Borehole Recordings Research Articles

Classifying Ground Rippability and Weathering Grades in a Sedimentary Rock Geological Environment Using Seismic Refraction Survey

Introduction An in-depth understanding of the ground subsurface is crucial for foundation design and excavation works and for avoiding potential hazards during land development.In this regard, the ground rippability and weathering grades are some of the ground information needed. While geotechnical works are preferred, their limited horizontal coverage and high cost are often constraints that limit their use. Aims To counter this, a geophysical survey is employed for its wider area coverage and cost-efficiency. Therefore, this study used the seismic refraction method to assess the rippability and weathering grades in a sedimentary rock geological setting (interbedded sandstone, siltstone, and shale) as a preliminary ground assessment. Methods A seismic refraction survey was carried out using Aktiebolaget Elektrisk Malmletning (ABEM) Terraloc Pro 2, where the survey line was 115m long. Rippability was obtained by correlating seismic values with the Caterpillar D10R rippability table. Meanwhile, the weathering grades of the ground were determined by correlating the study area with another study area of a similar geological setting. Results Within the 39m penetration depth, three layers can be classified from the ground’s P-wave velocity values and D10R Caterpillar rippability chart, which include rippable, marginal, and non-rippable layers. A break in the continuous ground layers could be seen, causing lower velocity values to be sandwiched between high velocities, which signified the presence of fracture. The weathering grades were also successfully classified from the seismic velocity values. Conclusion Using seismic refraction method, this study successfully employed seismic velocity values in determining the rippability and weathering grades of interbedded sedimentary rock without borehole record.

Generation of Site-Specific Accelerograms and Response Spectra Involving Sampling Information from Borehole Records

This paper is aimed at serving the needs of structural engineering designers of an important structure (or a group of structures located on the same site) who is seeking guidance on how to obtain accelerograms and/or derive response spectra that accurately represent the site subsoil conditions as informed by the borelogs. The presented site-specific seismic action model may be used to replace the default seismic action model stipulated for the designated site class. Presented in this article is a procedure for generating soil surface motions in an earthquake, and their associated site-specific response spectra, taking into account details of the soil layers. Dynamic site response analyses are involved. The conditional mean spectrum methodology is employed for selecting and scaling accelerograms for obtaining input motion on bedrock. The selection depends on the natural period of both the site and the structure. Multiple borelogs taken from within the same site are analysed to identify the critical soil column models without having to conduct site response analysis on every borelog. The technique for simplifying the soil layers utilising the shear strain profile is introduced to further cut down on the time of analyses. The procedures described in this article have been written into a web-based program that is freely accessible to engineering practitioners.

Horizontal seismic wave at ground surface from transfer function based on ambient noise

Earthquake detection can be improved by ensuring that seismometer sites experience little artificial noise in the surrounding environment. To minimize noise, seismological stations should be positioned in rocky mountainous areas without nearby valleys, away from significant human activity. However, such surface sites may be scarce when constructing dense monitoring networks, necessitating the use of underground sites to ensure low noise levels. The Korean Meteorological Administration is currently installing new underground seismometers to increase seismic monitoring capacity. However, seismic data on the ground surface are also required for engineering technological developments (to reduce damage to structural components). Therefore, borehole seismic stations without surface seismometers need to estimate ground surface motion from borehole record data. We propose a transfer function that converts motion within boreholes to surface seismic waves using ambient noise, thereby facilitating estimation of ground surface motions using borehole seismometers. As a result, predicting ground surface motion from borehole record data becomes possible.

Surface Excavation Assessment in Non-bedded Rock Mass Based on Borehole Information and Seismic Survey

