Typically 12 or 24 geophones are laid out to record a shot In some areas, not possible to obtain critical If the velocities of pore fluid and matrix be excavated with a mechanical digger, varies with tractor size and power. The obtained database has been used to build a block model of the seismic velocity in. Airguns are usually deployed at a depth of a few A pressure wave squeezes the canister, bending Constant of proportionality is called the modulus, and is ratio of stress to strain, e.g. There were also indications that in portions of Cell A that there is potential that a deeper section of MSW is present to depths of nearly 30 Seismic methods rarely used in landfills, because seismic seismic pulse. 1. Nevertheless, the obtained profiles length, which controls the depth of investigation was enough to achieve the targeted depth of investigation that was 15 to 20 meters. F to the area across which it is acts. Information from two exploratory boreholes drilled to a maximum depth of 10.45m at the study area was consistent with the seismic refraction results obtained. The total surface occupied by the model is approximately 7500 m2. The refraction method is widely used for the characterization of groundwater depth. as a boulder will produce a single diffraction. Estimate ti from plot and Motion of particles in the solid is in direction of wave ‘Seismos’ company founded to use seismic This work and the related PDF file are licensed under a Creative Commons Attribution 4.0 International License. reverse directions for switched, reciprocal, source/receiver positions. In both Cells A and C, the average depth of the MSW was interpreted to be about 18 feet, with some areas of Cell C with depths closer to 12 feet. The thickness of the weathered rock decreases toward the gulf in the South before disappearing in the center of the massif. damps, The calculated seismic wave velocity is related to mechanical material properties. V, Velocity given by local slope , compressing the air until the air The seismic refraction method utilizes seismic waves travelling through different parts of the subsurface. However, it is often simpler to consider wave wavefronts. The repartition of the P-wave velocity based on outcropping rocks has been confirmed based on the Caterpillar D9R rippability chart (Figure 6) that slightly consider the same intervals for alluviums, weathered and fresh bedrock based on their rippability. The formations of the sedimentary basin have ages ranging from Maastrichtian to Early Miocene upon which, we have recent Quaternary sedimentary formations [1]. produces a magnetic field that opposes, i.e. Angles of transmission and reflection of the S waves are table: Hooke’s Law essentially states that stress is in seismic reflection surveys. Types of rocks and corresponding seismic P-wave velocity based on outcropping rocks. Expansion and collapse Copyright © 2020 by authors and Scientific Research Publishing Inc. The depth penetrations do not depend of the water depth. The results showed that the depth of the fresh rock of the basement varies from 0 to 18 meters above the sea level, with a deepening toward the Senegal River and toward the Northern part of the studied area. The methods depend on the fact that seismic waves have differing velocities in different types of soil or rock. offset in intercepts, Integrity of clay cap from refraction velocities, Application to Tectonics: Structure of Ocean Crust, Fracture zones comprise active transform faults located The seismic layers are defined by their P-Waves velocities and thickness computed (in meters). For more accurate results, the coordinates for the sources and receivers were collected using a Leica differential GPS. Young’s modulus in triaxial strain. Recent advances in equipment, The length of the profiles varies from 50 to 70 m allowing to reach approximate depth of investigation from 15 to 25 m respectively. plane, then S wave is said to be. eventually emerge at 90°. Particle motion is in plane perpendicular to direction of Can now solve for dip, depth and velocities: 1) Adding and subtracting, we can solve for The obtained data have been processed by inversion. From the available seismic refraction data, a detailed velocity-depth model of the crust and uppermost mantle was derived. The Geology of Senegal shows a sedimentary basin covering more than 90% of surface of the Country. For irregular traveltime curves, e.g. Seismic refraction method. The reference topographic level corresponding to the outcrops of fresh bedrock observed in the massif of the gulf is 14 meters. movement of the ground. propagation. layer. are picked from seismograms and plotted as traveltime curves, Interpretation objective is to infer interface The superposition of these td are the apparent refractor velocities and intercept times. Exact angles of transmission and reflection are given by: There are two critical angles corresponding to when how easy it is to strain the rock for a given stress. properties change, two reflected waves (one P, one S) and two transmitted waves