PROfile Jean-Luc Mari and Christophe Vergniault Well seismic surveying and acoustic logging
Well seismic surveying and acoustic logging An overview of the current state of knowledge in the geotechnical field and possible methodology transfers from the oil industry to near surface studies
Well seismic surveying and acoustic logging An overview of the current state of knowledge in the geotechnical field and possible methodology transfers from the oil industry to near surface studies Jean-Luc Mari Christophe Vergniault QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE
DOI: 10.1051/978-2-7598-2263-8 ISBN(ebook) : 978-2-7598-2263-8 This book is published in under Open Access Creative Commons License CC-BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/en/) allowing non-commercial use, distribution, reproduction of the text, via any medium, provided the source is cited. © EDP Sciences, 2018
5 Contents Foreword 9 Introduction 11 Chapter 1 • Shear velocity measurement in boreholes 15 1.1 Context 15 1.1.1 Overview of invasive and non-invasive Vs measurements 15 1.1.2 Applications 17 1.1.3 Environmental conditions 18 1.2 Vs measurements by borehole seismic method 19 1.2.1 Downhole 21 1.2.1.1 The acquisition device 21 1.2.1.2 Analysis of a downhole (DH) 26 1.2.1.3 The specific example of offshore 29 1.2.2 Uphole 30 1.2.3 Crosshole 30 1.2.3.1 Prerequisites 31 1.2.3.2 The acquisition device 32 1.2.3.3 Crosshole analysis 36 1.3 Vs measurement by logging 39 1.3.1 Acoustic logging with high vertical resolution 39 1.3.2 PS Suspension Logging (PSSL) 41
6 Well seismic surveying and acoustic logging 1.4 Case study of a downhole acquisition, PSSL and acoustic logging, in the same borehole 44 1.5 Conclusion 46 References 47 Chapter 2 • Well seismic surveying 49 2.1 Introduction 49 2.2 Well seismic data acquisition 53 2.2.1 Necessary resources for data acquisition 53 2.2.2 Implementation in the field 53 2.2.2.1 Description of classical VSP operation in a vertical well 53 2.2.2.2 Well probes 54 2.2.2.3 Seismic source 56 2.2.2.4 Acquisition parameters 56 2.2.2.5 Security 57 2.2.2.6 Quality Control 57 2.2.2.7 Production 57 2.3 Seismic waves 57 2.4 Processing sequence 62 2.5 Application with a geotechnical dataset 71 2.6 Conclusion 74 References 76 Chapter 3 • Acoustic logging 77 3.1 Introduction 77 3.2 Acoustic logging data acquisition 79 3.2.1 Necessary resources for data acquisition 80 3.2.2 Implementation in the field 80 3.2.2.1 Description of an acoustic logging operation in a vertical borehole 80 3.2.2.2 Acoustic probes 81 3.2.2.3 Acquisition and visualization parameters 84 3.2.2.4 Acoustic logging in deviated well 84 3.2.2.5 Security 84 3.2.2.6 Quality control 84 3.2.2.7 Production 84 3.3 Acoustic waves 85 3.4 Processing sequence 88
7 Contents 3.5 Acoustic imaging 91 3.5.1 Refraction acoustic imaging 91 3.5.2 Reflection acoustic imaging 95 3.6 Characterization of a formation using Stoneley waves 97 3.7 Conclusion 100 References 101 Chapter 4 • Tying surface seismic data 103 4.1 Introduction 103 4.2 3D VHR seismic survey and VSP 104 4.3 Acoustic logging 108 4.4 Acoustic logs 109 4.5 Time conversion of acoustic logs and calculation of the synthetic seismogram 111 4.6 Integrated sonic time and VSP vertical time 113 4.7 Conclusion 116 References 116 Chapter 5 • Contribution of seismic and acoustic methods to the characterization of karstic formations 117 5.1 Introduction 117 5.2 Geological context 118 5.3 3D seismic acquisition and processing 120 5.4 Well seismic measurements 124 5.5 Monopole full waveform acoustic logging 127 5.6 Conclusion 131 5.7 Acknowledgements 132 References 132 Conclusion 135
9 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE Foreword 1 J.-L. Mari, C. Vergniault Based on their experience in geophysics applied to the oil and gas industry and the geotechnical field, the authors have set out to explain how conventional approaches used in deep exploration geophysics can be applied to certain geotechnical and hydrogeological surveys, and site characterizations in the framework of seismic hazard studies. After reviewing the current state of knowledge in the geotechnical field regarding borehole measurements of subsurface shear velocities, the book aims to illustrate the feasibility of carrying out vertical seismic profiles, logs and 3D seismic reflection blocks. In addition to these examples, the authors have sought to provide readers with guidelines to carry out these operations, in terms of acquisition, as well as processing and interpretation. The authors thank Françoise Coppens, Patrick Meynier and Gilles Porel for their contributions to this book. Many thanks to Jim Johnson and Katell Guernic from Tamarin (www.tamarintext.com) for the English translation of the book This chapter of Well seismic surveying and acoustic logging is published under Open Source Creative Commons License CC-BY-NC-ND allowing non-commercial use, distribution, reproduction of the text, via any medium, provided the source is cited. © EDP Sciences, 2018 DOI: 10.1051/978-2-7598-2263-8.c001
10 Well seismic surveying and acoustic logging The authors A graduate of the Institut Physique du Globe Strasbourg and IFP School (petroleum geosciences, major in geophysics in 1978), Jean-Luc Mari was employed by IFP Energies Nouvelles in 1979 as a research engineer in the Geophysics Department, where he worked on several research projects, such as high-resolution seismic surveying, reservoir monitoring, and the development of borehole tools, in collaboration with industrial partners GdF-Suez, CGG, Total and ELF Aquitaine. In 1986, he was seconded to ELF Aquitaine where he worked on reservoir geophysics. He joined IFPEN in 1987 and was seconded to the Reservoir Department, where he studied, in particular, the benefits of using geophysical methods in horizontal wells. In 1994 he was appointed to IFP School as a professor and obtained the accreditation to supervise research in earth sciences at the Université Pierre et Marie Curie. Currently geophysics professor at IFP School, and an expert in geophysics for IFP Energies Nouvelles, Jean-Luc Mari is a member of the EAGE. He is associate editor for Near Surface Geophysics. In 2010, he received a Knighthood from the Ordre des Palmes Académiques. After gaining experience in various roles involving physical measurements, Christophe Vergniault has worked as a geophysicist in the Geosciences Department at EDF’s Industrial Division (EDF-DIPNN-DI-TEGG) for eight years. Trained as a geophysics ingénieur (EOST) and with a geology degree from Montpellier (master in advanced studies in structure and evolution of the lithosphere), he worked as a geophysicist in a company specializing in offshore surveying (Géodia), as a petrophysicist and wellsite geologist at Gaz De France (Exploration & Production and underground storage department), and as physical measurement analyst for inspecting EDF hydropower. The result of this accumulation of experiences is reflected in this book through his outlook on geophysical measurements applied to the geotechnical field.
