Borehole Observations Research Articles

Microseismic Event Location Using Reverse‐Time Focusing and Interferometric Techniques for Surface and Borehole Observations

AbstractExploration and exploitation of unconventional reservoirs has become a new central issue of international hydrocarbon industry. For the unconventional reservoir has the characteristic of low‐permeability, multiple stages of hydraulic fracturing are usually required to create connected cracks through which hydrocarbons can flow. We can characterize the crack attributes (i.e., trending direction, density, etc.), evaluate the effect of hydrofracturing, analyze the rupture pattern, and infer the source mechanisms by locating and analyzing the induced microseismic events. Microseismic event locating can be achieved by using reverse‐time focusing technique. But many factors such as high noise, velocity errors, and sparsely‐sampled data will cause remarkable location errors. In this study, we combine the reverse‐time focusing technique with interferometric imaging, and discuss the multi‐sources locating method using multi‐component data acquired on the earth surface, in wells and jointly on the both conditions. Synthetic examples and case study reveal the advantage of suppressing artifacts and increasing accuracy. At the same time we point out the constraint factors of the reverse‐time focusing method in practical application, and provide some tactics.

A statistical assessment of the uncertainty in a 3-D geological framework model

Three-dimensional framework models are the state of the art to present geologists’ understanding of a region in a form that can be used to support planning and decision making. However, there is little information on the uncertainty of such framework models. This paper reports an experiment in which five geologists each produced a framework model of a single region in the east of England. Each modeller was provided with a unique set of borehole observations from which to make their model. Each set was made by withholding five unique validation boreholes from the set of all available boreholes. The models could then be compared with the validation observations. There was no significant between-modeller source of variation in framework model error. There was no evidence of systematic bias in the modelled depth for any unit, and a statistically significant but small tendency for the mean error to increase with depth below the surface. The confidence interval for the predicted height of a surface at a point ranged from ±5.6 m to ±6.4 m. There was some evidence that the variance of the model error increased with depth, but no evidence that it differed between modellers or varied with the number of close-neighbouring boreholes or distance to the outcrop. These results are specific to the area that has been modelled, with relatively simple geology, and reflect the relatively dense set of boreholes available for modelling. The method should be applied under a range of conditions to derive more general conclusions.

Past climate changes and permafrost depth at the Lake El'gygytgyn site: implications from data and thermal modeling

Abstract. This study focuses on the temperature field observed in boreholes drilled as part of interdisciplinary scientific campaign targeting the El'gygytgyn Crater Lake in NE Russia. Temperature data are available from two sites: the lake borehole 5011-1 located near the center of the lake reaching 400 m depth, and the land borehole 5011-3 at the rim of the lake, with a depth of 140 m. Constraints on permafrost depth and past climate changes are derived from numerical simulation of the thermal regime associated with the lake-related talik structure. The thermal properties of the subsurface needed for these simulations are based on laboratory measurements of representative cores from the quaternary sediments and the underlying impact-affected rock, complemented by further information from geophysical logs and data from published literature. The temperature observations in the lake borehole 5011-1 are dominated by thermal perturbations related to the drilling process, and thus only give reliable values for the lowermost value in the borehole. Undisturbed temperature data recorded over more than two years are available in the 140 m deep land-based borehole 5011-3. The analysis of these observations allows determination of not only the recent mean annual ground surface temperature, but also the ground surface temperature history, though with large uncertainties. Although the depth of this borehole is by far too insufficient for a complete reconstruction of past temperatures back to the Last Glacial Maximum, it still affects the thermal regime, and thus permafrost depth. This effect is constrained by numerical modeling: assuming that the lake borehole observations are hardly influenced by the past changes in surface air temperature, an estimate of steady-state conditions is possible, leading to a meaningful value of 14 ± 5 K for the post-glacial warming. The strong curvature of the temperature data in shallower depths around 60 m can be explained by a comparatively large amplitude of the Little Ice Age (up to 4 K), with low temperatures prevailing far into the 20th century. Other mechanisms, like varying porosity, may also have an influence on the temperature profile, however, our modeling studies imply a major contribution from recent climate changes.