One of the most popular indirect methods for investigating the subsurface information is through borehole record. The information obtained from a bore log will provide the subsurface scenario with regard to the soil and rock mass profile, stiffness of the material, weathering grade and quality of the rock material. It is wise to obtain as much as possible useful information from it for any civil engineering design including its excavatability assessment. On the other hand, seismic velocity is also commonly used to describe subsurface information as it furnish actual profile of the subsurface along the survey line. Seismic survey is assumed to be relevant geophysical method to characterize the boundary of soil-rock. Although the field borehole information and seismic method has been widely applied in the ground investigation, their applicability in assessing excavation performance is still debatable, especially when it involves thick soil-rock interaction zone in tropical region. The objective of this study is to investigate the applicability of ground profile obtained from the field borehole information and the seismic survey in surface excavation works. Field studies were carried out ontwo on-going excavation sites namely Nilai and Kota Tinggi involving non-bedded -bedded rock masses, namely granite. This study aims to present the relationship between seismic refraction method and boreholes were to investigate their effectiveness in assessing the ground information for excavation purpose. A sets of boreholeswere drilled approximately on the same path of seismic linesto obtain relationship between those methods. The seismic survey results are evaluated with Standard Penetration Test (SPT), strength index, core recovery (CR) and Rock Quality Designation (RQD) information. Upon obtaining that information, trial excavation were carried out using different size of excavator machines to determine its productivity rate. This study provides useful information on the excavatability of ground materials by using various type of excavating machines, based on the most commonly used of ground investigation tools, which are boreholes and seismic velocity method.

Spatiotemporal Variability of Earthquake Source Parameters at Parkfield, California, and Their Relationship With the 2004 M6 Earthquake

AbstractEarthquake stress drop is an important source parameter that directly links to strong ground motion and fundamental questions in earthquake physics. Stress drop estimations may contain significant uncertainties due to such factors as variations in material properties and data limitations, which limit the applications of stress drop interpretations. Using a high‐resolution borehole network, we estimate stress drop for 4551 (M0‐4) earthquakes on the San Andreas Fault at Parkfield, California, between 2001 and 2016 using spectral decomposition and an improved stacking method. To evaluate the influence of spatiotemporal variations of material properties on stress drop estimations, we apply different strategies to account for spatial variations of velocity and attenuation changes, and divide earthquakes into three separate time periods to correct temporal variations of attenuation. These results show that appropriate corrections can significantly reduce the scatter in stress drop estimates, and decrease apparent depth and magnitude dependence. We find that insufficient bandwidth can cause systematic underestimation of stress drop estimates and increased scatter. The stress drop measurements from the high‐frequency borehole recordings exhibit complex stable spatial patterns with no clear correlation with the nature of fault slip, or the slip distribution of the 2004 M6 earthquake. Temporal variations are significantly smaller, less well resolved and varying spatially. They do not affect the long‐term stress drop spatial variations, suggesting local material properties may control the spatial heterogeneity of stress drop.

A Local Magnitude Scale from Borehole Recordings with Site Correction of the Surface to Downhole

Abstract The Central Weather Bureau of Taiwan deployed 62 borehole seismic stations equipped with accelerometers and broadband seismometers by the end of 2018. Currently, a large number of earthquake records has been accumulated because of the high seismicity rate of Taiwan. These records can be used to detect microseismicity because the background seismic noise level is lower at downhole stations than surface stations. However, the magnitudes of the waveforms recorded by the downhole stations are lower than those of surface stations because of near-surface site effects. In Taiwan, local magnitude (ML) determinations use an attenuation function derived from surface stations, and ML will be underestimated if this attenuation function is used for downhole stations. Records of 14,653 earthquakes were used to determine site amplifications corresponding to site corrections of the surface to downhole. For all available earthquake–station pairs, a total of 5470 amplification factors satisfied condition of the incident angle less than 35°, which are used for further discussion in this work. The results reveal that amplification factors ranging from 1.11 to 6.07 can be used to describe site effects. These amplification factors are strongly correlated with VS30 (average shear-wave velocity for the top 30 m of strata). The amplification factor was used to revise station ML for downhole accelerometer (denoted MLB), and revised MLB (MLBNEW) was compared with ML from the surface accelerometer (denoted MLS). Results show that MLBNEW and MLS are similar. Using the derived site corrections of surface to downhole, the downhole seismograms can be used to directly determine ML, especially for the borehole broadband waveforms, enhancing detection capabilities for microearthquakes and improving the Taiwanese earthquake catalog.