11 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE Introduction 2 J.-L. Mari, C. Vergniault In the geophysics of oil exploration and reservoir studies, the seismic method is the most commonly used method to obtain a subsurface model. This method plays an increasingly important role in soil investigations for geotechnical, hydrogeological and site characterization studies regarding seismic hazard issues (Mari et al, 1999). The surface seismic method involves: • Seismic refraction (P or S waves), which provides a subsurface velocity model. This method, applied to P waves, is commonly used in the geotechnology field to identify changes in the position of the bedrock, as well as longitudinal changes of its physical state or that of its overburden (see AGAP’s Guide Sismique réfraction, O. Magnin, Y. Bertrand, 2005). • Seismic reflection, a type of two or three-dimensional subsurface ultrasound method, which initially provides an image of the acoustic impedance contrasts of the subsurface. Depending on the means implemented, an investigation can reach hundreds of meters to several thousand meters in depth. However, the method does not perform well in the first 20 to 50 meters. This chapter of Well seismic surveying and acoustic logging is published under Open Source Creative Commons License CC-BY-NC-ND allowing non-commercial use, distribution, reproduction of the text, via any medium, provided the source is cited. © EDP Sciences, 2018 DOI: 10.1051/978-2-7598-2263-8.c002
12 Well seismic surveying and acoustic logging • Multiple Analysis of Surface Waves (MASW) which, by analyzing the Rayleigh or Love wave phase velocity in the frequency domain (scatter diagram), enables the calculation of the evolution of the shear wave velocity (VS) within the first tens of meters of subsurface. This method is increasingly used in geotechnology in combination with the seismic refraction method to determine the shear modulus. The vertical resolution of all surface geophysical methods decreases as a function of the depth investigated. To obtain a precise model of the deep subsoil’s seismic parameters (propagation velocities of P waves (VP) and S waves (VS), and density), geophysicists use borehole data such as those provided by the well seismic and acoustic logging methods, in particular to carry out the tying and calibration at depth of surface measurements. In addition, processing provides both a model for the propagation velocities of waves (P and S waves) and also for density, such as the examples presented at the end of this introduction. The examples presented in Figure 1 are extracted from 3D seismic data. Figure 1(a) is a near-surface example (Mari and Porel, 2007). The P velocity distribution was obtained by seismic refraction (tomography) for the very near surface (up to 30 m deep) and by seismic reflection (acoustic inversion) for the deep seismic horizons (20 to 120 m). This first example is the subject of the case study in Chapter 5. It should be noted that a similar approach could be made by combining the MASW method and the S-wave seismic method. Figures 1(b), 1(c) and 1(d) are derived from the processing of a seismic reflection survey carried out to map horizons down to 1,500 m deep (Mari and Yven, 2014). The distribution of velocity (VP and VS) and density were obtained by elastic inversion. The examples presented in this introduction already make it possible to highlight the fact that surface and well seismic methods combined with acoustic methods can be used successfully to estimate mechanical modules (Poisson’s ratio, shear modulus and Young’s modulus...). The objective of this book is to illustrate that the processes applied in deep geophysical exploration, combining different seismic and logging methods, can be applied to certain geotechnical and hydrogeological surveys, and site characterization in the context of seismic hazard studies. This book, which is composed of five chapters, aims to present some of these approaches and their applications for near surface surveys (<150 m): • The first chapter provides an overview of the state-of-the-art technology in the geotechnical field regarding borehole measurements of subsoil shear wave velocity. It highlights the benefits of combining different methods: VSP-type well survey measurement with SH waves, generally called downhole, transmission between boreholes generally called crosshole, and dipole type acoustic logging (PSSL). • The second chapter is devoted to the well seismic method. It describes the implementation procedure, the means of acquisition (sources and sensors) used in the civil engineering field, the different types of waves that make up the well seismic recordings (volume waves and guided modes) and the processing sequences. For more information, see “Well seismic surveying” by J.L. Mari and F. Coppens, 2003, Editions Technip.