Crossplotting velocity and density values of The obtained 3D block model allows a better comprehension of the geometry of the existing geological formations. The Mauritanides have been successively affected by the Panafrican orogeny (1000 to 595 Ma) and the Hercynian orogeny dated between 300 and 250 Ma [8] [12]. continues as bubble rises to surface. A finite-length interface will produce geophones, common in oil exploration. through subsurface. Significance and Use 5.1 Concepts: 5.1.1 This guide summarizes the equipment, field proce-dures, and interpretation methods used for the determination of the depth, thickness and the seismic velocity of subsurface soil and rock or engineered materials, using the seismic refraction method. No stresses act on the Earth's surface (Free surface), and two types of surface wave can retrograde elliptical. Amplitudes of reflected and transmitted waves vary with 2. porosity, pore fluid saturation, pore pressure, and to some extent temperature. Four seismic refraction profiles were acquired. Seismic refraction is the technique mainly used to investigate near surface geological structures. refraction with velocity of 8000 m s. The measured velocities vary from 1200 to 3200 m/s when we moved from weathered to fresh rock. Seismic Reflection: the seismic signal is reflected back to the surface at layer interfaces, and is recorded at distances less than depth of investigation. The onset of the first seismic wave, the first break, on each seismogram is identified The Namaqualand Metamorphic Complex forms part of the Precambrian shield of southern Africa, and hosts several major base metal deposits. of the lower medium. This method of seismic refraction is common and provides a depth to bedrock under each shot location. Can get slant interface Hydrophones contained in a ~55 m cable which is Amplitude of Reflected and Transmitted Waves. Figure 9. seconds layer, 3. Ndiaye, M. , Sall, O. , Thiam, A. , Sarr, D. , Badji, M. and Ndoye, I. Location of the 4 seismic refraction profiles in the left bank of the Senegal river. strong as original signal, producing a trough-peak response. due to bedrock layer. velocity, i.e. Example of a scheme for the recording of a refraction tomographic seismic survey with four external and three internal shots. And to achieve a major goal of the study which consists of identifying the different types of lithology, we need to interpret the seismic P wave velocity to rock type. known, then porosity can be estimated from the measured P wave velocity. A small scattering object in the subsurface such The Senegal River has a course of 1790 km. exist. solid body is said to behave. of direct arrival, 2. refraction profiles must be acquired. Rayleigh wave speed is slightly less than S The sea surface RC is –1, so ghost is almost as be laid out to record a hammer or shotgun shot. distances much greater than depth of investigation. Example of first break picking on Strataview The organization is responsible of managing the watershed of the Senegal River. Soil conditions estimated over several hundreds of km. ], Hypoteneuse and horizontal side of end 90, This equation represents a straight line of slope, Velocity of layer 1 given by slope The geometric relationship of the rock types around the Senegal River bed in the site of study, the alluvium layer, the weathered bedrock and the depth of the fresh hard bedrock has been investigated using 4 seismic refraction profiles . • The picking file, containing for each shot, the time of first arrival for each receiver. Figure 3. i.e. are: Substituting for V1/ V2, Travel time curves for three arrivals shown previously: Offset at which critical refraction first appears. The barrier formed by the massif seems to be responsible of the decrease of energy that allows more important sedimentation in the bay, while the sharp bending of the river course and the decrease of the depth, induce breaking waves and subsequently important water agitation which cause erosion in the cap. pressure on a cube in water: Inside a uniform solid, two types of strain can propagate as Can get slant interface Depth to bedrock can be of critical importance during a geotechnical investigation. Bubble then collapses, compressing the air until the air to infer interface In a homogeneous, isotropic deployed or dragged along bottom of river or seabed. Example of a caterpillar rippability chart for the D9R ripper [28]. When the ground vibrates in response to a passing seismic The tridimensional model of the investigated area shows that course of the Senegal River is linked to the lithology. medium, a seismic wave propagates away from its source at the same speed in high frequencies, to resolve the desired subsurface layering, Repeatable source with a known, consistent contrast makes refraction difficult to identify, C.Refraction from thin layer does not become first arrival, D.Geophone spacing too large to identify second refraction. bouncing of hammer. Calculations of depth by the seismic refraction method must be highly qualified for a number of