality, low‐amplitude earthquake data recorded by the Treasure Island (TI) downhole array between 1993 and 2010 were used for comparisons with the two versions of the equivalent linear (EQL) site‐response methods implemented in the computer program STRATA (Kottke and Rathje, 2008; Rathje and Kottke, 2013): Use of the TS approach in STRATA matches well the empirically determined SAF between the bedrock and the surface. The STRATA version of the RVT approach can produce a significantly different SAF from the empirically determined and the TS approach. Further testing of different realizations of the RVT method is desirable to assess the method’s reliability and limitations. The paper of Kottke and Rathje (2013) raises a very important issue of comparison of the two site‐response analysis approaches: classical TS SHAKE‐type (Idriss and Seed, 1968; Idriss and Sun, 1992) and RVT (Vanmarcke, 1975; Der Kiureghian, 1980; Hanks and McGuire, 1981). These approaches represent two versions of EQL site‐response analyses. I found this discussion timely, because more and more probabilistic seismic‐hazard projects, including those for critical facilities, use the RVT approach. Kottke and Rathje (2013) properly pointed out that the fact that the RVT site‐response analysis does not require input TS makes it an attractive alternative to the TS approach. Nevertheless, the main questions are:

A microseismic experiment in Abu Dhabi, United Arab Emirates: implications for carbonate reservoir monitoring

A microseismic experiment utilizing a single downhole array of eight 3-component receivers was conducted in an offshore oilfield in the emirate of Abu Dhabi in the United Arab Emirates. The Lower Cretaceous Thamama Group is the major carbonate reservoir in the field producing from six zones. Microseismic data was acquired in conjunction with gas injections that took place in the Thamama IV A and IV B reservoirs during 9 days of acquisition. The aim of the experiment was to monitor the microseismic activity arising in the carbonate reservoirs as a result of gas injection. A total of 103 microseismic events were detected from two of the receivers. For the majority of these events, there were no well-defined P wave arrivals, probably due to the presence of strong background noise and the very weak microseismic signals. The results from this experiment indicate that the detected events are probably related to the microseismic activities caused by the gas injection within the Thamama IV B reservoir. Therefore, downhole microseismic monitoring methods can potentially provide valuable information about the fracture systems within the carbonate reservoirs of Abu Dhabi's oilfields.

A Study on the Variability of Kappa ( ) in a Borehole: Implications of the Computation Process

Knowledge of the acceleration spectral shape is crucial to various applications in engineering seismology. Spectral amplitude decays rapidly at high frequencies. Anderson and Hough (1984) introduced the empirical factor κ to model this attenuation. This is the first time κ is studied in a vertical array consisting of more than two stations. We use 180 earthquakes recorded at a downhole array with five stations in soils and rock to investigate the effect of soil conditions on κ . Given that κ computation processes vary across literature when following the classic Anderson–Hough method, we investigate its variability with the different assumptions that can be made when applying the method. The estimates of κ range between 0.017 and 0.031 s at the surface and between 0.004 and 0.024 s at rock. This variability due to the assumptions made is larger than the error of each estimate and larger than the average difference in values between sediment and rock. For this data set, part of it can be attributed to the type of distance used. Given this variability, κ values across literature may not always be comparable; this may bias the results of applications using κ as an input parameter, such as ground‐motion prediction equations. We suggest ways to render the process more homogeneous. We also find that κ at rock level is not well approximated by surface records from which we deconvolved the geotechnical transfer function. Finally, we compute κ on the vertical component and find a dependence of the vertical‐to‐horizontal κ ratio on site conditions. Online Material: Table of regression parameters and figure showing the regressed lines.

Site Response Analysis Using Downhole Array Recordings during the March 2011 Tohoku-Oki Earthquake and the Effect of Long-Duration Ground Motions

Downhole arrays provide enhanced understanding of dynamic soil behavior and site response. Historically, downhole array recordings have been available only for earthquakes with relatively limited durations. New recordings from a number of KiK-net downhole arrays during the 11 March 2011, Mw 9.0, subduction zone earthquake near the east coast of Honshu, Japan, allow us to investigate dynamic soil characteristics and site response due to long-duration subduction zone earthquakes. Using these recordings, we perform one-dimensional site response analyses to evaluate the applicability of commonly used analysis approaches under long-duration earthquakes. We find that site response analyses capture key features of measured surface response spectra particularly at soft rock/stiff soil sites subject to long-duration motion. However, at softer soil sites, it appears that the modulus reduction is overestimated and site-specific characterization is needed.