13 Introduction Near surface example P velocity distribution (a) P velocity distribution (b) Density distribution (c) S velocity distribution (d) Figure 1 Distributions of velocities and densities obtained by seismic surveying Near surface example (a), petroleum type example (b, c, d) • The third chapter is about full waveform acoustic logging and its main applications in the civil engineering field. It briefly describes: the logging tools implemented (monopole or dipole), the different wave types that make up acoustic recordings, and the contribution of acoustic measurements to the description of geological formations (mechanical parameters). There is also a discussion on the contribution of Stoneley waves for the estimation of S velocities of formations
14 Well seismic surveying and acoustic logging and fractured zone detection. In addition, it shows how acoustic logging can be used to evaluate the quality of well cementation. • The fourth chapter describes the benefits of combining measurements of formation velocities provided by VSP-type well tools and acoustic (sonic) tools. Based on a near surface example, it shows a tying method between sonic and check shots (VSP), which is used to obtain a time-depth relationship, tied to the seismic data and used for the conversion of logs into time and the calculation of synthetic seismograms. • The fifth chapter is an integrated case study of a karstic limestone aquifer that is relatively close to the surface (20 to 130 m). We show how multi-scale description of the reservoir can be realized by integrating the information provided by different 3D-THR surface seismic methods, full waveform acoustic logging, VSP with hydrophones, borehole optical televiewer and flow measurement. Note: In the oil sector, the word ‘well’ is commonly used to mean borehole. This notion is not ideal for geotechnology usage, where boreholes are drilled for investigation and not production. In this document, we will therefore use the word ‘well’ for all descriptions relating to a transfer of technology from the oil world to geotechnology. However, ‘borehole’ will be used in relation to common geotechnical methods. References Mari J. L., Arens G., Chapellier D., Gaudiani P., 1999, Geophysics of reservoir and civil engineering, Éditions Technip, Paris, ISBN 2-7108-0757-2. Mari J.L., Porel G, 2007, 3D seismic imaging of a near – surface heterogeneous aquifer: a case study, Oil and Gas Science and Technology, Rev IFP 63, 179-201. Doi: 10.2516/ogst/2007077. Mari J.L., Yven B., 2014, The application of high-resolution 3D seismic data to model the distribution of mechanical and hydrogeological properties of a potential host rock for the deep storage of radioactive waste in France, Marine and Petroleum Geology 53, 133-153.
15 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE 1 Shear velocity measurement in boreholes 3 C. Vergniault, J.-L. Mari 1.1 Context 1.1.1 Overview of invasive and non-invasive Vs measurements Measurement of shear waves (S) can be invasive (borehole seismic surveys and logging) or non-invasive (surface waves (Multi analysis of surface waves: MASW) and ambient noise (Ambiance measurement vibration: AMV)). Obviously, each method has advantages and disadvantages that the project manager (client or subcontractor) must evaluate to select the most suitable method for the project. To assist in this process, Table 1.1 summarizes the pros and cons of each method. This chapter of Well seismic surveying and acoustic logging is published under Open Source Creative Commons License CC-BY-NC-ND allowing non-commercial use, distribution, reproduction of the text, via any medium, provided the source is cited. © EDP Sciences, 2018 DOI: 10.1051/978-2-7598-2263-8.c003
16 Well seismic surveying and acoustic logging Table 1.1 Pros and cons of invasive and non-invasive methods. Method Target depth Vertical resolution Limitations Offshore application Non-invasive method S refraction 50 m, but sometimes less than 10 m in an industrial environment Several meters Affected by noise in an industrial environment, limited by velocity reversals Yes MASW 15 to 20 m Several meters Need for a tabular medium in the investigated area Yes AMV Several hundred meters Several meters to decameters Need for a tabular medium in the investigated area Not currently Logging Monopole sonic logging >1,000 m, for deep boreholes, of prospective exploration Several decameters Requires an uncased borehole and for Vs to be greater than Vp in the borehole fluid Yes PSSL or dipole sonic logging Up to 300 m for geotechnical deep boreholes 1 m Preferably in an uncased borehole but it’s also possible to use PSSL through a sealed case Yes, but with uncased holes Borehole seismic surveying Crosshole Up to 50 m, exceptionally 100 m 1 m Requires 2 boreholes, either uncased or with sealed casings and limited deviation Generally too expensive Downhole Up to 50 m, exceptionally 100 m, for S-waves. Can also be very deep for P-waves (> 1,000 m) Several meters Requires 1 borehole, uncased or sealed cased Yes for P-waves. Remains too complicated for S-waves Uphole 10 m for S-waves, 50 m for P-waves Several meters Difficult to have a powerful S source without damaging the casing Difficult to have an adapted S source Based on the capabilities and limitations of each method, described in Table 1.1, several choices are possible depending on the project’s objective and the level of knowledge about the site. These choices are summarized in Table 1.2. Finally, the economic aspect is also a key factor in choosing between the possible options.