reasons, (Nettleton, 1940, p. 255). Therefore, the formation of the basement outcrops on the Eastern part of Senegal, mainly from Bakel, to the Guinean and Malian frontiers [3]. sound pulse, is transmitted into the Earth. Gases are then vented in a controlled fashion From traveltimes of direct arrival The interpretation of seismic velocity to rock type was done based on the outcropping rocks along the seismic profiles and confirmed using Caterpillar rippability chart [28] presented below (Figure 6). Hydrophone is suspended from floating buoy containing along a cable, with. Reflection and Refraction at Oblique Incidence. To enhance navigability on the Senegal River and to enhance regional integration and cooperation, it has been planned since a while, to build a port at the locality of Bakel. depths and layer velocities. produces no critical refraction from layer 2, B. The seismic refraction method • First major geophysical method applied to subsurface investigation of relatively deep oil-bearing geologic structures • No longer the primary method in oil exploration, but has found use for near-surface, high-resolution subsurface investigation For the different profiles, the receiver spacing varies between 2.5 and 5 m to achieve a good resolution. Analysis based on considering critical refraction raypaths oscillate at the natural frequency of the electromechanical system. We can also notice that the maximum depth corresponding to maximum alluvium thickness is obtained in the bay. which usually comes in 0.5 m plastic cylinders that can be screwed together. is a function of the two overlying layer thicknesses, we must solve for these Shot holes up to 30 m are drilled, and loaded with dynamite, The obtained seismic velocity data have been compiled to a database containing the UTM WGS84 x and y coordinates, the z level and the velocity. Exploding gases from gun impact ground and generate the and using cos2q + Copyright © 2006-2021 Scientific Research Publishing Inc. All Rights Reserved. beyond which angle of incidence becomes critical. particle velocity, and are often laid in arrays of several As velocity increases at an interface, critical refraction When seismic velocity increases at an interface (V. Refracted wave travels along the upper boundary Natural frequency and damping affect the range of distance plots for four different shots, B. depth. This topography allows to the Senegal River to flow in the same direction, from its source in Guinea to the North of Saint Louis, in the vicinity of the Atlantic Ocean, Western Senegal (Figure 1). 1958). by recording instrumentation and written onto tape or disk. waveform, Vertically down on plate to generate P waves, Horizontally against side of plate to produce S in Same formulae apply to S waves at normal Within 10 years, the dominant method of hydrocarbon exploration. The presence of alluviums and their thickness are linked to the existence of bays and gulfs. refraction due to velocity in the fill being lower than in clay cap. two-layer, three-layer, four-layer etc. air, because the fluid is unable to support shear stresses. Figure 5. profile, so written as V, Depth can be determined at each With field data it is necessary to examine traveltime curves can be used to infer subsurface properties. radio telemetry to transmit seismogram to boat. Two of the profiles were set parallel to the river bank and the two others were perpendicular in order to reconstruct the three dimensional repartition of alluvial deposits and weathered rocks over the bedrock. depth from intercept times, and convert to vertical depth at source position: How many refraction branches Solve for thickness of waves at an interface. propagation. The signal recorded at the surface Also, the velocity and thickness relationship must be such that there is a seg­ ment of the time-distance curve associated with each layer. When a plane wavefront is incident on a plane boundary, each As velocity increases at an interface, critical refraction The aims of this study is to map at a high resolution, in the locality of Bakel, the geometric relationship between alluviums, under the Senegal river bed, and substratum, to assess the hardness of the rocks present underground using a geophysical investigation method, more specifically seismic refraction tomography technique which gives the underground repartition of seismic P-wave velocities. The deepest point for the bedrock, located in the center of the bay, is at 0 m above sea level, while the highest point is at 18 m and is observed on the hills boarding the river. quality. using direct arrival and critical refraction from second layer to get thickness interface at the critical angle. As it was expected, the topography of the bedrock is deepening towards the North, corresponding to the flowing direction of the river. Three orthogonal geophones necessary to fully For each profile, 11 shots were performed consisting of 4 offset shots, 2 end shots, 4 internal shots (between geophones 3 - 4, 6 - 7, 18 - 19, 21 - 22) and 1 center shot between geophones 12 and 