Compressional and EM wave velocity anisotropy in a temperate glacier due to basal crevasses, and implications for water content estimation

AbstractWe have conducted a series of experiments designed to investigate elastic and electromagnetic (EM) velocity anisotropy associated with a preferentially aligned fracture system on a temperate valley glacier in south-central Alaska, USA. Measurements include a three-dimensional compressional wave (P-wave) seismic reflection survey conducted over a 300 m x 300 m survey patch, with uniform source grid and static checkerboard receiver pattern. Additionally, we acquired a multi-azimuth, multi-offset, polarimetric ground-penetrating radar (GPR) reflection experiment in a wagon-wheel geometry with 94° of azimuthal coverage. Results show azimuthal variation in the P-wave normal-moveout velocity of >3% (3765 and 3630 m s–1 in the fast and slow directions respectively) and difference of nearly 5% between the fast (0.164 m ns–1) and slow (0.156 m ns–1) EM velocities. Fracture orientations estimated from the GPR and seismic velocity data are consistent and indicate a preferred fracture orientation that is 30-45° oblique to glacier flow; these measurements agree with borehole observations. Anisotropic analysis of the polarimetric data gives a single volumetric water content estimate of 0.73 ±0.11%. We conclude that meaningful estimates of physical properties in glaciers based on EM or seismic velocity measurements require collecting data such that the presence of anisotropy can be evaluated and an anisotropic analysis employed when necessary.

Optimizing CSG development: Quantitative estimation of lithological and geomechanical reservoir quality parameters from seismic data

SUMMARY Work over the last decade on seismic azimuthal anisotropy has identified a link between fracture density and orientation observed by well logs and the intensity and orientation of the observed anisotropy. Recent work has correlated these measurements to provide quantitative estimates of fracture density from 3D wide-azimuth seismic data for tight gas sands. The work highlights the impact of advanced seismic processing in successfully recovering reliable fracture estimates which correlate well with borehole observations. These kind of areal, quantitative estimates of fracture density provide a valuable tool to guide drilling and completion programs in tight reservoirs. Building upon this work and considering Coal Seam Gas plays in particular we need to consider some additional reservoir quality parameters whilst trying to impose the same quantitative approach on the interpretation of seismic data and correlation with borehole logging observations. The characterization of CSG plays involves the understanding of the reservoir matrix properties as well as the in-situ stresses and fracturing that will determine optimal producing zones. Pre-stack seismic data and azimuthal WAZ (wide azimuth) seismic processing can help in the identification of sweet spots in CSG resource plays through detailed reservoir-oriented gather conditioning followed by prestack seismic inversion and multi-attribute analysis. This analysis provides rock property estimates such Poisson’s ratio, and Young’s modulus, amongst others. These properties are in turn related to quantitative reservoir properties such as porosity and brittleness. In this presentation we show an integrated approach based on pre-stack azimuthal seismic data analysis and well log information to identify sweet spots, estimate geomechanical properties and in situ principal stresses.

Interpretation of magnetic gradient tensor for automatic locating a dipole source

In this paper, I propose the algorithm that the location of a magnetic dipole can be detected from the magnetic gradient tensor. I derive the location vector of a vertically magnetizated dipole from magnetic gradient tensor. Deficit of magnetic moment of magnetic dipole makes the induced location information incomplete. However if the observation of magnetic gradient tensor would be collected on one more points, the algorithm is able to point the location of magnetic dipole by clustering the solution of the proposed method. For example, I show that magnetic gradient tensor can be converted as the source location successively by picking common solution area in synthetic case of borehole observation.

Effective porosity of a carbonate aquifer with bacterial contamination: Walkerton, Ontario, Canada