17 1. Shear velocity measurement in boreholes Table 1.2 Method suitability according to objective. Requirement Modeling the ground response for the purposes of sensitive construction design Designing a construction to conform to the Eurocode 8 soil classification Level of knowledge about the site Overall knowledge of the site is insufficient to establish velocity logging of the ground under the proposed building Detailed knowledge (velocity logging) available at several points around the site, but not in the ground below the proposed building No special knowledge The Vs 30 is known in several points around the site, but not for the ground below the proposed building Recommended measurement Crosshole coupled with downhole or PSSL coupled with crosshole in the top 10 meters Downhole or PSSL coupled with MASW and AMV Downhole coupled with MASW and AMV MASW and AMV if space is sufficient and unaffected by multiple underground networks This chapter addresses invasive geophysical methods to obtain a Vs log, i.e. the following methods: downhole, uphole, crosshole and logging. The case studies presented are from EDF feedback from numerous soil surveys conducted over the last 5 years, as well as from the Inter Pacific research project whose objective was to compare geophysical methods for site seismic characterization (Garofalo, 2016). Non-invasive methods are not covered in this document. 1.1.2 Applications The most common application concerns the design and sizing of civil engineering structures. Indeed, in the Eurocode 8 soil classification, one of the three main parameters for classifying soils is based on a average of shear wave propagation velocity within the top 30 meters. This parameter is conventionally called “Vs 30”. Table 1.3 Mechanical parameters according to Vp, Vs and ρ (from Bourbié, 1986). Mechanical parameter Equation with Vp, Vs and r Young’s modulus, Edmax (Pa): ρV V V V V S P S P S 2 2 2 2 2 3 4− − Lamé constant, l (Pa) ρ V V P S 2 2 2− ( ) Bulk modulus, K (Pa) ρ V V P S 2 2 4 3 − ( ) The shear modulus, rigidity or Coulomb’s coefficient (Gdmax or µ in Pa) ρVS 2 Poisson’s ratio, g (without dimension) V V V V P S P S 2 2 2 2 2 2 − − ( )
18 Well seismic surveying and acoustic logging Moreover, for a given density, the P and S measurements in boreholes also enable the determination of in-situ parameters necessary to define a model that remains elastic under small deformations (Table 1.3). There are essentially two moduli: the Young’s modulus and the shear modulus. 1.1.3 Environmental conditions It is important to note that, in the geotechnics field, drilling for invasive measurements generally crosses unstable terrain. Hence, they are often lined with a casing or PVC tubing that is sealed to the formation with cement grout. Seal quality is paramount to enable a good transmission of seismic signals. Therefore, drilling and sealing must meet the standards described in ASTM D7400 for downhole and D4428 for crosshole. The key points of this phase can be summarized in Table 1.4. Table 1.4 Summary of hole parameters according to ASTM standards for geophysical borehole measurements. Max drilling diameter 175 mm (7 in) Internal casing diameter 50 to 100 mm (2 to 4 in) Tubing type PVC or aluminum, closed at the base Sealing grout Bentonite-cement grout with a density close to that of the surrounding soil to limit the energy loss by this waveguide For PVC-cased boreholes, it is possible to carry out a cementation control through a Cement Bond Log and Variable Density Log (CBL-VDL), even though this tool was initially developed for steel casings (see Chapter 3). The CBL-VDL log is a recording carried out with an acoustic tool centered in the borehole, with a piezoelectric source and receiver that are distinct with a distance between them of 3ft and 5ft for CBL and VDL respectively. If the coupling is good (low acoustic velocity contrast) most of the energy is transmitted and the energy of the refracted waves is low, which visually translates into low CBL and VDL amplitudes (gray rectangles in Figure 1.1). If this is not the case, the amplitude of the recorded waves is high. This signal is somewhat “rectilinear” in nature (red rectangle in Figure 1.1). It should be noted that if the formation has a high velocity compared to that of the casing (2,100 to 2,200 m/s for PVC compared to 5,600 m/s for steel), then the refracted waves in the formation will arrive first, thus masking the refracted waves in the casing (orange rectangle in Figure 1.1). On the other hand, if these waves dominate, it means that the entirety of the casing, grout and formation is adherent.
19 1. Shear velocity measurement in boreholes Figure 1.1 Illustration of a Full wave sonic log used as a CBL-VDL to evaluate the quality of the cementation (LIM Logging acquisition for EDF). This logging can be carried out with an acoustic tool of the sonic full waveform type. This is a necessity for a crosshole through an unstable formation (sandygravelly). Indeed, depending on the results, the allocation of emitting and receiving boreholes may be reviewed. 1.2 Vs measurements by borehole seismic method Borehole transmission seismic surveying can be illustrated as a special type of Vertical Seismic Profiling (VSP, see Chapter 2) where the focus of interest is not the waves reflected at the interfaces, but the time of first arrival of the transmitted wave free pipe good cimentation waves of formation
20 Well seismic surveying and acoustic logging between the source at the surface and the receiver in the borehole. This is illustrated in Figure 1.2 below. Figure 1.2 Illustration of the seismic raypath during a VSP or check shot (left) and a downhole (right). Reflected waves shown in purple; direct waves shown as dotted lines. Regarding the study of transmitted or direct waves, in petroleum seismic prospecting, we refer more to check shots and also to seismic coring. • In general, a check shot is carried out on the whole depth of an oil borehole (kilometric scale). It is used for seismic calibration, i.e. the geological characterization of seismic markers. Indeed, check shot results are presented in the form of graphs: time-depth, mean velocities of intervals, mean quadratic velocities as a function of the depth and the geological formations encountered in the borehole. • Seismic coring is usually carried out in a shallow borehole (hectometric or decametric scale). It is used to determine the weathering zone (WZ) parameters, particularly its thickness and velocity, which are essential parameters for static corrections. Often its acquisition follows an uphole configuration (source in borehole, such as dynamite) rather than downhole (source on surface). Geotechnics tends to focus on downhole (DH), and also on shear waves (S) in particular. For applications requiring a detailed evaluation of the Vs logging as a function of depth (modeling ground response for construction design purposes), we often favor the measurement of transmission between boreholes, enabling the establishment of a velocity profile with a metric step, referred to as crosshole.
