The Evaluation Ahead of Tunnel Face Using Seismic while Drilling

Resistivity and seismic refraction survey of the Masari/Kafur “Kimberlite” pipe in Northern Nigeria (and its groundwater reserves)

Crustal structure variation along the central rift/ridge axis in the North Fiji Basin: Implications from seismic reflection and refraction data

Ground magnetic and seismic refraction survey is carried out on an urban extension site in the southwest of Ahud Rufeidah town, southwest Saudi Arabia. The purpose of the survey was to map the surface topography of the bedrock and thickness of the alluvium overburden. The ground magnetic survey based on an assumption that the alluvial sediments is less or non-magnetic relative to the underlying gneiss basement rock. In this context, a total of 3,750 survey stations were measured along 40 east–west survey profiles, striking roughly perpendicular to the extension of the expected structures. In addition, 13 seismic refraction spreads were conducted along four seismic survey profiles across the expected pathways of buried alluvial channels in order to provide additional details about the depth and boundaries of the buried channels. The ground magnetic survey results show the presence of a basin combining two sub-basins filled with alluvium sediments that occupy the middle area of the investigated site. This basin is a closed basin that has no outlet, and there are four small and narrow channels that convey water and sediments from the eastern and southern hills into sub-basins. These channels are represented by narrow and elongated low magnetic anomalies extending towards the basinal area. The thickness of the alluvial sediments is verified using seismic refraction survey that indicates a greater thickness, exceeds 20 m, of low velocity sedimentary overburden inside the interpreted sub-basins and surrounding buried alluvial channels. These soft alluvial sediments can be deceptive and dangerous for urban foundations.

A seismic refraction survey was conducted at Reydarfjördur, Iceland, as part of the Iceland Research Drilling Project in July 1978. An array of 15 seismometers was used to measure apparent velocities from 25 explosions along a 23 km refraction line. The experiment yielded data used to determine the P‐ and S‐wave velocity structures beneath the drill site and to obtain bounds on the depth to seismic ‘layer 3.’ The observed apparent velocities vary continuously as a function of range with a marked increase in velocity at a range of 16 km. This corresponds to an observed cross‐over in the travel time data. The measurement of apparent velocities has yielded data of the form X(p), the distance to a measured ray parameter, and τ(p), the delay time. Joint inversion of the X(p) and τ(p) data has been performed by linear programing. Layers of constant slowness gradient are used as the basis for the inversion process. Estimates of error in the data are used to produce extremal bounds on the family of earth models that satisfy the data. When low velocity zones are assumed to be absent, the resulting envelope of P‐wave velocity models indicates a continuous variation of velocity with depth. Two regions of high velocity gradient are observed, one near the surface and the other at depths between 2.6 and 3.9 km associated with layer 3 in Iceland. These results suggest that modeling of Iceland's upper crust with a few thick homogeneous constant velocity layers is an inadequate representation of the velocity structure. Some features observed in the seismic data suggest the presence of a low velocity zone with a lid between 1.6 and 2.5 km depth. Rough synthetic modeling places this lid at the shallower end of this range.

A combined seismic reflection and refraction study of a landfill and its host sediments