Summary Preferential flow through solutionally enlarged fractures can be a significant influence on travel times and source area definition in carbonate aquifers. However, it has proven challenging to step beyond a conceptual model to implementing, parameterizing and testing an appropriate numerical model of preferential flow. Here both porous medium and preferential flow models are developed with respect to a deadly contamination of the municipal groundwater supply at Walkerton, Ontario, Canada. The preferential flow model is based on simple orthogonal fracture aperture and spacing. The models are parameterized from borehole, gamma, flow and video logs resulting in a two order of magnitude lower effective porosity for the preferential flow model. The observed hydraulic conductivity and effective porosity are used to predict groundwater travel times using a porous medium model. These model predictions are compared to a number of independent estimates of effective porosity, including three forced gradient tracer tests. The results show that the effective porosity and hydraulic conductivity values closely match the preferential flow predictions for an equivalent fracture network of ∼10 m spacing of 1 mm fractures. Three tracer tests resulted in groundwater velocities of hundreds of meters per day, as predicted when an effective porosity of 0.05% was used in the groundwater model. These velocities are consistent with a compilation of 185 tracer test velocities from regional Paleozoic carbonate aquifers. The implication is that carbonate aquifers in southern Ontario are characterized by relatively low-volume dissolutionally-enlarged fracture networks that dominate flow and transport. The porous matrix has large storage capacity, but contributes little to transport. Numerical models based on much higher porosities risk significantly underestimating capture zones in such aquifers. The hydraulic conductivity – effective porosity prediction framework provides a general analytical framework for a preferential flow carbonate aquifer. Not only is the framework readily parameterized from borehole observations, but also it can be implemented in a conventional porous medium model, and critically tested using simple tracer tests.

Identification of permeable zones based on the borehole observations of seismoacoustic emissions and helium concentrations

Measurements of seismoacoustic emissions in boreholes allow researchers to identify fault zones and zones of cracking and rock shattering. These zones are usually identified from the maximal values of seismoacoustic emission. However, the permeability of the identified zones cannot be estimated solely from the seismoacoustic emission data, especially in the case when seismoacoustic emission sources are located in isolation in the vicinity of a borehole. The experiments have shown that dynamically active permeable zones of tectonic failures can be identified based on joint measurements of seismoacoustic emissions and helium concentrations in fluid.

Analysis on control technology and coordination deformation mechanism of rise entry group with high ground stress

Based on engineering practice of Wuyang coal mine, we have done X-ray diffract researches to No. 3 coals and its roof and floor rocks by XRD meter, and simulated the interactive effect of surrounding rock deformation by FLAC5.0 numerical simulation software under the condition of different tunneling method of multimine roadway in parallel. The internal structure of surrounding rock of 76 belt roadway was being observed by borehole observation instruments; and then, we has analyzed the reason of failure and deformation of surrounding rocks of several rise entry, and has proposed the technical measures for controlling interactive effect of several rise entry surrounding rock deformation at last. For the thickness seam rise roadway, two conclusions were drawn: one is that the co-deformation among roadway group mainly reflects on that both shear failure and deformation in coal pillar among roadways has decreased the width of pillar core region and clamping action of coal pillar to roof strata, and has increased the actual span of roof strata, and has intensified the flexural failure of roof strata, and has prized the bed separation of roof deep rock strata; the other is that the control effects of interactive deformation among roadways is obvious when appropriate readjusting construction sequence in tunneling of multimine parallel roadways because the construction sequence among roadways also has greater effects on deformation of the surrounding rock in roadway.

Applicability of the multiple yield model for estimating the deformation of vertical rock walls during large-scale excavations

This paper describes the validity of the multiple yield model (MYM) based on a comparison between predictions by MYM analysis and in-situ measurements of two large-scale vertical excavations, about 30m in depth and 100m in width, for nuclear power plants. MYM is a finite element method for modeling the mechanical properties of intact rock and joint systems in rock mass. The method can analyze the non-linearity of deformation under loading and unloading stress paths. The analysis was conducted by determining the geometrical model of rock mass from test adit and borehole observations of discontinuities in conditions such as orientation, spacing, and persistence. The physical parameters of intact rock and discontinuities were determined by laboratory tests using sample specimens, taking the scale effect into account. The deformation mode and displacement value determined by the MYM analysis both corresponded well with in-situ measurements. We have confirmed that MYM can estimate the actual behavior of discontinuous rocks with adequate accuracy for practical application.