21 1. Shear velocity measurement in boreholes 1.2.1 Downhole During a downhole acquisition, the seismic receiver is inside the borehole and the source is on the surface. Alternative configurations can be implemented with a seismic receiver in a cone penetrometer (Seismic Cone Penetration Testing - SCPT) or in a dilatometer (SDMT). In addition, a configuration with two receivers separated by a fixed distance is also possible. Depending on the type of strike on the ground surface (vertical or horizontal), the seismic source can generate a signal containing a maximum of energy in the compression wave (P) or the shear wave (S), see Figure 1.3. Figure 1.3 Typical configuration of a downhole where the shear waves are mainly recorded by horizontal components and the compression wave by the vertical component of the geophone in the borehole (according to SeisImager technical documents, 2013). 1.2.1.1 The acquisition device It is necessary to dissociate the acquisition and analysis of compression waves (P) and shear waves (S).
22 Well seismic surveying and acoustic logging 1.2.1.1.1 Sources The standard ASTM D7400-08 (Standard Test Methods for Downhole Seismic Testing) clearly defines the most common S source. It utilizes a beam of 2.4 m long and 15 cm thick, on which a vehicle can be positioned to ensure good coupling. An SH wave-rich signal is then obtained by lateral strikes on the sides of the beam, with a hammer of 5 to 15 kg. The hammer can be mounted onto a pivoting axis as shown in the photos in Figure 1.4. Figure 1.4 Illustration of an S source used in downhole testing which complies with the ASTM standard (IgeoTest source on the left, IMG source on the right). To generate P waves, an anvil can be vertically struck, either next to the beam on the axis between the midpoint and the borehole head, or next to each end of the beam. Choice of source location The distance between the midpoint of the beam and the probe (offset) must be carefully selected to limit the impact of tube waves, refracted waves and the angle of incidence. This distance is generally between 1 and 3 m. If a larger offset is required (4 to 6 m) or if the medium has several different velocity layers in the first 10 meters, then the refracted waves and actual paths (Snell-Descartes law) must be taken into account in the analysis (see Figure 1.5). The impact of tube waves is a reality that must be taken into account. Figure 1.6 shows downhole data from a borehole full of water (blue signal), and then without water (black signal). This shows the complexity of the tube wave signal: a time of 60 ms at 30 m is not consistent with a simple signal reflected at the bottom and transiting through the water. The velocity corresponds more to that of a guided mode transiting at the water-casing interface (see Chapter 2, paragraphs on acoustic waves).
23 1. Shear velocity measurement in boreholes Figure 1.5 Illustration of the difference between actual path and straight rays in a classic downhole analysis (left). On the right, a ray model after joint inversion (Fugro for EDF) of a crosshole and 2 downholes with different offsets, illustrating that refracted waves are more present when the source offset is increased. Figure 1.6 Illustration of a tube wave in a water-filled borehole.
24 Well seismic surveying and acoustic logging Figure 1.7 shows that air waves also exist in a borehole without water, but they are significantly less energetic (95 ms reflection at 30 m). The effect can be reduced by placing a plug at the borehole head. Figure 1.7 Illustration of a tube wave in a borehole without water. Depending on the velocity of the medium, the tube wave in the water can mask P or S arrivals (see Figure 1.8). To avoid this it is essential to empty the borehole as much as possible or to increase the distance between the source and the borehole head. However, this last option will facilitate the generation of refracted waves (see Figure 1.5). Figure 1.8 Illustration of tube wave impact (InterPacific Project), with a 3 m offset and borehole that has been emptied of water in the first example; with a 5 m offset and a borehole with water for the second example; and finally a third example showing a 3 m offset and borehole with water. This last example shows tube waves (InterPacific Project).
25 1. Shear velocity measurement in boreholes Seismic source coupling The coupling of the seismic source is also an important point (Figure 1.9). Ideally, the source should be positioned on natural ground (Figure 1.9a). In the presence of backfill, which strongly attenuates seismic waves (Figure 1.9b), it may be advisable to dig out the area until the natural terrain can be accessed. It should be noted that drilling and cementing techniques have a strong influence on signal transmission quality, as shown in the example in Figure 1.9c. a b c Figure 1.9 Effect of source coupling in downhole operation; a: Downhole with a source positioned on natural ground; b: Downhole with a source positioned on backfill (water table at 9.8 m); c: Downhole on the same site as for a and b, but drilling carried out with sonic corer (4 m water table, outside of water table the signal is polluted by refracted waves because the offset is too large).