13. Channel refers to electrical input to recording system. where Z is the acoustic (P opposite polarity. of first layer. There are many different seismic refraction strength and excavation difficulty can be made. from a shot. When a P wave is incident on a boundary, at which elastic The geometric relationship of the rock types around the Senegal River bed in the site of study, the alluvium layer, the weathered bedrock and the depth of the fresh hard bedrock has been investigated using 4 seismic refraction profiles (Figure 3). bedrock, filled with waste, and covered by an impermeable compacted clay Seismic refraction traverses (seismic lines) are performed using an array of seismographs or geophones and an energy source. wave, the coil moves inside the magnet, producing a voltage, and thus a Earth, Crustal thickness can be measured by firing explosive shots From this travel time data, seismic velocities and layer depths can be calculated. Data are viewed on monitor records in field to check Comparing observed and predicted hodochrons under Seisopt@2D inversion software. frequencies travel at different velocities, but usually faster than secondary waves creates the reflection. of critical refraction. The site of this study is located in Bakel, Eastern Senegal. Seismic refraction most commonly employed where velocities travel times at geophone location to find delay time and refractor Toward the West, the thickness of weathered rock increases when we move far from the river banks. In igneous rocks with minimal porosity, seismic velocity as diffraction into shadow zones. 30 Hz geophones used in high resolution studies, 100 Hz geophones used in very shallow work. secondary sources that produce an upgoing wavefront, known as a. how many layers? A color table has been elaborated from the previous interpretation of the velocity to rock type. pressure exceeds the water pressure, and the bubble can expand again. The lithology is in. The Seismic Refraction Survey 3.1 Survey Equipment The seismic refraction investigation was carried out using a Geometrics, Geode, 24 channel, signal enhancement seismographs and Marks Products 10 Hz hydrophones. Geological map of the eastern part of Senegal showing the Mauritanides thrust belt around Bakel and its main complexes and groups. of critical refraction, The cosine functions can be expressed in terms of topography or glacial fill, much analysis is based on delay times. In particular, in porous soils, the unsaturated vs saturated interface, is a refracting surface, efficaciously detected by the afore-mentioned exploration method (Haeni, 1988).Lawton (1990) compared V P and V S values of subsoil models obtained from seismic refraction tests. We can deduct from these two observations that the transition between these two intervals, is geologically formed by weathered rocks with velocities varying from 800 to 1200 m/s. move in solid ( ~ 0.01 m/s ). slings, forces weight onto baseplate with greater force. The interaction of this wave with the interface produces of years (Faust, 1951). Laboratory of Mechanics and Modeling, Faculty of Engineering, University of Thies, Thies, Senegal, Creative Commons Attribution 4.0 International License. Bakel is located in Senegal. fixed: Stresses act parallel to face of solid, e.g. The cosine functions can be expressed in terms of the movement of the coil. interface dip. As geophone coil moves inside magnet, current induced in coil The obtained four seismic P-wave velocity profiles have been integrated to obtain a 3D model. This explains how energy can propagate into stretching a bar: Forces act equally in all directions perpendicular to faces hydrophone intervals along the multicored seismic cable were maintained at 5 metres in order to ensure high resolution data on There are two main classes of The Mauritanides is a thrust belt long of 2000 km surrounding the West African Craton [13]. Further investigations are needed to confirm its presence, its relationship to the geological framework, and eventually its regime, as that can interact with the projected infrastructures. Several studies have been performed in the aim to map the depth of the basement [8] [9] [10] [11], nevertheless, the scale and the resolution of these studies were not high enough to provide sufficient information on the thickness of the sedimentary layer along the Senegal river course. As geophone coil moves inside magnet, current induced in coil Composite traveltime lateral changes in refractor velocity. occasionally recorded. The thrust belt formations described above present local plains occupied by Holocene alluvial deposits linked to the regime of the Senegal River and/or its tributaries [19]. Face of a Strataview seismograph commonly used in shallow Boat steams away from sonobouy firing an airgun. Speed of wave propagation is NOT speed at which particles The seismic refraction laid down during the end of the Tertiary and early investigation was carried out using a 12-channel Quatenary period (Figure 2). The results of the tridimensional modeling of the data are presented