A shallow geothermal experiment in a sandy aquifer monitored using electric resistivity tomography

Groundwater resources are increasingly used around the world for geothermal exploitation systems. To monitor such systems and to estimate their governing parameters, we rely mainly on borehole observations of the temperature field at a few locations. Bulk electric resistivity variations can bring important information on temperature changes in aquifers. We have used surface electric resistivity tomography to monitor spatially temperature variations in a sandy aquifer during a thermal injection test. Heated water (48°C) was injected for 70 hours at the rate of [Formula: see text] in a 10.5°C aquifer. Temperature changes derived from time-lapse electric images were in agreement with laboratory water electric conductivity-temperature measurements. In parallel, a coupled hydrogeologic saturated flow and heat transport model was calibrated on geophysical data for the conceptual model, and on hydrogeologic and temperature data for the parameters. The resistivity images showed an upper flow of heated water along the well above the injection screens and led to a new conceptualization of the hydrogeologic source term. The comparison between the temperature models derived from resistivity images and from the simulations was satisfactory. Quantitatively, resistivity changes allowed estimating temperature changes within the aquifer, and qualitatively, the heated plume evolution was successfully monitored. This work demonstrates the ability of electric resistivity tomography to study heat and storage experiments in shallow aquifers. These results could potentially lead to a number of practical applications, such as the monitoring or the design of shallow geothermal systems.

자력 변화율 텐서를 이용한 자기 쌍극자 위치 결정

이 논문에서는 자력 변화율 텐서를 이용하여 자기 쌍극자의 위치 정보를 파악하는 알고리즘에 대하여 기술하였다. 수직으로 자화된 자기 쌍극자에 의한 자력 변화율 텐서에서 출발하여 자기 쌍극자의 위치 벡터를 유도하였다. 그러나 이 경우 자기 쌍극자의 모멘트에 대한 정보가 주어지지 않았으므로 자기 쌍극자의 위치 벡터가 불완전하게 유도된다. 이를 극복하기 위하여 여러 측정점에서 측정된 자력 변화율 텐서값이 있다고 가정하고 이를 이용하여 자동으로 자기 쌍극자의 위치를 찾아내는 알고리즘을 제안하였다. 시추공에서 자력 변화율 텐서가 측정되었다고 가정한 합성 모델 실험에서 자력 변화율 텐서와 자기 쌍극자 자동 탐지 알고리즘을 이용하여 자기 쌍극자의 위치를 정확하게 찾을 수 있음을 확인하였다. In this paper, I propose the algorithm that the location of a magnetic dipole can be detected from the magnetic gradient tensor. I induce the location vector of a vertically magnetizated dipole from the magnetic gradient tensor. Deficit of magnetic moment of magnetic dipole makes the induced location information incomplete. However, if the observation of magnetic gradient tensor would be collected on more points, the algorithm is able to catch the location of the magnetic dipole by clustering the solution of the proposed algorithm. For example, I show that the synthetic case of borehole observation of magnetic gradient tensor can find the source location successively by picking common solution area.

Hydrological modelling of drained blanket peatland

Open ditch drainage is a commonly implemented land management practice in upland blanket peatlands, particularly in the UK, where policy decisions between the 1940s and 1970s led to widespread drainage of the uplands. The change in the hydrological regime associated with the drainage of blanket peat is poorly understood, yet has perceived importance for flooding, low flows and water quality. We propose a new simplified physics-based model that allows the associated hydrological processes and flow responses to be explored. The model couples four one-dimensional models to represent a three-dimensional hillslope, allowing for the exploration of flow and water table response throughout the model domain for a range of drainage configurations and peat properties. The model is tested against a data set collected from Oughtershaw Beck, UK, with results showing good model performance for wet periods although less conformity with borehole observations during rewetting periods. A wider exploration of model behaviour indicates that the model is consistent with the hydrological response reported in the literature for a number of drained blanket peat sites, and therefore has potential to provide guidance to decision makers concerning the effects of management practices. Through a global sensitivity analysis, we conclude that further field investigations to assist in the surface and drain roughness parameterisation would help reduce the uncertainty in the model predictions.

Challenges in Understanding the Hydrologic Controls on the Mobility of Slow‐Moving Landslides

Slow‐moving landslides are a wide‐spread type of active mass movement, can cause severe damages to infrastructure, and may be a precursor of sudden catastrophic slope failures. Pore‐water pressure is commonly regarded as the most important among a number of possible factors controlling landslide velocity. We used high‐resolution monitoring data to explore the relations of landslide mobility and hydrologic processes at the Heumöser landslide in Austria, which is characterized by continuous slow movement along a shear zone. Movement rates showed a seasonality that was associated with elevated pore‐water pressures. Pore pressure monitoring revealed a system of confined and separated aquifers with differing dynamics. Analysis of a simple infinite slope mobility model showed that small variations in parameters, along with measured pore pressure dynamics, provided a perfect match to our observations. Modeling showed a stabilizing effect of snow cover due to the additional load. This finding was supported by a multiple regression model, which further suggested that effective pore pressures at the slip surface were partially differing from the borehole observations and were related to preferential infiltration and subsurface flow in adjacent areas. It appears that in a setting like the Heumöser landslide, hydrologic processes delicately influence slope mobility through their control on pore pressure dynamics and the weight of the landslide body, which challenges observation and modeling. Moreover, it appears that their simplicity, and especially their high sensitivity to parameter variations, limits the conclusions that can be drawn from infinite slope models.