3D Active Source Seismic Imaging of the Alpine Fault Zone and the Whataroa Glacial Valley in New Zealand

AbstractThe Alpine Fault zone in New Zealand marks a major transpressional plate boundary that is late in its typical earthquake cycle. Understanding the subsurface structures is crucial to understand the tectonic processes taking place. A unique seismic survey including 2D lines, a 3D array, and borehole recordings, has been performed in the Whataroa Valley and provides new insights into the Alpine Fault zone down to ∼2 km depth at the location of the Deep Fault Drilling Project (DFDP)‐2 drill site. Seismic images are obtained by focusing prestack depth migration approaches. Despite the challenging conditions for seismic imaging within a sediment filled glacial valley and steeply dipping valley flanks, several structures related to the valley itself as well as the tectonic fault system are imaged. A set of several reflectors dipping 40°–56° to the southeast are identified in a ∼600 m wide zone that is interpreted to be the minimum extent of the damage zone. Different approaches image one distinct reflector dipping at ∼40°, which is interpreted to be the main Alpine Fault reflector located only ∼100 m beneath the maximum drilled depth of the DFDP‐2B borehole. At shallower depths (z < 0.5 km), additional reflectors are identified as fault segments with generally steeper dips up to 56°. Additionally, a glacially over‐deepened trough with nearly horizontally layered sediments and a major fault (z < 0.5 km) are identified 0.5–1 km south of the DFDP‐2B borehole. Thus, a complex structural environment is seismically imaged and shows the complexity of the Alpine Fault at Whataroa.

Soft soils: A study on their electrical resistivity values and geotechnical properties (porosity, SPT and particle size distribution)

Soft soils pose abundant engineering issues due to its low bearing capacity and shear strength. Comprehensive study on soft soil’s physical properties such as shear strength and ability to store water (porosity) could help in devising the optimum ground improvements and foundations techniques. Therefore, physical properties of soft marine clay in Nibong Tebal were thoroughly studied using 2-Dimensional Resistivity Imaging (2-DRI) method in conjunction with porosity measurements, standard penetration test values (SPT-n) and particle size distribution (PSD) analysis. The 2-DRI profile depicts three lithologies, which are unsaturated topsoil, saturated soft clayey soil and saturated sandy soil in the area. The soft soil extends up to 32 m in thickness where it overlies the sandy layer and could be correlated back to lithology profile from borehole record. Additionally, soil samples were collected at three locations along the survey line for porosity measurements via saturation porosimetry method. The samples demonstrate that the clay layer has a very large porosity range and signifies that the soil will compress tremendously under load. On the other hand, SPT-N values of the soft clay is also very low; thus, could be classed as very soft to soft cohesive soil with very low shear strength as compared to a higher range SPT-n values of the sandy layer. The PSD result also compliments the 2-DRI, porosity and SPT results to show distinct differences between topsoil and the soft clay layer in terms of the presence of fine grains. These results further indicate that the thick upper layer is not capable of bearing immense loads such as high-rise infrastructures due to the soil’s high porosity and low shear strength. Hence, the area must undergo ground remediations prior to any infrastructure developments on the land.

Application of geophysical methods to evaluate soil dynamic properties in Penang Island, Malaysia

Abstract The essential criterion of selecting the site for building foundations is the underlying soil. This vital requirement stems from the associated potential for disaster should the ground condition be unstable. Conventionally, the method of geotechnical drilling is used in soil investigation work. However, this method is limited to providing subsurface information only at one particular point. This research aims to utilise geophysical methods to investigate the soil dynamic properties of various lithology types via application of electrical resistivity method (ERM), seismic refraction (SR), and multichannel analysis of surface wave (MASW) methods. The soil dynamic properties (bulk density, Poisson’s ratio, Young’s modulus, shear modulus, bulk modulus) and material bearing capacity (subgrade reaction coefficient, ultimate and allowable bearing capacity) were computed using a relative formula by using compressional wave (VP) and shear wave velocities (VS). The calculated values of soil dynamic properties and material bearing capacity for the three study areas are capable in determining the competency and stiffness of subsurface layers for building foundation. The results leading to this conclusion can be verified by the values of standard penetration test value (SPT-N) from borehole record. From this study, it is apparent that the soil dynamic properties and material bearing capacity values are essential to determine the competency of soil, in turn aiding in determining the area suitable for the shallow foundation to rest. Thus, geophysical methods are time- and cost-saving, as well as non-intrusive, recommended for site investigation before starting any construction work.