Effect of lattice-shaped ground improvement geometry on seismic response of liquefiable soil deposits via 3-D seismic effective stress analysis

This manuscript presents the results of an investigation into the effect of various configurations of lattice-shaped soil improvement on the seismic response of liquefiable soil deposits using 3-dimensional seismic effective stress analysis with an advanced constitutive model for sandy soils. The particular problem considered is based on the soil stratigraphy of the down-hole seismic array site at Port Island, Kobe, and the input ground motion recorded at this location in the 1995 Kobe earthquake. Nine different soil improvement geometries are considered by changing the number and wall thickness of improved soil cells for the site. For each of the different improvement geometries, the salient features of the ground response are presented, including: (i) peak surface acceleration, displacement and response spectra; (ii) excess pore pressure, stress path, and stress–strain response of the enclosed unimproved soil; and (iii) peak deformations of the improved soil, among others. Finally, the obtained results for all considered improvement geometries are summarised based on improved area ratio (RIA); improvement length-to-height ratio (RLH); and normalised construction cost (RCC). The results demonstrate the complexity of the seismic response and interaction between improved and enclosed liquefaction-susceptible native soils when subjected to strong ground shaking; the dependence on both the improved area and length-to-height ratios; as well as the wealth of insightful information that advanced effective stress analyses can provide for assessing the seismic response of such soil deposits.

Development of an inverse analysis framework for extracting dynamic soil behavior and pore pressure response from downhole array measurements

SUMMARYObservations from earthquakes over the past several decades have highlighted the importance of local site conditions on propagated ground motions. Downhole arrays are deployed to measure motions at the ground surface and within the soil profile, and also to record the pore pressure response within the soft soil profiles during excitation. The degradation of soil stiffness as excess pore pressures are generated during earthquake events has also been observed. An inverse analysis framework is developed and demonstrated to directly extract soil material behavior including pore water pressure (PWP) generation from downhole array measurements that can then be readily used in 1D nonlinear site response analysis. The self‐learning simulations (SelfSim) inverse analysis framework, previously developed for total stress site response analysis, is extended to extract PWP generation behavior of the soil in addition to cyclic response during ground shaking. A Neural Network based constitutive model is introduced to represent PWP generation during cyclic loading. A new analysis scheme is introduced that can use data from co‐located piezometer and accelerometer sensors. The successful performance of the proposed framework is demonstrated using four synthetic vertical array recordings. Copyright © 2013 John Wiley & Sons, Ltd.

Elastic and Large-Strain Nonlinear Seismic Site Response from Analysis of Vertical Array Recordings

AbstractStrong ground motions from the Mw=6.6 2007 Niigata-ken Chuetsu-oki earthquake were recorded by a free-field downhole array at a nuclear power plant. Site conditions consist of about 70 m of medium-dense sands overlying clayey bedrock, with groundwater located at 45 m. Ground shaking at the bedrock level had a geometric mean peak acceleration of 0.55g, which reduced to 0.4g at the ground surface, indicating nonlinear site response. One-dimensional ground response analysis of relatively weak motion aftershock data provides good matches of the observed resonant site frequencies and amplification levels, provided small-strain damping levels somewhat larger than those from laboratory tests are applied. Nonlinear ground response analyses of strong-motion data using laboratory-based modulus reduction and damping relations valid up to moderate strain levels (<∼0.5%) produce unrealistic strain localization at a velocity contrast. A procedure is presented to more realistically represent the large-strain por...

A taxonomy of site response complexity

A few extensively studied downhole seismic arrays are commonly used in detailed site response studies. Thus, there is a critical need to increase the number of sites that are used to compare soil constitutive models. Toward this end, we develop a classification scheme for downhole arrays that identifies stations where common wave propagation assumptions are valid. For stations where the one-dimensional (1D) assumption does not hold, we identify different levels of complexity that must be accounted for, which is a function of the inter-event variability and the similarity between the empirical and one-dimensional theoretical transfer functions. The classification is based on 100 seismic arrays in Japan that have recorded surface accelerations in excess of 0.3g, 69 of which exhibit low inter-event variability. The response at 16 of these sites resembles the one-dimensional response, while the others deviate from one-dimensional behavior, indicating that the one-dimensional assumption is not acceptable in most cases. We check our interpretation of the taxonomy with field investigations at two stations. The field observations show large lateral variations of the velocity profile across distances of hundreds of meters at the station where we expect the one-dimensional assumption does not hold.