Low strength of deep San Andreas fault gouge from SAFOD core

The San Andreas fault accommodates 28-34 mm yr(-1) of right lateral motion of the Pacific crustal plate northwestward past the North American plate. In California, the fault is composed of two distinct locked segments that have produced great earthquakes in historical times, separated by a 150-km-long creeping zone. The San Andreas Fault Observatory at Depth (SAFOD) is a scientific borehole located northwest of Parkfield, California, near the southern end of the creeping zone. Core was recovered from across the actively deforming San Andreas fault at a vertical depth of 2.7 km (ref. 1). Here we report laboratory strength measurements of these fault core materials at in situ conditions, demonstrating that at this locality and this depth the San Andreas fault is profoundly weak (coefficient of friction, 0.15) owing to the presence of the smectite clay mineral saponite, which is one of the weakest phyllosilicates known. This Mg-rich clay is the low-temperature product of metasomatic reactions between the quartzofeldspathic wall rocks and serpentinite blocks in the fault. These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms. The combination of these measurements of fault core strength with borehole observations yields a self-consistent picture of the stress state of the San Andreas fault at the SAFOD site, in which the fault is intrinsically weak in an otherwise strong crust.

Using apparent soil electrical conductivity (ECa) to characterize vineyard soils of high clay content

The adoption of precision viticulture requires a detailed knowledge of variation in soil chemical, physical and profile properties. This study evaluates the usefulness of apparent electrical conductivity (ECa) data within a GIS framework to identify variations in soil chemical and physical properties and moisture content. The work was conducted in a vineyard located in the Carneros Region (Napa Valley, California). The soil was sampled using 44 boreholes to quantify chemical and physical characteristics and 9 open pits to verify the borehole observations. Moisture content was determined using time domain reflectometry (TDR). To characterize soil ECa, three campaigns were undertaken using a soil electrical conductivity meter (EM38). Linear regressions between soil ECa and soil properties were determined. Boreholes and TDR data were interpolated by kriging to characterize the spatial distribution of soil variables. The resulting maps were compared to the results obtained using the best ECa linear regressions. Using ECa measurements, soil properties like extractable Na+ and Mg2+, clay and sand content were well estimated, while best estimates were obtained for extractable Na+ (r 2 = 0.770) and clay content (r 2 = 0.621). The best estimates for soil moisture content corresponded to moisture in the deeper soil horizons (r 2 = 0.449). The methods described above provided maps of soil properties estimated by ECa in a GIS framework, and could save time and resources during vineyard establishment and management.

물리탐사 자료의 해상도 향상을 위한 지구통계학적 다운스케일링

Inversion result of geophysical data given as a block type was geostatistically simulated with borehole observation given as a point type and was applied to the rock classifying map. The geophysical data generally involved secondary information for the target material and were obtained for overall region. In contrast, borehole data provided direct information for the target material, but tended to be effective only for a narrow range of region and were dealt as a point type. Integrated simulation or kriging interpolation of these two different kinds of information required the covariance for point-point, point-block and block-block. Using the Bssim module included in SGeMS software, integrated result of geophysical data and borehole data were obtained. The results were then compared with the method of geostatistical inversion proposed by authors. Downscaling method used in this study showed relatively more flexible than the geostatistical inversion.