Repeating Earthquakes With Remarkably Repeatable Ruptures on the San Andreas Fault at Parkfield

AbstractWe calculate rupture directivity and velocity for earthquakes in three well‐recorded repeating sequences (2001–2016) on the San Andreas Fault at Parkfield using P waves from borehole recordings and the empirical Green's function method. The individual events in each sequence all show the same directivity; the largest magnitude sequence (M ~ 2.7, 8 events) ruptures unilaterally NW (at ~0.8Vs), the second sequence (M ~ 2.3, 9 events) ruptures unilaterally SE, and the smallest magnitude sequence (M ~ 2, 11 events) is less well resolved. The highly repetitive rupture suggests that geometry or material properties might control nucleation of small locked patches. The source spectra of the M ~ 2.7 sequence exhibit no detectable temporal variation. The smaller M sequences both exhibit a decrease in high‐frequency energy following the M6 earthquake that recovers with time. This could indicate a decrease in stress drop, an increase in attenuation, or a combination of the two, followed by gradual healing.

Ground-Motion Attenuation, Stress Drop, and Directivity of Induced Events in the Groningen Gas Field by Spectral Inversion of Borehole Records

ABSTRACTProduction-induced earthquakes in the Groningen gas field caused damage to buildings and concerns for the population, the gas-field owner, and the local and national authorities and institutions. The largest event (ML=3.6) occurred in 2012 near Huizinge, and, despite the subsequent decision of the Dutch government to reduce the gas production in the following years, similar magnitude events occurred in 2018 and 2019 (ML=3.4). Thanks to the improvement of the local seismic networks in the last years, recent events provide a large number of recordings and an unprecedented opportunity to study the characteristics of induced earthquakes in the Groningen gas field and related ground motions.In this study, we exploit the S-wave Fourier amplitude spectra recorded by the 200 m depth borehole sensors of the G network from 2015 to 2019 to derive source and attenuation parameters for ML≥2 induced earthquakes. The borehole spectra are decomposed into source, attenuation, and site nonparametric functions, and parametric models are then adopted to determine moment magnitudes, corner frequencies, and stress drops of 21 events. Attenuation and source parameters are discussed and compared with previous estimates for the region. The impact of destructive interference of upgoing and downgoing waves at borehole depth on the derived parameters is also discussed and assessed to be minor.The analysis of the apparent source spectra reveals that several events show rupture directivity and provides clear observations of frequency-dependent directivity effects in induced earthquakes. The estimated rupture direction shows a good agreement with orientation of pre-existing faults within the reservoir. Our results confirm that rupture directivity is still an important factor for small-magnitude induced events, affecting the amplitude of recorded short-period response spectra and causing relevant spatial ground-motion variability.

Assessing the Applicability of Deconvolution of Borehole Records for Determining Near‐Surface Shear‐Wave Attenuation

Assessing the Applicability of Deconvolution of Borehole Records for Determining Near‐Surface Shear‐Wave Attenuation

Constructing the Apparent Geological Model by Fusing Surface Resistivity Survey and Borehole Records.

We constructed an apparent geological model with resistivity data from surface resistivity surveys. We developed a data fusion approach by integrating dense electrical resistivity measurements collected with Schlumberger arrays and wellbore logs. This approach includes an optimization algorithm and a geostatistic interpolation method. We first generated an apparent formation factor model from the surface resistivity measurements and groundwater resistivity records with an inverse distance method. We then converted the model into a geology model with the optimized judgment criteria from the algorithms relating the apparent formation factors to the borehole geology. We also employed a non-parametric bootstrap method to analyze the uncertainty of the predicted sediment types, and the predictive uncertainties of clay, gravel, and sand were less than 5%. Overall, our model is capable of capturing the spatial features of the sediment types. More importantly, this approach can be arranged in a self-updated sequence to enable adjustments to the model to accommodate newly collected core records or geophysical data. This approach yields a more detailed apparent geological model for use in future groundwater simulations, which is of benefit to multi-discipline studies.