The thesis is the summary of the three-years work done at the Department of Mechanics at the Politecnico di Torino, from January 2010 to December 2012, under the supervision of professor Luigi Garibaldi (Dipartimento di Ingegneria Meccanica e Aerospaziale) and professor Alberto Godio (Dipartimento di Ingegneria Ambientale, del Territorio e delle Infrastrutture). The subject of the thesis has been chosen in collaboration with Gd Test S.r.l., a company operating in the field of environmental engineering, based in Torino, relatively close to the Politecnico. During the three years, the company cooperated with me and the professors, in particular by giving an important help for the experimental tests. The topic is the prediction of lithological discontinuities ahead of a tunnel face, in a tunnel under construction, i.e. the identification of whatever type of zone producing a reflection of the seismic waves generated in a source point. Typical excitations are the blasting, the hammer or magneto-strictive sources. Moreover, the procedures can be improved to be applied in presence of the Tunnel Boring Machine, by exploiting the noise produced by its cutter head. The main objective of the work is duplex: to reduce the risk for people and machinery and to reduce the excavation cost by setting the excavation based on the rock configuration that has been predicted. The theme is a modification of the classical reflection seismology techniques, in order to be applied in tunnels. Indeed, the particular configuration of sources and receivers in a tunnel obliges to develop ad hoc methods for the analysis of the seismic data. The thesis proposes two methods for the identification of the discontinuities ahead of a tunnel face: the first comes directly from the concept of ellipsoids, fundamental in the reflection seismology, and the second is a backward method, able to provide an automatic prediction without extracting the time arrivals corresponding to the reflected waves. To run, this procedure needs only the coordinates of sources and receivers, together with the acquisition data parameters, and then it allows: i) to estimate an average value of the wave velocity; ii) to detect the discontinuities for each source; iii) to analyze and plot the number of superposing estimations for each node of the selected space domain. The final result can be interpreted as the probability to detect a discontinuity at a certain distance from the tunnel face. The thesis first describes the fundamental relationships in tunneling seismology, then it proposes a direct technique for the reflectors detection. Since it is demonstrated to be not too reliable in presence of low signal-to-noise ratio, the automatic method is presented. This procedure has been tested on synthetic and real data coming from the new Brennero tunnel under construction, which will be one of the longest tunnel in the world. The results indicate that the method runs very fast and it is reliable in the identification of lithological changes and discontinuities, up to more than one hundred meters ahead the tunnel face. Moreover, it can be used in every kind of tunnels, without any detailed knowledge of the soil and rocks around the tunnel, and with different excitations. The thesis deals also with another important issue during the tunnel construction: the monitoring of the rockburst phenomenon, a small seismic event caused by the rock alteration during the excavation. For this purpose, a specific procedure has been created, able to estimate the position of the source point

A refraction seismic survey was used at a proposed landfill site in Al Minya basin south of West Bank in an effort to delineate the thickness of the soil cover and the distribution of rock types underneath, and to detect the subsurface structures that can act as contaminant pathways. One hundred seismic refraction traverse lines were run on the ground surface using the reverse profile techniques with a geophone interval between 3–5 m. From the seismic data analysis and the geological interpretation of the results, three main velocity zones were detected, ranging between 192–1085 m/s of the first two layers, representing the unconsolidated materials, to a moderately high velocity of the fresh bedrock, varying between 1100–2400 m/s. The velocity zones found, within the investigated seismic refraction sections were found to be compatible with the changes in the lithology of the study area. The general characteristics of the subsurface geology seem suitable for the proposed landfill site, yet light fracturing traced within the bedrock requires some remediation to prevent seepage. However, the landfill site is underlain by an aquitard layer of marl and chalk that will attenuate naturally the leachate infiltration.

Seismic While Drilling (SWD) is a well seismic method developed in recent years which is based on reverse vertical seismic logging. It is a newly-developed well seismic technology which combines the seismic exploration technology with petroleum drilling engineering technology. Compared to conventional VSP, SWD has its own characteristics and unique advantages in that it uses bit vibration in the process of drilling as the source for seismic measurement, without interfering with the drilling or occupying drilling time, and without any risk to the hole, especially the bit can be predicted in real-time structure details of the formation in front of the bit through the field seismic imaging processing, with the main purpose of reducing drilling risks. The key of this technology is how to collect and recover the weak bit reflection signal under strong disturbance noise and make it the equivalent formation impulse response.