Using integrated near‐surface geophysical surveys to aid mapping and interpretation of geology in an alluvial landscape within a 3D soil‐geology framework

ABSTRACTAn integrated geological, geophysical and remote sensing survey was undertaken as part of the construction of a high‐resolution 3D model of the shallow subsurface geology of part of the Trent Valley in Nottinghamshire, UK. The 3D model was created using the GSI3D software package and the geophysical techniques used included ground‐penetrating radar (GPR), electrical resistivity tomography (ERT) and automated resistivity profiling. In addition, the remote sensing techniques of light detection and ranging (LIDAR) and airborne thematic mapping (ATM) were used. The objective of the study was to assess the contribution of these techniques to improve the geological mapping and interpretation of terrace deposits and other geological features. The study site had an area of ~2 km2 and consisted of a Triassic mudstone escarpment, overlain first by a sand and gravel river terrace that extended to the modern floodplain of the River Trent. Automated resistivity profiling mapping proved to be the central tool in identifying and positioning geological features at a greater resolution than would be obtained through traditional geological mapping and borehole observation. These features included i) a buried cliff delineating the south‐eastern limits of the incised Trent valley, ii) siltstone beds within the Gunthorpe Member of the Mercia Mudstone Group and iii) the variability of the sediments within the river terrace. A long ERT transect across the site successfully imaged the buried cliff and outcropping siltstone beds on the escarpment. Combined ERT and GPR transects revealed the depth of the sand and gravel deposits (Holme Pierrepont sands and gravels), whilst the GPR provided information about the depositional environment. Remote sensing using light detection and ranging proved essential in the original geological survey because it confirmed the absence of a second river terrace that had been previously thought to exist. This case study demonstrates the importance of combining geophysical techniques with traditional geological survey and borehole analysis, in order to create high‐resolution 3D geological models, which are increasingly being used as a platform to understand and solve environmental problems.

Spatial Variations in Fault-Zone Structure along the Nojima Fault, Central Japan, as Inferred from Borehole Observations of Fault-Zone Trapped Waves

Abstract We investigated the fault-zone structures (fault-zone width, shear-wave velocity, and Q s) of the Nojima fault, Japan, using the Love-wave-type fault-zone trapped waves (LTWs) recorded at two borehole stations—TOS2 and HRB—located along the fault. TOS2 (depth: 1673 m) is located at an end of the fault, while HRB (depth: 600 or 720 m) is located in the middle section of the fault where the greatest surface displacement of 2 m was recorded during the 1995 Kobe earthquake. The distance between the two stations is about 4 km. We found 22 records that exhibit typical LTW. We assumed the fault-zone structure to be a 2D uniform low-velocity wave guide and estimated the averaged fault-zone structure from the hypocenter to receiver by modeling the LTWs. Because we can infer the low-velocity fault zone to a depth of 8 km by observing the duration of LTWs, we obtain the average fault-zone structure from the borehole stations to a depth of approximately 8 km. The width, shear-wave velocity, and Q s of the fault zone beneath HRB are 100 m, 2.9 km/sec, and 60, respectively. On the other hand, the average width of the fault zone beneath TOS2 is 220 m, which is larger than that beneath HRB. We also conducted 3D finite-difference modeling of LTWs to confirm the spatial variations in the fault-zone structure to a depth of 8 km. The numerical simulation suggests that the Nojima fault has uniform elastic and attenuation properties along the fault zone to a depth of 8 km, while the width of the fault zone increases with the distance from the HRB to the south.

Borehole observations of fluid flow from South Chamorro Seamount, an active serpentinite mud volcano in the Mariana forearc

A sealed borehole observatory (CORK) was deployed on South Chamorro Seamount, an active serpentinite mud volcano in the Mariana forearc to explore subduction-related processes on a non-accretionary, convergent plate margin. Formation fluid was overpressured relative to ambient hydrostatic conditions. Fluid flowed from the borehole at ~ 0.2 L/s when the observatory was opened to recover instruments 2 yr after it was installed. The chemical composition of the formation fluid is similar to that extrapolated from trends in pore water data collected during Ocean Drilling Program Leg 195 when the observatory was established. Reduced sulfur is present in this highly-alkaline (pH 12.4) formation fluid, indicative of microbial activity at or below the depth of the screened casing, 149–202 m below the seafloor. Discharge from the open borehole continued for 37 days, until the observatory was resealed. This discharge requires significant permeability at depth (> 6 × 10 − 14 m 2). Zones of high permeability may be associated with the formation of headwall scarps, consistent with numerous slumps on the southeastern flank of the seamount, and likely shape a geochemical environment suitable for an active microbial community.