2-D Electrical Resistivity Tomography (ERT) Assessment of Ground Failure in Urban Area

This study was carried out to assess the foundation defects around an urban area in Selangor, Malaysia using 2-D electrical resistivity tomography (ERT). The affected structure is a three storey houses and having severe foundation-based cracks. Six 2-D ERT survey lines with 5 m minimum electrode spacing using Pole-dipole array were executed parallel to building’s wall. Four boreholes were conducted to identify the depth to competent layer to verify the 2-D ERT results. Inversion model of 2-D resistivity show that the study area consists of two main zones. The first zone is a low resistivity value (<100 Ωm), which appears to be a zone that is fully saturated with sandy silt and this could be an influence factor the increasing water level because sandy silt is highly permeable in nature and alluvium (silt, sand and clay), boulder (1200-3500 Ωm) or highly weathered with the resistivity values of 100-1000 Ωm at 20-70 m depth. The second zone is the granite bedrock of more than 3500 Ωm with depth greater than 70 m. These results were complimented and confirmed by borehole records. The ERT and borehole record suggest that the clay, sand, saturated zone, highly weathered zone and boulders at foundation depths may lead to ground movements which affected the stability of the building.

Correlation of Geophysical and Geotechnical Methods for Sediment Mapping in Sungai Batu, Kedah

Exploration geophysics is widely used to map the subsurface characteristics of a region, to understand the underlying rock structures and spatial distribution of rock units. 2-D resistivity and seismic refraction methods were conducted in Sungai Batu locality with objective to identify and map the sediment deposit with correlation of borehole record. 2-D resistivity data was acquire using ABEM SAS4000 system with Pole-dipole array and 2.5 m minimum electrode spacing while for seismic refraction ABEM MK8 seismograph was used to record the seismic data and 5 kg sledgehammer used as a seismic source with geophones interval of 5 m spacing. The inversion model of 2-D resistivity result shows that, the resistivity values <100 Ωm was interpreted as saturated zone with while high resistivity values >500 Ωm as the hard layer for this study area. The seismic result indicates that the velocity values <2000 m/s represent as the highly-weathered soil consists of clay and sand while high velocity values >3600 m/s interpreted as the hard layer in this locality.

Identification of Sungai Batu Sediment using 2-D Resistivity Imaging and Seismic Refraction Methods for Ancient River Mapping at Kedah, Malaysia

Sungai Batu is an earliest civilization in Southeast Asia with evidenced by the discovery of riverside jetty, iron smelting, and ritual monuments. The evidences can lead to prediction of buried river caused by geological and sedimentation process. This study was conducted to study sediment deposit characteristic and to map thickness of the sediments using 2-D resistivity imaging and seismic refraction for ancient river mapping. A total of thirty, 2-D resistivity and nine seismic survey lines were conducted at the study area. Four of the lines R1-R4 and S1-S4 were correlated and validated with existing on site boreholes BH1-BH4 to identify sediment type and thickness. The validated values applied to the remaining survey lines which no borehole record to map the subsurface of the study area. Based on the results, Sungai Batu area consist of clay with resistivity value of 6.6-25.9 Ω.m and velocity value of 716.9-1606.9 m/s; sandy clay with resistivity value of 6-265.1 Ω.m and velocity value of 1003.6-1901.4 m/s; while shale was identified with resistivity value of >668.6 Ω.m and velocity value of >2051.7 m/s. Boundary between clay/sandy clay with shale was identified with resistivity value of 314 Ω.m and velocity value of 1822 m/s. The integration of the 2-D resistivity and seismic refraction identified that the thickness of Sungai Batu sediment is 0-150 m and Sungai Batu ancient river was successfully map based on thickness of sediment which is >45 m.

Extreme aridification since the beginning of the Pliocene in the Tarim Basin, western China

Mid-latitude Central Asia is characterized by an extreme arid landscape. Among the Central Asian deserts, the Taklimakan Desert is the largest shifting sand desert which is located in the rain shadow of the Tibetan Plateau and of the other central Asian high mountains. The formation of this desert is important for placing widespread aridification into a regional tectonic context at the western end of the Himalayan-Tibetan orogen. However, there still exists considerable controversy regarding the timing of desert formation, thus impeding our understanding of the climatic effects of the Tibetan uplift and regional environmental changes. Here we report new biostratigraphic age control and multiple high-resolution climatic records from the center of the Taklimakan Desert. Our results reveal dramatic environmental changes at ~5Ma, suggesting that an extremely dry climate has prevailed since the beginning of the Pliocene, which is consistent with findings from a high-resolution borehole record about 670km to the east in the same basin. The two records combined demonstrate that an extremely dry environment prevailed in the entire Tarim Basin from approximately 5Ma. This was related to the reduced transport of water vapor by westerlies in response to the retreat of the Paratethys Ocean driven by global climatic cooling, and the closed oceanic water-vapor pathway between the Pamir and the Tian Shan ranges driven by the ongoing India-Eurasia collision.