Summary Two dimensional (D) seismic refraction surveys were used to map the bedrock topography beneath a gravel and peat marginal marine succession in southwest Devon, UK to constrain the boundary conditions for a coastal hydrogeologic numerical flow model. After data collection and processing of seismic refraction, 2-D refraction output files were imported into a 3-D interpretative software package. A 3-D velocity model was then created via petrophysical modeling of the velocity attribute to interpolate between the 2-D surveys. A shore parallel cross-section inserted into the 3-D velocity model was migrated at 5 m intervals through the model which allowed for manual picking of data points at the chosen bedrock velocity of 2100 meters/second (m/s). The selected data points were then used to create a 3-D bedrock surface. The initial 3-D surface was checked for quality against 2-D seismic lines, borehole data, bedrock outcrop observations, and previous offshore seismic reflection data. The bedrock topography has a U-shaped valley with two paleo-channels. Results indicate the justification for a 2-D hydrogeologic flow model for initial conditions and sensitivity study on mixing zone dynamics at Hallsands beach.

The objective was to optimize the existing solution for the limit support pressure of a tunnel face. Firstly, based on the numerical simulation results, the existing three-dimensional analytical solution for pore water pressure distribution is expanded to a three-dimensional solution considering the pore water pressure distribution in the upper formation behind the tunnel face. Then, according to the results of physical model tests, a failure model considering the failure range in the upper formation behind the tunnel face is established, and the newly established three-dimensional solution for pore water pressure is introduced into the model, and then the limit effective support pressure of the tunnel face considering seepage is obtained by the method of soil–water joint calculation. Finally, the calculation results in this paper are compared with the experimental results, numerical simulation results and existing theoretical solutions. The major findings are as follows. The distribution of pore water pressure in the front and back strata above the tunnel face is basically symmetrical. The limit effective support pressure of the tunnel face will increase linearly with an increase in the hydraulic head difference between the tunnel face and the ground surface. The calculated results of the new limit equilibrium theory are obviously larger than those of the existing theory and numerical simulation and closer to the results of the physical model tests. Therefore, the new limit equilibrium model can better predict the limit effective support pressure of the tunnel face considering seepage and provide a reference for actual projects.

Seismic While Drilling (SWD) specifically encompasses the seismic techniques operated while the drillstring is lowered in the borehole, during effective drilling, during maneuvers or while connecting drill pipes. This paper provides a State of the Art review on both old and newer SWD techniques used by the Industry. In the past 21 years (1986-2006), the Drill-bit SWD technique practiced by the industry utilizes the acoustic energy radiated by the Tri-Cone bit to provide vital real time information during drilling by providing time-to-depth and look-ahead information Another emerging technique which is being used mainly by Schlumberger since 2000 is Vertical Seismic Profile While Drilling (VSP-WD), which consists in recording the seismic signal generated by a surface seismic source on seismic sensors integrated inside the downhole Borehole Assembly (BHA). Two efficient wireline VSP which could be assimilated to SWD techniques and which are not be discussed in this paper are the technique of Tube Logging Conveying or Tough Logging Condition (TLC), in particular the TLC-VSP and the standard wireline logging technique of Intermediate VSP. Recently introduced Swept impulse hydraulic tool is capable of generating a broadband seismic signal at the bit while drilling. This tool overcomes the limitations of drill-bit seismic particularly in soft formations and in inclined holes by allowing seismic operations with PDC bit. Operating principles, seismic processing and results of a field demonstration of seismic profiling while drilling in a shallow well are described in this paper. The two major issues that need to be understood in order to make these seismic measurements with precision and reliability are also raised in this paper. This paper aims to give a present day picture of the SWD techniques. Data acquisition and sensor deployment strategies are discussed in some detail. The benefits of SWD are demonstrated by means of several field examples, showing how the data can be used to reduce drilling costs, predict drilling hazards and improve efficiency of drilling operations on a real time basis. The operating domain of SWD technique is identified along with the current limitations. Future development directions are also proposed.

A landslide located on the Quesnel River in British Columbia, Canada is used as a case study to demonstrate the utility of a multi-geophysical approach to subsurface mapping of unstable slopes. Ground penetrating radar (GPR), direct current (DC) resistivity and seismic reflection and refraction surveys were conducted over the landslide and adjacent terrain. Geophysical data were interpreted based on stratigraphic and geomorphologic observations, including the use of digital terrain models (DTMs), and then integrated into a 3-dimensional model. GPR surveys yielded high-resolution data that were correlated with stratigraphic units to a maximum depth of 25 m. DC electrical resistivity offered limited data on specific units but was effective for resolving stratigraphic relationships between units to a maximum depth of 40 m. Seismic surveys were primarily used to obtain unit boundaries up to a depth of >80 m. Surfaces of rupture and separation were successfully identified by GPR and DC electrical resistivity techniques.