A Comparison of Hydraulic-Fracture Modeling With Downhole and Surface Microseismic Data in a Stacked Fluvial Pay System

Summary This paper presents a study that combines and compares the results of hydraulic-fracture mapping and modeling using both downhole and surface microseismic arrays. There were three objectives for the study: developing detailed post-treatment models of hydraulic-fracturing treatments in the subject well, Well D1, which was monitored with downhole microseismic tools; developing detailed post-treatment models of the hydraulic-fracturing treatments in the subject well, Well S1, which was monitored with surface microseismic tools; and determining the match characteristics of the downhole and surface microseismic data to hydraulic-fracture models developed for both Wells D1 and S1. Input data for this project were obtained from two wells in the Greater Natural Buttes field, Uinta basin, Utah. Ten fracture models were built using five stages each from the two subject wells, Wells D1 and S1, and detailed pressure matches were made. Comparisons of the match characteristics from the multiple inputs were then developed. The hydraulic-fracture-stimulation models were graphically integrated with the microseismic events using visualization software. This software allowed the final model-simulated fracture geometries to be plotted along with the microseismic events in 3D space, thus allowing the viewer to see a full integration of data in a stacked fluvial pay system. Results from the integration process show good agreement in geometries and depth for most stages in the downhole-monitored well, whereas comparisons of surface microseismic-mapping measurements with the simulated fracture geometries yielded variable results. When combined with additional inputs such as geologic models, the integration methodology used in this project provides an excellent tool for hydraulic-fracture modeling and reservoir management in stacked pay systems.

Inferring rock fracture evolution during reservoir stimulation from seismic anisotropy

We have analyzed seismic anisotropy using shear-wave-splitting measurements made on microseismic events recorded during a hydraulic fracture experiment in a tight gas reservoir in Carthage, east Texas. Microseismic events were recorded on two downhole arrays of three-component sensors, the geometry of which provided good ray coverage for anisotropy analysis. A total of 16,633 seismograms from 888 located events yielded 1545 well-constrained shear-wave-splitting measurements. Manual analysis of splitting from a subset of this data set reveals temporal changes in splitting during fracturing. Inversion using the full data set allows the identification of fracture strike and density, which is observed to vary during fracturing. The recovered fracture strike in the rock mass is parallel to directions of regional borehole breakout, but oblique to the hydraulic fracture corridor as mapped by the microseismic event. We relate this to en-echelon fracturing of preexisting cracks. The magnitude of shear-wave splitting shows a clear temporal increase during each pumping stage, indicating the generation of cracks and fissures in a halo around the fracture corridor, which thus increase the overall permeability of the rock mass. Our results show that shear-wave-splitting analysis can provide a useful tool for monitoring spatial and temporal variations in fracture networks generated by hydraulic stimulation.

Site- and ground motion-dependent nonlinear effects in seismological model predictions

We investigate the empirical relationship between site response nonlinearity, soil properties, and ground motion characteristics, as a first step to enable efficient integration of nonlinear analyses in broadband ground motion simulations. For this purpose, we use 24 downhole array sites with detailed geotechnical information and subject the profiles to broadband ground motion synthetics. We quantify the extent of soil nonlinearity in site-specific response analyses by estimating the divergence between linear and nonlinear ground surface predictions. We show that the parameters controlling the nonlinear response are V S 30 (weighted averaged shear wave velocity in the top 30 m of the soil profile), the site amplification at the fundamental frequency (Amp), the peak ground acceleration (PGA), and the frequency index (FI), a quantitative measure we define to characterize how well the incident motion can “see” the near-surface soil layers. Using the synthetic results, we quantitatively describe the error introduced in ground motion predictions when nonlinear effects are not accounted for, and show that the error is both site and ground motion dependent. Our study indicates that to characterize the susceptibility of a soil profile to nonlinear effects, V S 30 should be complemented with measures of the soil–rock impedance contrast, as well as measures of the ground motion intensity and frequency content.

Effective VTI anisotropy for consistent monitoring of microseismic events

The monitoring of induced or triggered microseismic events increasingly is being used to inform the efficient production of unconventional reservoirs. A key aspect of economic production in these low-permeability rocks is hydraulic fracture stimulation, usually in horizontal wells. To evaluate the success of the stimulation, engineers rely on monitoring the induced (or triggered) microseismic events that are then interpreted to map the stimulated reservoir volume and likely drainage area of the well. These microseismic events can be mapped either from downhole or surface monitoring arrays. In this study, we discuss a newly developed methodology that allows economic and consistent mapping of microseismic events from multiple stimulated wells across an entire field. This approach allows better comparison of stimulation techniques between wells in order to optimize long-term development of the reservoir. As well, the method enables a relatively robust observation of velocity anisotropy that leads to better wave-propagation modeling and more accurate event locations