3D numerical simulation and ground motion prediction: Verification, validation and beyond – Lessons from the E2VP project

The Euroseistest Verification and Validation Project (E2VP) is part of a series of complementary benchmarking exercises launched to better assess the ability of numerical simulation to accurately predict seismic ground motion. E2VP targeted more specifically the current, most-advanced numerical methods applied to realistic 3D, linear models of sedimentary basins through a quantitative comparison of the recorded and numerically-simulated ground motions. The target site, located within the Mygdonian basin near Thessaloniki, Greece, has been thoroughly investigated for two decades and a detailed, realistic 3D model has been derived from geological, geophysical and geotechnical investigations, while a dedicated instrumentation provided a significant number of surface and borehole recordings. Verification and validation tests up to a frequency of 4Hz, much beyond the 0.4Hz fundamental frequency of the deepest part of the graben, have been performed for a set of 19 local, small to moderate magnitude events. For careful and accurate enough computations, the model-to-model differences are smaller than the model-to-observations differences, the latter being controlled by uncertainties primarily in the crustal propagation model and source properties, and secondarily in the shallow structure. It is therefore recommended to prefer distant and/or deep events (R>10–20km, Z>8–10km) for validation exercises. Additional sensitivity tests illustrate the ability of carefully verified numerical simulation tools to provide an instructive insight at the structure of the so-called “aleatory” variability of ground motion, for both its within- and between-event components. The between-event variability is shown to be very sensitive to hypocenter location errors (even as low as ±2km), and to uncertainty in magnitude estimates. It explains the increase of aleatory variability for small magnitude events and emphasizes the usefulness of dense seismological networks. The within event, single-site variability is shown to be associated to an “epistemic” dependence of the 3D site response on the event back-azimuth, distance and depth, and calls for caution when interpreting single-station variabilities derived from a too small number of events.

Shallow velocity model in the area of Pozzo Pitarrone, Mt. Etna, from single station, array methods and borehole data

&lt;p&gt;Seismic noise recorded by a temporary array installed around Pozzo Pitarrone, NE flank of Mt. Etna, have been analysed with several techniques. Single station HVSR method and SPAC array method have been applied to stationary seismic noise to investigate the local shallow structure. The inversion of dispersion curves produced a shear wave velocity model of the area reliable down to depth of about 130 m. A comparison of such model with the stratigraphic information available for the investigated area shows a good qualitative agreement. Taking advantage of a borehole station installed at 130 m depth, we could estimate also the P-wave velocity by comparing the borehole recordings of local earthquakes with the same event recorded at surface. Further insight on the P-wave velocity in the upper 130 m layer comes from the surface reflected wave observable in some cases at the borehole station. From this analysis we obtained an average P-wave velocity of about 1.2 km/s, compatible with the shear wave velocity found from the analysis of seismic noise.&lt;/p&gt;

Site response analyses using downhole arrays at various seismic hazard levels of Singapore

Local site conditions can significantly influence the characteristics of seismic ground motions. In this study, site response analyses using one-dimensional linear elastic (LE), equivalent-linear (EQL) and nonlinear (NL) approaches are performed at different seismic hazard levels of Singapore. Two seismic stations, namely, the KAP and BES stations located at soft soil sites, are selected from the national network of Singapore. Firstly, site response estimates using the LE, EQL (SHAKE04) and NL (DEEPSOIL) approaches are compared with the borehole recordings. Results show favorable matches between the predictions and the observations at the KAP site, while under-predictions are observed for all the three site effect approaches at the BES site. Secondly, the applicability of the LE, EQL and NL models is examined at different hazard levels of Singapore. It is found that for the hazard level at a return period of 475 years, the computed maximum strain (γmax) is 0.06% and then the EQL model can provide accurate site response predictions. However, for the hazard level at a return period of 2475 years, the calculated γmax is larger than 2%, resulting in notable differences in the predictions of different site response models. This study highlights the importance of site effects in seismic hazard analysis of Singapore.