26 Well seismic surveying and acoustic logging However, it is not always possible to situate the source on natural ground, but it is possible to make a short borehole to be able to use a borehole source directly under a backfilled area, in addition to that on the surface. However, it is important to use the surface source, because borehole sources are usually less powerful and there can be too much attenuation to make measurements for paths greater than 10 m (limitation on uphole measurement without the addition of a large number of stacks). 1.2.1.1.2 Receivers In downhole surveys, P-wave energy arrives mainly on the vertical component of the receiver and SH wave energy is distributed on the horizontal components. Given that a short “offset” was applied (1 to 3 m), it was necessary to pump the water out of the borehole to avoid tube waves. However, for a deep downhole, the casing should not be emptied to a depth greater than 50 m, otherwise the risk of crushing the tube becomes too great. Consequently, it is not possible to use hydrophones near to the surface to make a P-wave downhole survey. A 3-component receiver anchored to the borehole wall, either a geophone or accelerometer type, is therefore recommended. 1.2.1.2 Analysis of a downhole (DH) The initial step of the analysis involves picking the first P and S arrivals. A good quality signal will ensure that P-waves can be picked without difficulty. For S-waves, it is essential to exploit the polarization property of the wave according to the direction of the strike (Figure 1.10). For this purpose, the phase opposition of shots in the opposite direction makes it possible to unambiguously identify S arrivals (blue and black signals are of opposite sign, in Figure 1.10). Without this opposition, the recorded signal should be viewed with caution, because it may translate tube wave pollution or P-wave interference. Figure 1.10 illustrates that with a conventional source, the phase opposition characterizing the S-waves can be observed down to 50 m deep. In favorable terrain, the same types of signals are observed down to a depth of 100 m. As with seismic refraction, the first step of the analysis is to view the distance-time graph (source-receiver distance curve as a function of time). In the first 10 meters this curve differs significantly from the depth-time curve due to the offset of the source (see Figure 1.5). At this point, the path between the source and the receiver is considered linear. However, this approximation is false if the medium has noticeable velocity variations in the first 10 meters. The impact increases with the offset of the source. From the distance-time graph, a downhole analysis is carried out in depth ranges (slices). The division into slices must be related to the slope break on the distance-time graph, but also and especially in connection with the geological log obtained from cores. On the slices thus defined, the slope between the variations of distance and time of each segment provides the average velocity over the corresponding interval (see Figure 1.11). The slope of the radius linking a point to the origin gives the average velocity of the ground at the corresponding depth. Due to the picking uncertainty,
27 1. Shear velocity measurement in boreholes each slice must contain a minimum of 3 measurement points, ideally 4 or 5 (see Figure 1.12). As a result, it is not recommended to carry out downhole acquisition with 2 receivers simultaneously to perform velocity calculations based on these 2 measurements. Finally, we also note that a downhole analysis, even with a metric measurement interval, is more integrative than a crosshole measurement. Figure 1.10 S downhole, up to 50 m in depth (InterPacific). Figure 1.11 Illustration of a downhole (DH) analysis in P and S-waves based on 4 slices.
28 Well seismic surveying and acoustic logging Figure 1.12 Illustration of the sensitivity of the analysis of the same Downhole (DH) according to slice selection. Figure 1.13 Uphole and downhole result after inversion with GEOTOMGC software (Terradata for EDF).
29 1. Shear velocity measurement in boreholes To ensure the reliability of the analysis for the first 10 meters, a direct modeling or iterative inversion approach is possible (Figure 1.13). That is to say that, the direct calculation of travel times, for a given velocity model, must integrate the direct and refracted paths. Then, depending on the difference between the measured times and the calculated times, the model is adjusted. This iteration is repeated until the error criterion is acceptable. Currently, software packages available on the market are not optimized for borehole measurements and do not directly differentiate between direct and refracted waves but take straight and curved raypaths into account. The straight rays correspond to the transmitted travel paths and the curved rays to the refracted waves. 1.2.1.3 The specific example of offshore It is possible to carry out a P downhole survey at sea in an uncased borehole, using a source such as an air gun or sparker near to the surface. It is, however, much more complicated to find an S source that is operational underwater and a 3-component receiver. Finally, if the ground is unstable, a P downhole survey through an unsealed protective casing can work if the terrain crossed is sufficiently plastic to establish an acceptable coupling with the casing (see Figure 1.14). Figure 1.14 Downhole at sea in an uncased hole and through an unsealed protective casing (Fugro for EDF).
30 Well seismic surveying and acoustic logging 1.2.2 Uphole An uphole is similar to a downhole, but with the source in the borehole and the receiver on the surface. The practice is more limited because S sources in boreholes generate a signal that fades over a relatively short distance. Even for P-waves, with a relatively powerful source (sparker) it is necessary to perform a large number of stacks. However, Figure 1.13 shows that depending on the context it is possible to obtain interesting results with P-waves (left graph). In addition, it can be useful to carry out an uphole when the measurement is coupled to crosshole acquisition, because this only requires the addition of a 3-component receiver on the surface. 1.2.3 Crosshole The principle consists of measuring the travel time of compressional and shearing seismic waves, transmitted directly between a seismic source in a borehole and a 3-component receiver in a neighboring borehole. The compressional wave (P) propagates through the horizontal plane as well as the horizontal component of the shear wave (SH). The vertical component of the shear wave (SV) propagates in the vertical plane. Therefore, the energy of the P and SH wave is mainly distributed on the horizontal receivers and that of the Sv wave on the vertical receiver. The result of a crosshole measurement is often blindly accepted as a reference, but the method can face the following limitations, it can: • be strongly affected by refracted waves in surrounding indurated strata, • present “spatial aliasing” if bed thickness is less than that of the intervals, • be affected by highly inclined formations, • be strongly affected by poor cementation (casing-sealant-formation connection), • be strongly affected by damage in the vicinity of the borehole. Consequently, it is recommended to perform a downhole and/or uphole recording to complete and validate the crosshole, assuming that the anisotropic attenuation is negligible. Specific cases: • With highly heterogeneous geology, only seismic tomography can provide reliable information for the P-wave profile. Therefore, in an environment with a known heterogeneous nature, it is also recommended to carry out an additional P crosshole acquisition with a hydrophone array so as to exploit the tomography data. For S-waves, the implementation of more than 2 anchored sensors is complex in the geotechnical field and therefore downhole or uphole recording generally has to suffice. • Similarly, if the test is carried out in an environment with high horizontal anisotropy (e.g. beds fractured in a certain direction), the measurement system and
31 1. Shear velocity measurement in boreholes results interpretation must be adapted (receiving boreholes at 90° with directions parallel and perpendicular to those of the anisotropy). • Finally, if the test is carried out in a medium with strong vertical anisotropy related to thin beds, the measurement system must also include a specific SH source (sparker P-SH). 1.2.3.1 Prerequisites The quality of the casing sealing has a much stronger impact on the crosshole measurement than it does on the downhole measurement, because the signals are of higher frequency. Velocity calculation requires the precise knowledge of the distance between the boreholes. Human error is possible with inclination measurements (see Figure 1.15) and a calibration defect can affect the trajectory measurements (see Figure 1.16). It is therefore vital to apply a procedure on the surface to control for the directions given by these measurements. Figure 1.15 Example of a result of 2 distinct trajectory measurements and one inclination measurement in the same borehole (InterPacific Project). Figure 1.16 Example of a result of 3 distinct trajectory measurements in the same borehole (InterPacific Project).