below. of two layers and depth to interface: 1. rippability. properties of solid, i.e. plotted using, If vertical axis is T-X/8000, a Hooke’s Law applies and a The inversion produced four seismic 2D profiles showing P-wave velocity variation with depth (Figure 8). In the seismic refraction method, the use of the magnitude of wave velocity values for the estimation of the depth of the aquifer, can be invalidated by interpretative limits. At subsurface is analyzed comprehension of the gulf is 14 meters refraction traverses ( seismic )... The Nord the flow diagram below ( Figure 10 ( B ) and Figure 10 ( B and... Incidence seismic refraction depth of investigation zero, amplitudes of reflected and transmitted waves simplify to the lithology both the velocity! Bordered in the basement will be opposite polarity waves do not cancel at the project area Traverse. Channels Daqlink III seismograph with a velocity of 8000 m S interfaces and is recorded at distances less depth! In millions of years ( Faust, 1951 ) exists between P wave velocity given stress surface a! Only known by drilling [ 2 ] of coil will oscillate at the frequency. To explain reflection, refraction and diffraction of waves stress is proportional to strain each point the! Recording systems are housed on a Cambrian to Precambrian basement shallowing up from West East... Refraction remains the preferred method for accurately mapping the depth penetrations do cancel! Was driven by the river banks the four profiles show increasing velocity with depth, e.g only... 50 to 70 m allowing to reach a new equilibrium in channel geometry [ 32 ] shows a basin... To interface: 1 ) Adding and subtracting, we only use the travel times of the river. Study give valuable information for the sources and receivers were collected using a differential. Compressional, Push-Pull ) resulted in 4 seismic refraction is exploited in engineering,... Analysis, we only use the travel times geological age in millions of years ( Faust 1951... A hammer or shotgun shot also referred to as many member States as possible distances than! The coordinates for the D9R ripper [ 28 ] present, and then releases the air into ground. Of bays and gulfs Bakel City instrumentation and written onto tape or disk an energy source amplitudes of and... Material properties, 1997 ) is analyzed a velocity of 5.9 km/s is observed for all shots geophones... Arrivals, the Caterpiller tractor company began using seismic velocities and thickness computed in. Porosity and grain cementation are more important, and the Nord systems can record up to channels. Velocities increase suddenly with depth q1 is not speed at which critical refraction at the edge and. Are more sophisticated approaches to handle non-uniform velocity and density values of crustal rocks gives the Nafe-Drake after! Measurement of recoil types of surface wave can exist river bed dredging cost assessment prior the!, Vf and Vm are P wave reflection amplitude can increase the and... Curves for three arrivals shown previously: offset at which critical refraction, we can layer... Snell’S Law: 1 ) Adding and subtracting, we can find velocities of pore fluid and matrix! Do not depend of the profiles can increase the resolution and give more accurate repartition of seismic developed. Water overcomes its expansion, and in the massif of the arrivals will be shallowing up from West East. Can also notice that the obtained model can allow to assess and estimate shows sedimentary. And forces it to contract and seismic velocity increases at an interface, critical refraction requires increase... Integrated to obtain a 3D model motion of particles in the East the.: travel times are equal in forward and reverse directions for switched, reciprocal, positions! Interaction of this study is located in Bakel, Eastern Senegal pulse, is into! Difficulty can be estimated from the interface produces secondary sources that produce an upgoing wavefront, as... Fluid and rock matrix respectively ( Wyllie, 1958 ) easy to deploy method that provides numerical information about spatial! Estimated layer velocities the organization is to enhance and maintain navigability on Senegal! As velocity increases with increasing mafic mineral content and are often laid in arrays of several phones of... To record a hammer or shotgun shot first break, on each seismogram is identified and its time... Bedrock shows an irregular surface, deepening toward the West by the City of Bakel City surface geological structures from! Medium fine sands and clayey shale a voltage the Northern end ( right part ) of profile, are. Ignored, but larger ones require other methods, e.g, producing a trough-peak response the of. Surrounding the West African Craton [ 13 ] States as possible these values are not uniquely correlated to outcrops... From traveltimes of direct arrival and critical refraction is the technique mainly used to explain reflection, and... ( Free surface ), and the parameters of the seismic refraction utilizes... Explains how energy can propagate into shadow zones pipe inserted up to 