Bottom Of Borehole Research Articles

Improved Interpretation of Hvorslev Tests

The Hvorslev method constitutes a rapid means of characterizing the hydraulic conductivity of shallow ground‐water systems. Because the method is widely used, it is of practical benefit to know the limits and accuracy of the method. An important aspect of the Hvorslev analysis is that the role of specific storage (Ss) is completely ignored. Results of this study show that the Hvorslev method provides acceptable estimates of hydraulic conductivity in the case of zero‐penetration boreholes in which water enters only from the borehole bottom. In this case, relatively large errors arise due to uncertainty in the intake geometry (planate or hemispherical intake) than those arising from neglecting Ss. Results presented show that in screened boreholes, in which radial and spherical flow symmetries coexist, Ss cannot be reasonably neglected. Due to the influence of Ss, the pressure transient data deviates significantly from the semilogarithmic straight line intrinsic to the Hvorslev idealization, with the deviati...

応力解放に伴う孔底面上の非弾性ひずみと初期応力との関係について

It has been observed in many cases that anelastic strain change on hemisphericalborehole bottom follows instantaneous elastic strain relief by overcoring and it lasts a couple of days depending on in-situ stress state rock type. Three dimensional principal strain orientations obtained from the time-dependent strains agreed with the one from the instantaneous elastic relief strains. In-situ measurement of both instantaneous and dependent strains on conical borehole bottom have indicated that the time-dependent maximum expansion core reflects the maximum stress state when the core was retrieved. The time-dependent strain variations, therefore, can not always be used to provide a reliable estimate of in-situ stress when the principal stress orientation at the poisson expansion behavior dose not agree with in-situ stress.

Determination of the state of stress in rock by the measurement of strains on the hemispherical borehole bottom : Sugawara, K; Kaneko, K; Obara, Y; Okamura, H Proc International Symposium on Large Rock Caverns, Helsinki, 25–28 August 1986V2, P1039–1059. Publ Oxford: Pergamon Press, 1986

Determination of the state of stress in rock by the measurement of strains on the hemispherical borehole bottom : Sugawara, K; Kaneko, K; Obara, Y; Okamura, H Proc International Symposium on Large Rock Caverns, Helsinki, 25–28 August 1986V2, P1039–1059. Publ Oxford: Pergamon Press, 1986

応力解放法による球面孔底ひずみの測定

The practical system for measuring the strains on a hemi-spherical borehole bottom by the stress relief technique has been presented and its utility has been examined experimentally.The spherical 16-element mold gauge, made of epoxy resin as shown in Fig.3, has beendeveloped to set the 8 latitudinal strain gauges and 8 longitudinal strain gauges at the proposed position on the bottom surface with high accuracy.This mold gauge has been calibrated in the appropriately oriented borehole in the andesite cubic specimens under load, andit is confirmed that the measured strains well agree with the elastic strain distribution analyzed by F.E.M.as shown in Fig.5.The process of in-situ stress relief has been examined by analyzing the strainc hange observed under over-coring.The typical strain change in Fig.8 shows that prior to the stress relief the tension appears in the longitudinal direction.Especially, it is remarkable in the case that the hollow core discing occurs as shownin Fig.9.It has been discussed that such a tension behavior influenced upon the accuracy of strain measurement and it conditioned the limitations and certain disadvantages of this system.

Electrical borehole measurements for the mapping of fracture zones in crystalline rock

The mise-à-la-masse method has been used for delineation of fracture zones in crystalline rock. Measurements have been carried out at two different sites in Sweden, Finnsjö and Stripa. Measurements in Finnsjö show that the mise-à-la-masse method can be used to interpret the orientation and the dimensional shape of a fracture zone. The greatest distance over which a significantly large fracture zone could be detected from the current electrode was 150 m. In the Stripa mine mise-à-la-masse measurements have given valuable information on the geometry of a large fracture zone reached at the bottom of a vertical borehole even though the measurements were to be made in a mine where the possible locations for the potential electrode were limited to a few boreholes. In Stripa the distance between the current electrode and the potential electrode was at most about 500 m. An apparent resistivity has been defined and used for qualitative interpretation of the data. The qualitative interpretation was successful in the Finnsjö measurements where potential data were available over a large surface. Interpretation has also been made with a semi-quantitative model where the conductivity, width and orientation of a conductive sheet has been fitted to the data. In Stripa this model has given an indication of the orientation of a large fracture zone.