32 Well seismic surveying and acoustic logging 1.2.3.2 The acquisition device It is necessary to dissociate the acquisition and the analysis of the compression (P) and shear (S) waves. Until the start of the 1980s (Figure 1.17), the crosshole test was carried out as the source moved forward by striking the head of the drill string. This method has evolved through the development of an emission probe pressed against the borehole wall. This has the major advantage of separating the activities of the borehole drilling group from those of the geophysics measurements team. However, this brings source coupling problems. Figure 1.17 Historical crosshole measurement method, applied in France until the beginning of the 1980s. The typically recommended crosshole method (Figure 1.18) requires 3 boreholes to calculate the velocity between two receiving boreholes without needing precise knowledge of the wave generation time (t0, zero time). Today, this approach is questionable because very often the attenuation is such that the signal of the second borehole is difficult to exploit, and also the shot recording is no longer a technical limitation. Nevertheless, working with only one receiving borehole requires greater rigor, in particular, a calibrated measurement chain is required. As an example, there can be drift between the electric impulse and the generation of a sparker signal (Figure 1.19) due to the wearing of electrodes.
33 1. Shear velocity measurement in boreholes Figure 1.18 Illustration of a 3 borehole crosshole system. Figure 1.19 Illustration of a P source: sparker (SolGéo). Moreover, having a single receiver borehole can be disadvantageous if we want to pick the first arrival time at the maximum wave amplitude (apex), rather than at its beginning. Admittedly, this technique makes it possible to work with signals with a signal-to-noise ratio that is not so good. Moreover, due to the signal attenuation,
34 Well seismic surveying and acoustic logging too much noise can make it difficult to identify the start of the wave, which is the case particularly with the signal in the most distant borehole. 1.2.3.2.1 Sources To maximize the energy of shear waves, it is recommended that a source that is mechanical (hammer-anvil) or electromechanical (similar to the sources of the PS Suspension Logging (PSSL) tool) should be anchored to the wall (Figure 1.20). Figure 1.20 Illustration of a mechanical (Ballard anvil hammer) and an electrodynamic (SolGeo, mass moved up or down by an electromagnet) S source. 1.2.3.2.2 Receivers For P-waves, energy is distributed over the horizontal components, which can generate difficulties in picking the first arrivals and requires a mathematical polarization operation to project the energy in the source-receiver direction. Some receivers can be oriented in a given direction. However, the most simple and robust method is to use hydrophones to measure the P arrivals. It should be noted that some hydrophones are directly equipped with an amplification system to optimize recording dynamics. For S-wave analysis, only a single 1-component receiver is needed. However, having a 3-component receiver associated with the mechanical source makes it possible, if required, to verify P measurements with the sparker-hydrophone combination (Figure 1.21).
35 1. Shear velocity measurement in boreholes Figure 1.21 Hydrophone with an amplification system and 3C geophone with an anchoring system (Solgéo). Sampling Crosshole data acquisition requires small time sampling intervals. 50 μs sampling is acceptable for recording S-waves, whereas higher frequency P-waves require a faster sampling rate until 2.5 μs (Figure 1.22). a b c Figure 1.22 P recording in the same borehole with a sampling rate of (a) 2.5, (b) 32.25 and (c) 62.5 µs.