Istanbul geotechnical downhole arrays

Geotechnical downhole arrays that record seismic ground response in near-surface layers are increasingly becoming one of the best sources of information for developing, verifying, and calibrating engineering analyses and design methods. Recently, three geotechnical downhole arrays have been installed in the European side of Istanbul, Turkey with efforts of Kandilli Observatory and Earthquake Research Institute (KOERI). The existing high seismic activity of the region increases the scientific importance of these downhole arrays. Each array is composed of one accelerometer on the ground surface and three or four borehole accelerometers at various depths along the soil profile with the deepest sensor located at the engineering bedrock level (Vs > 750 m/s). The arrays also provide reference bedrock motion for Rapid Response Network composed of 55 surface instruments operated by KOERI in the west European side of city. An important aspect of this paper is to introduce the recently-completed test sites emphasizing on their potential to generate valuable data for scientific community. Preliminary analysis of the recorded data provides evidence for significant site amplification of ground motion by the surface layers. Comparisons between the recorded and modeled response indicate that Vs 30 alone is a poor indicator of the amplification potential emphasizing the importance of site-specific analyses in predicting the ground response as opposed to empirical approaches.

Prediction of strain energy-based liquefaction resistance of sand–silt mixtures: An evolutionary approach

Liquefaction is a catastrophic type of ground failure, which usually occurs in loose saturated soil deposits under earthquake excitations. A new predictive model is presented in this study to estimate the amount of strain energy density, which is required for the liquefaction triggering of sand–silt mixtures. A wide-ranging database containing the results of cyclic tests on sand–silt mixtures was first gathered from previously published studies. Input variables of the model were chosen from the available understandings evolved from the previous studies on the strain energy-based liquefaction potential assessment. In order to avoid overtraining, two sets of validation data were employed and a particular monitoring was made on the behavior of the evolved models. Results of a comprehensive parametric study on the proposed model are in accord with the previously published experimental observations. Accordingly, the amount of strain energy required for liquefaction onset increases with increase in initial effective overburden pressure, relative density, and mean grain size. The effect of nonplastic fines on strain energy-based liquefaction resistance shows a more complicated behavior. Accordingly, liquefaction resistance increases with increase in fines up to about 10–15% and then starts to decline for a higher increase in fines content. Further verifications of the model were carried out using the valuable results of some downhole array data as well as centrifuge model tests. These verifications confirm that the proposed model, which was derived from laboratory data, can be successfully utilized under field conditions.

Evaluation of undrained shear strains in multi-directional horizontal shaking

The evaluation of shear strains under multi-directional shaking is an important issue in interpreting dynamic soil behavior for both laboratory physical modeling and in situ monitoring. Shear strain components evaluated from Cartesian coordinates in undrained conditions have limitations to fully capture the coupled shear strain-pore pressure responses with an individual expression. In the present study, radial and rotational shear strain components derived from particle motions described with cylindrical polar coordinates are proposed. The proposed radial and rotational shear strains are verified with data from a bi-directional laminar shear box and a free field downhole array. Comparison results show that the proposed expressions of shear strain effectively capture the coupled strain-pore pressure responses in terms of the frequency content, amplitude variation, phase difference, and oscillation behavior. Comparison results reveal that the radial shear strain is the dominant shearing mode and the amplitude of the rotational shear strain is only 6.5–14.5% of the radial component. This provides quantitative data for the correction factor for multi-directional shaking and suggests that a simple shear system capable of inducing the radial shear strain on the vertical plane is a better approach than other shearing modes for physically modeling the behavior of soil subjected to undrained seismic loadings.

Down-hole seismometer arrays near Tokyo Bay Area

Down-hole seismometer arrays near Tokyo Bay Area

A Wavelet-based Seismogram Inversion Algorithm for the In Situ Characterization of Nonlinear Soil Behavior