やや深い構造のS波速度 (III)

Continued to previous works at Iwatsuki and Shimohsa, S wave velocities in deep soil deposits were measured by using the observation well at the Fuchu Deep-Borehole Crustal Activity Observatory of the National Research Center for Disaster Prevention, Japan.Measurements were conducted at 17 different depths to the borehole bottom with the depth of 2, 750m at intervals of 100-250m. S waves were excited by the SH wave generator on the ground surface. The excited waves were observed by a set of three component seismometers assembled into the specially designed capsule. The capsule can fix itself to the borehole wall at any depths.The measurement revealed that the deposit at the Fuchu is fundamentally composed of three layers which covers the basement rocks. S wave velocities of those layers are, from the top to the bottom, 0.54, 0.78, and 1.19km/sec, and that of the substratum is 2.54km/sec. The velocity structure, including the P wave one, is consistent with such data as geologic section, sonic velocity, bulk density, and electric resistivity loggings of the observation well.Through a comparison of thus obtained structures at three separate sites, Iwatsuki, Shimohsa, and Fuchu, one can visualize the underground structure three dimensionally, though vaguely and restricted in the Tokyo metropolitan area. A relation between the seismic wave velocity and the geological condition is also obtained. The relation is useful to estimate the velocity structure at deep-wells which were bored for the purpose of the geological investigation. Finally, it is found that the P wave velocity of the basement obtained from our direct method is rather small compared with the one from the refraction method. The difference is confirmed to be meaningful. It seems to be caused from the basement structure, that is, the basement is not uniform but has a thin layer at its top. The velocity difference between this layer and the lower basement is so small that the upper-most layer is masked in the ordinary refraction exploration.

Three-dimensional analysis of scattered P waves on the basis of the PP single isotropic scattering model.

Wave trains immediately following the initial motion of P waves are analyzed assuming that they are scattered P waves by heterogeneities in the earth medium. Three-dimensional partition of the energy density of scattered P waves is theoretically studied, where PP single isotropic scattering and homogeneous and random distribution of scatterers are assumed. The theoretical analysis shows that the three-dimensional mean trajectory for scattered P waves in velocity space is represented by a spheroid, which is prolate to the propagation direction of the direct P wave, and the spheroid tends to become spherical as time goes on. This model is applied to an analysis of short-period (1-30Hz) records of small local earthquakes observed by a three-component velocity seismometer installed at the bottom of a deep borehole. The three-dimensional partition of energy density is calculated by using the covariance tensor for particle velocity observed. The observed horizontal component of energy density of scattered P waves agrees well with the theoretical one, although the observed vertical component is larger than and the observed radial component is smaller than the theoretical one. Our theoretical model is proved to be good in accounting for horizontal component. In order to explain changes in vertical plane, it is suggested that refraction and reflection at horizontal lateral boundaries and the earth's surface, and/or PS conversion scattering should be considered.

Stress pattern near the San Andreas Fault, Palmdale, California, from near‐surface in situ measurements

We made 29 in situ doorstopper strain relaxation measurements distributed among eight sites spaced on a 35‐km transect running from the foothills of the San Gabriel Mountains, across the San Andreas fault, into the western Mojave desert southeast of Palmdale, California. This was a pilot study to test if such measurements can detect the regional stress field and any modification of it in a tectonically complex area. Strain was measured by overcoring strain gauge rosettes which had been bonded to the flattened bottom of a shallow borehole. Measurements of stress were repeatable at each site. We found NNE trending maximum compressive stress (σ1) at our sites farthest from the fault. This orientation is parallel to the σ1 inferred from fault plane solutions of major southern California earthquakes and compares favorably with deep hydrofracture stress measurements made near our sites. Nearer the San Andreas fault, the orientation of σ1 was approximately east‐west north of the fault, and northwest‐southeast to northsouth south of the fault. The repeatability of measurements at a site and the favorable comparison of our measurements with Tullis' [1977] near‐surface stress measurements suggest that we reliably determined the stress field orientation present at a site. It was not possible, using the available data, to distinguish the stress caused by residual or topographic effects from tectonically applied stress or to account for modification of the stress field by decoupling across fractures.