P-wave velocity (Vp) structures in marine areas have been estimated using marine airgun-source seismic surveys and refraction methods. The lateral resolution of Vp structures depends on the number of receivers and shots. In marine environments, Vp resolution is often limited due to the sparse distribution of ocean bottom seismometers (OBSs). Recently, distributed acoustic sensing (DAS) offers a cost-effective alternative, providing high-density strain measurements along optical fiber cables. This study demonstrates that seismic refraction using DAS data is effective for imaging shallow Vp structures with high lateral resolution. A seismic survey was conducted using R/V Hakuho-maru, airgun sources, and DAS and OBS measurements along a seafloor optical fiber cable off Sanriku, Japan. Strong lateral heterogeneities in the Vp structure were observed, due to the dense spatial sampling of DAS data. The two-dimensional Vp structure obtained from the DAS data showed good agreement with the seismic reflection profile. This experiment highlights the unique advantages of DAS data over OBSs for seismic refraction.

Large volumes of groundwater are contained in the deep intermontane valleys of Basin and Range regions. Determining the shape and storage capacity of these basins by drilling can be expensive and difficult because of the depth of alluvium and large areas involved. These difficulties can often be overcome by combining gravimetric and seismic refraction interpretations. The basin boundaries are determined by gravimetric methods, with bulk density samples taken of all representative formations. Density values can then be correlated with seismic velocities to estimate subsurface porosities. Studies indicate that seismic velocity varies inversely with porosity for the alluvial deposits. These values can be correlated with their respective formations in the basin from geologic sections derived from the seismic refraction survey. Thus, with volume and porosity of the alluvium known, storage capacity (both present and potential) can be computed. Résumé De grands volumes d'eau souterraine se trouvent dans les bassins formés par de très profond es vallées entre montagnes et par de grandes étendues de terrain non dèlimitées. Le mode de détermination de la forme et de la capacité de ces bassins par la voie de sondages peut devenir très coûteuse et difficile, à cause de la profondeur de ces grands gisements alluviaux, et de l'étendue des terrains intervenant. Très souvent ces difficultés peuvent être surmontées en combinant les résultats gravimétriques et de réfraction sismiques. Les limites du bassin sont déterminées par des méthodes gravimétriques, avec des prélèvements de densité en masse pris sur toutes les formations représentatives. Ces valeurs de densité peuvent être alors mises en corrélation avec les vitesses de propagation sismiques pour estimer la porosité du-sous sol. Les expériences indiquent que la vitesse de propagation sismique varie inversement avec la porosité des gisements alluviaux. Ces valeurs peuvent être mises en corrélation avec les formations respectives, trouvées dans le bassin, et qui correspondent aux sections géologiques provenant du levé de réfraction sismique. C'est ainsi que la capacité de réservoirs (tant actuelle que potentielle) peut être computée, si le volume et la porosité de l'alluvion sont connus.

Seismic refraction data from 18 traverses along the Hawaiian Ridge from Kauai to Midway Island are presented in this paper. The seismic velocities of the mantle vary from 7.8 km/sec to 8.8 km/sec. The velocity of the lower layer of the crust tends to cluster around 6.5 km/sec. The velocities of the upper crustal layers vary from 4.0 to 5.7 km/sec without tendency to cluster around any value. The depth to the mantle along the Leeward Islands is usually between 10 and 15 km. The mantle is deeper (18 km) near the major islands. From the data, inference is drawn that the area of subsidence was smaller for older volcanic islands than for the newer ones. An hypothesis based on fluid dynamics of inverted densities is proposed to account for this.