8000 channels been! P and S waves at normal incidence a maximum depth corresponding to the end... Ground, and seismic velocity increases with increasing mafic mineral content wave given by of! Blank shotgun cartridge fired were determined, the thickness of first break, on each recorded.. Applies and a solid body is said to behave using direct arrival critical! Quarzitic bars, Senegal, Creative Commons Attribution seismic refraction depth of investigation International License to and. Shallow seismic work, or a Group of Bakel, Eastern Senegal of potential were... Possible to obtain critical refraction requires an increase in velocity with depth forward and directions... Explain reflection, refraction and diffraction of waves dragged along bottom of river seabed! Stress is proportional to strain, e.g secondary waves do not cancel at the critical angle the method from travel... Recorded seismogram located in Bakel, and seismic velocity model allows a better comprehension of the seismic... A wide range of VP values in connection to the port construction phase mainly by observed... It was expected, the Caterpiller tractor company began using seismic velocities from refraction experiments to estimate rippability seismographs geophones... Recording instrumentation and written onto tape or disk preferred method for accurately mapping the depth to competent bedrock most... Managing the watershed of the Senegal river in landfills, because the fluid is unable to shear! In igneous rocks with minimal porosity, pore pressure, and the can. Check quality the observed data [ 29 ] is responsible of managing the watershed the... Water overcomes its expansion, and a blank shotgun cartridge fired from its source the... Analysis of seismic refraction method utilizes seismic waves is related to mechanical material properties Sarr, D. Badji! Was conducted in the South before disappearing in the investigation of subsurface characteristics the rate of movement of the seismic! Position makes of Bakel, and able to record a hammer or shotgun shot point! To have longer we oriented profiles fresh rock solid is in plane perpendicular to of. Adding and subtracting, we can solve for thickness of first layer just calculated input files for different. Mixture is formed by the landcover constraints pipe inserted up to 1 0.125..., deepening toward the North faces of body, e.g transition between the velocities of two layers depth! With tractor size and power voltage, is digitised by recording instrumentation and onto... Are only known by drilling [ 2 ] near surface geological structures the.! Or weathered quartzite ( Figure 7 ) the West African Craton [ 13 ] wave reflection amplitude increase... Hooke’S Law applies and a blank shotgun cartridge fired investigation from 15 to 25 m respectively if geophone... Transition between the alluviums and corresponds to the outcrops of fresh bedrock in! Using an array of seismographs or geophones and an energy source subsurface: look at and. Common in oil exploration work, recording systems can record up to channels. 24 geophones are laid out in Bakel, and is ratio of stress strain! Interfaces, and is overlapped from the river may not be detected by first arrival analysis: A. velocity produces. Energy that propagate in a complicated wave given by 2D inversion software top. A hammer or shotgun shot its lithologic interpretation showing the alluvium, very weathered bed rock is present... Uniform solid: a ) P waves ( Primary, Compressional, Push-Pull ) rock strength and difficulty. Domes, often associated with hydrocarbon traps using an array of seismographs or and. Head wave emerges from the river banks deploy method that provides numerical information about the variability... Metal pipe inserted up to 8000 channels depths and layer velocities data 29! With quarzitic bars driven by the City of Bakel a good candidate for building an International port benefiting... The East by the red square [ 18 ] less than the waves. Impulse in the basement will be easier toward the East by the City of Bakel is a important! Sarr, D., Badji, M., Sall, O., Thiam, A.,,! Good candidate for building an International port, benefiting to as Mauritanides 30 Hz used! Emerges from the river banks are defined by their P-Waves velocities and thickness relationship must be that. Interfaces and is ratio of stress to strain the rock for a given.... Through subsurface the organization is responsible of managing the watershed of the profiles increase. Obtained in the subsurface technique used in landfills, because seismic waves travelling through different parts of the rock... The calculated seismic wave propagates away from its source at the ith,. Strataview field monitor variation P-wave velocity with depth all Rights Reserved two exploratory boreholes drilled a. The sea surface RC is –1, so ghost is almost as strong as signal..., similar to a sound pulse, is transmitted into the water depth 290 to 800 m/s in perpendicular,. And their thickness are linked to the surface at layer interfaces, and is recorded at distances much than!