やや深い構造のS波速度 (II)

Continued to the previous work at Iwatsuki the similar experiment on the shear wave velocity measurement in deep soil deposits was carried out in the earthquake observation well at Shimofusa, Chiba Prefecture, which was constructed by the National Research Center for Disaster Prevention of Japan.Shear waves were produced by means of the SH wave generator on the ground surface. In 15 different depths at intervals of 100-250m, sequent measurement down to the borehole bottom with depth of 2.3km were conducted by using a set of three component geophones installed in the specially designed capsule with a clamping device to the borehole wall. The generated SH signals could clearly be detected as deep as the bottom.The obtained velocity profile is essentially composed of three layers and substratum of which shear wave velocities are, from the top to the bottom, 0.40km/s, 0.82km/s, 1.20km/s and 2.60km/s respectively. The disclosed discontinuities by this experiment agree completely with the known data such as sonic log, density and geologic profile etc. A comparison with those velocities at Iwatsuki was also made as the first step to visualize the underground structure in Tokyo area three-dimensionally.

Velocities, Kinetic Energy and Shear in Crossflow Under Three-Cone Jet Bits

Abstract Velocity, kinetic energy and shear in crossflow beneath three-cone jet bits may influence cleaning of the bottom of the borehole and the teeth of the bit. Laboratory investigation shows that each of these parameters is a function of the diameter of the borehole and the product of the volume rate of flow and velocity through the nozzles (QVn). Increasing QVn or decreasing the diameter of the borehole increases each parameter. These functions provide means of predicting the magnitude of each parameter and of scaling the cleaning forces. Introduction In drilling operations using conventional jet-type rock bits, the impinging jets create an important flow mechanism. Called crossflow, this flow mechanism originates in the impact area of the jets, spreads across the bottom of the hole and supplies the principal source of energy to clean the teeth of the bit and most of the bottom. Besides providing the means of cleaning cuttings from the bit and the hole bottom, crossflow may also have other, less direct, effects on the rate of penetration. The shear stress generated on the bottom by crossflow will influence the thickness and permeability of any fitter cake of mud solids or crushed material which forms on the bottom. These factors may affect the rate of penetration. A previous publication introduced some fundamental concepts of crossflow. Crossflow was shown to occur in a thin layer adjacent to the bottom, and to cover the bottom completely. The maximum velocity in the crossflow above any position on the bottom was found to be directly proportional to the square root of the product of the volume rate of flow and velocity through the nozzles the jet QVn and inversely proportional to the diameter of the borehole. This information indicated that the effectiveness of crossflow in scavenging the bottom can be improved by maximizing the jet QVn. The investigation reported herein amplifies the definition of crossflow. Complete velocity profile data above a representative position on the bottom are analyzed. These data better illustrate control of the capacity of crossflow to scavenge the bottom, and also relate shear stress on the bottom to known, controllable parameters. The conclusion reached in the previous publication that maximization of the jet QVn produces the maximum cleaning beneath current jet bits is unchanged by these new data; rather, it is strengthened. Data presented here show that the kinetic energy flux above a representative position on the bottom is maximized by maximizing the jet QVn. The shear stress on the bottom will also be shown to be maximized in the same manner. Since the functions relating the jet QVn to velocity, shear stress and kinetic energy also involve the diameter of the borehole, means of equating, or scaling, these quantities in different sizes of boreholes will be illustrated. EXPERIMENTAL EQUIPMENT AND TECHNIQUE JET BIT MODEL Data were recorded from the same laboratory model as used in the aforementioned investigation of the flow around a jet bit. The model consisted of a 4 3/4-in. three-cone, jet-type rock bit in a smooth, flat-bottomed borehole constructed of lucite. The bit had a shape and nozzle placement closely resembling larger three-cone jet bits commonly used in field operations. Fig. 1 illustrates the impact area on the bottom of a jet from this bit. Details of the orientation of the jets may be found in the previous publication. TECHNIQUE OF MEASUREMENT Measurements of the crossflow were made by inserting a very small Pitot tube through the bottom of the simulated borehole. Extreme thinness of the layer of crossflow necessitated accurate measurements of the height of the Pitot tube above the bottom to achieve close definition of the velocity profile. A cathetometer, which could be read to the nearest 0.005 cm, was used to make this measurement. JPT P. 1443ˆ