We present a full waveform inversion algorithm of downhole array seismogram recordings that can be used to estimate the inelastic soil behavior in situ during earthquake ground motion. For this purpose, we first develop a new hysteretic scheme that improves upon existing nonlinear site response models by allowing adjustment of the width and length of the hysteresis loop for a relatively small number of soil parameters. The constitutive law is formulated to approximate the response of saturated cohesive materials, and does not account for volumetric changes due to shear leading to pore pressure development and potential liquefaction. We implement the soil model in the forward operator of the inversion, and evaluate the constitutive parameters that maximize the cross-correlation between site response predictions and observations on ground surface. The objective function is defined in the wavelet domain, which allows equal weight to be assigned across all frequency bands of the non-stationary signal. We evaluate the convergence rate and robustness of the proposed scheme for noise-free and noise-contaminated data, and illustrate good performance of the inversion for signal-to-noise ratios as low as 3. We finally employ the proposed scheme to downhole array data, and show that results compare very well with published data on generic soil conditions and previous geotechnical investigation studies at the array site. By assuming a realistic hysteretic model and estimating the constitutive soil parameters, the proposed inversion accounts for the instantaneous adjustment of soil response to the level and strain and load path during transient loading, and allows results to be used in predictions of nonlinear site effects during future events.

Comparison of surface and borehole locations of induced seismicity

ABSTRACTMonitoring of induced microseismic events has become an important tool in hydraulic fracture diagnostics and understanding fractured reservoirs in general. We compare microseismic event and their uncertainties using data sets obtained with surface and downhole arrays of receivers. We first model the uncertainties to understand the effect of different acquisition geometries on location accuracy. For a vertical array of receivers in a single monitoring borehole, we find that the largest part of the final location uncertainty is related to estimation of the backazimuth. This is followed by uncertainty in the vertical position and radial distance from the receivers. For surface monitoring, the largest uncertainty lies in the vertical position due to the use of only a single phase (usually P‐wave) in the estimation of the event location. In surface monitoring results, lateral positions are estimated robustly and are not sensitive to the velocity model.In this case study, we compare event location solutions from two catalogues of microseismic events; one from a downhole array and the second from a surface array of 1C geophone. Our results show that origin time can be reliably used to find matching events between the downhole and surface catalogues. The locations of the corresponding events display a systematic shift consistent with a poorly calibrated velocity model for downhole dataset. For this case study, locations derived from surface monitoring have less scatter in both vertical and horizontal directions.

Site- and Motion-Dependent Parametric Uncertainty of Site-Response Analyses in Earthquake Simulations

We investigate the propagation of uncertainty in site-response analyses from the soil model parameters to the ground surface motion at three downhole array sites in the Los Angeles (LA) Basin. For this purpose, we develop realistic stochastic models of elastic and nonlinear dynamic soil properties using extensive site-specific and generic geotechnical data on the variability of soil properties. We also generate synthetic ground motions using a finite source dynamic rupture model over a wide range of magnitudes and distances and use this statistically significant number of ground motions in the analysis. For each of the three sites, we evaluate the effects of soil parameter uncertainty as a function of the seismic input intensity and frequency content. We show that the frequency range, where the ground-motion variability due to soil parameter uncertainty is maximized, is a function of both the site and the seismogram characteristics. We compare our results with previously published studies and show that different soil models, statistical descriptions of soil parameters, or ground-motion scenarios may yield substantial differences in the estimated site-response scatter. We conclude that the effects of nonlinear soil property uncertainties on the ground-motion variability strongly depend on the seismic motion intensity, and this dependency is more pronounced for soft soil profiles. By contrast, the effects of velocity profile uncertainties are less intensity dependent and more sensitive to the velocity impedance in the near surface that governs the maximum site amplification.

Simulating soil stiffness degradation in transient site response predictions

A series of nonlinear seismic ground response analyses are conducted to investigate the effect of soil degradation on the seismic response of typical sedimentary deposits in Southern California. For this purpose, three downhole array station sites at the Los Angeles basin are subjected to broadband synthetic ground motion excitations. A characteristic strain measure, defined as the averaged maximum shear strain in the top 30 m of the soil profile ( γ 30 ), is proposed to enable the correlation between the ground shaking intensity level (here measured by means of the peak ground acceleration on the surface of rock-outcrop, PGA RO ) and the average deviation in ground motion response spectral predictions (denoted as e SA ) attributed to the effect of modulus degradation due to cyclic loading. It is shown that strong linear correlations exist among log ( PGA RO ) , log ( γ 30 ) and log ( e SA ) for the sites investigated. On the other hand, the number of loading cycles typically employed to indicate the importance of soil degradation is shown to play a much less important role. This becomes more apparent for near-field large amplitude ground motions characterized by a dominant pulse and overall low number of cycles, which nonetheless impose strains large enough to trigger modulus degradation effects. Based on this observation, it is suggested that the soil degradation effect in seismic ground motion is primarily controlled by the ground shaking intensity. The level of PGA RO beyond which soil degradation should be accounted for and incorporated in seismic ground response analysis is also depicted for the sites investigated.