36 Well seismic surveying and acoustic logging 1.2.3.3 Crosshole analysis At first glance crosshole analysis appears simple and robust, and only requires the picking of the first arrival of the selected wave (P or S), and to divide this travel time by the distance between the source and the receiver, to obtain the velocity of the medium. However, several cautionary notes, described below, should be considered. As discussed in the section about receivers, P interpretation using geophones requires the polarization of horizontal traces to optimize the signal amplitude. However, sometimes the result of this operation is less clear than when looking at the results of one of the two components, and sometimes the operation is complex, such as when the receiver has six horizontal components. Moreover, in alluviums, particularly outside of the water table, sometimes the attenuation of P-waves is such that they are no longer distinguishable from noise. Two causes explain this problem: • Firstly, drilling and cementing are delicate in this type of terrain and the result of cementing can be highly variable according to the drilling technique used. For this type of terrain there is a risk of forming a thick cement annular that does not adhere to the formation, which can create a waveguide that filters P-waves more than S-waves, because they are of higher frequency (often 1,000 Hz compared to 200 Hz). To limit this effect, the use of a sonic corer is recommended, to avoid destructuring the terrain. • Secondly, the mechanical source generates insufficient P-wave energy for this type of very attenuating terrain, so a more powerful P-wave source, such as a sparker, should be used. Figure 1.23 illustrates the above comments. The two crossholes were made in distinct sites, but both had sandy-gravelly alluvial cover down to 22 m and 21.6 m respectively, over a clay or marly substratum. In both cases, there is a change in the frequency content between the alluvium (low frequency signal) and the substratum (high frequency signal). However, in the second case the first identifiable arrivals in the first 19 meters are not P-waves, because the times correspond with those of the S-waves probably picked on the horizontal components. On the other hand, in the first case it is possible to identify P arrivals over the entire depth. Attenuation mainly affects distant receivers in crossholes with several receiving boreholes. This justifies making an acquisition with a specific P source that is more powerful than a mechanical source, which is made to maximize the energy of the S-waves. In addition, it is entirely possible to limit the crosshole to two boreholes, but only on the condition that the results are consistent with those of a downhole and/or uphole made in one of the two boreholes. The crosshole method can also be affected by refracted waves (especially when the offset is greater than 6 m). The example presented in Figure 1.24 illustrates that it
37 1. Shear velocity measurement in boreholes is possible to detect refracted waves and confirm their existence solely based on the visual analysis of the signals, crossed with the information from the cores. Figure 1.23 Examples of crosshole recording. a) left, P Crosshole (InterPacific, water table at 3 m), b) right EDF site (water table at 6 m). On the S-wave crosshole recording, on the left of Figure 1.24a, there is a fast S first arrival at 22 and 23 m in depth (shown in red). At greater depths, this S arrival is later. There are therefore two areas for which S velocities can be calculated at 500 and 260 m/s (see velocity model on the right). Directly under the fast formation, at 24 and 25 m deep, we observe a first arrival with little energy (shown in green). This is a refracted arrival, with the transmitted arrival (shown in red) keeping the same phase. The phenomenon can be explained by calculating the distancetime graph for example at 2 m from the interface between the two media (i.e. 25 m deep, see Figure 1.24b). Figure 1.24b shows that at the distance of the first borehole (X = 3 m) the direct wave (P: solid blue line, S: solid green) arrives first, while in the second borehole (X = 6 m) it arrives after the refracted wave (blue dotted line for the P-wave and green for the S-wave). By carrying out this analysis at depths of 24 and 26 m, we can then draw the fastest paths, which are illustrated by the green arrows on the left of Figure 1.24a.
38 Well seismic surveying and acoustic logging a b Figure 1.24 Crosshole examples; a: left, S recordings between 22 and 29 m in depth and, right, associated geological model; b: P and S time-distance curve, with a measurement level at 2 m from the interface with a faster strata (based on the bottom right model). The above approach requires time and experience. It is therefore difficult to generalize to apply to all applications. The ASTM standard recommends the implementation of a computer analysis to take into account the different possible paths (inversion). In practice, this was not carried out in France, but has started to be used since 2015. Figure 1.25 shows an example of tomography results obtained with inversion software.
39 1. Shear velocity measurement in boreholes Figure 1.25 Crosshole result after inversion with GEOTOMGC software (Terradata for EDF). 1.3 Vs measurement by logging 1.3.1 Acoustic logging with high vertical resolution In geotechnology, the logging tools that can be used are generally of a small diameter (<50 mm) and non-stackable, as is practiced in the field of oil exploration. The conventionally used acoustic probes are therefore short probes (3.5 m), composed of a high frequency monopole-type piezoelectric transmitter (10 to 40 kHz) (see Chapter 3). It is important to remember that this means that the source is omnidirectional and that it generates P-waves in the fluid. These waves are refracted (P, S) at the level of the borehole wall and guided modes also appear. These are, in particular, Rayleigh waves for a solid-air interface and the Scholte-Stoneley wave for a solid-fluid interface. Therefore, the existence of refracted shear waves implies that the P velocity of the fluid is lower than the S velocity of the formation for an uncased borehole. Vs measurement with acoustic logging is therefore recommended for fast formations.
40 Well seismic surveying and acoustic logging When the formation is slow (Vs formation < Vp fluid), the Scholte-Stoneley waves can enable the estimation of S velocities with an acoustic tool. It is nevertheless necessary to occasionally have the means of tying the conversion between the Scholte-Stoneley velocities and the shear velocities (for example, a downhole log with multi-metric measurement levels). Figure 1.26 is an example of Full Wave Sonic (FWS) logging obtained with a monopole tool. On the right the figure shows the acoustic section obtained with a sourcereceiver offset of 4 ft and on the left the velocity logs of the different wave trains (P refracted, S refracted and Stoneley). Figure 1.26 Example of sonic FW recording (monopole), showing a low energy P first arrival, for which the arrival time decreases significantly after 225 m (entry into the rock), the S arrival is only visible below 225 m where the medium has a velocity that is considered as fast compared to the mud (rock) [SEMM logging for EDF].
41 1. Shear velocity measurement in boreholes In slow formations, the measurement of S velocities cannot be obtained by using monopole-type tools. The oil industry has developed dipole-type tools including polarized transmitters and receivers that generate and record bending waves, the propagation velocity of which is close to the S velocity of the formation (see Chapter 3). The problem is that these oil industry tools are rigid and very long. However, a flexible alternative exists for the geotechnology field: PSSL. 1.3.2 PS Suspension Logging (PSSL) PS Suspension Logging (PSSL) is a flexible logging probe for non-continuous measurement with an electromechanical drill source and two receivers (see Chapter 3). It should be noted that the frequencies used by the PSSL enable measurements to be made in boreholes with PVC casing. Seismic wave velocities are calculated from the difference between the arrival times between the two receivers (Near and Far) spaced 1 m apart. As a result, the tool needs to be centered in the borehole, but since it is flexible (see Figure 1.27) and it is difficult to find centering rings that are adapted to the borehole, this is not always possible. a b Figure 1.27 PSSL tool a) set up of a PSSL tool with a continuously active drill rig (Fugro for EDF), b) typical centering ring component of PSSL tool.
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