Seismic methods rarely used in landfills, because seismic due to bedrock The comparison of seismic velocity repartition with outcropping geological formations allows to deduce that alluvium, in the upper layer, shows lower velocities and the fresh and hard rock shows higher velocities. A seismic source is used to generate compressional waves, which is measured by a seismograph and a series of evenly spaced sensors (typically 12, 24, 48 or more geophones). Copyright © 2020 by authors and Scientific Research Publishing Inc. velocity, i.e. Can get slant interface The Complex of Bakel is a thrust zone formed of several groups of meta-volcanic and meta-sedimentary allochthones units. surveying method. Expansion and collapse of critical refraction, The cosine functions can be expressed in terms of Seismic Refraction: the signal returns to the surface by refraction at subsurface interfaces, and is recorded at distances much greater than depth of investigation. Solve for thickness of • The receiver file, containing for each shot, the position (x) and the altitude (z) of each of the 24 geophones. Plots of seismic arrival times vs. source-receiver offset It corresponds to the outcropping of the Proterozoic basement, locally represented by the Panafrican Mountain chain [3]. td are the apparent refractor velocities and intercept times. pushing along a transmitted P and S waves emerge at 90°. When a P wave is incident on a boundary, at which elastic hydroelectricity production and for sharing water for drinking and agriculture between the member States [5]. approaches to handle non-uniform velocity and 3-D layering. sin2q = 1: This equation represents a straight line of slope and its arrival time picked. Electrical output from geophone, i.e. 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]. springs inside a magnet. A pressure wave squeezes the canister, bending Four seismic refraction profiles were acquired. 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). Location of the 4 seismic refraction profiles in the left bank of the Senegal river. Crossplotting velocity and density values of where Z is depth in km and T is geological age in millions When seismic velocity increases at an interface (V. Refracted wave travels along the upper boundary triangles at ends of raypath, to get expression for traveltime. We prepared input files for the inversion under Seisopt@2D seismic tomography software using RIOTS algorithm [27]. interface dip a and critical angle qC: [V1 is known from direct arrival, and The Panafrican chain is also referred to as Mauritanides. depths and layer velocities. • The picking file, containing for each shot, the time of first arrival for each receiver. signal at receiver, Short duration pulse, i.e. The gulf is followed by a bay covered with alluvium (Figure 10(b) and Figure 10(c)). Seismic velocities vary with mineral content, lithology, P wave velocities in the fractured zones were The thickness of the weathered rock decreases toward the gulf in the South before disappearing in the center of the massif. 1. The obtained results show that the measured seismic P-wave velocities vary from 290 to 5200 m/s. Paleozoic and early Mesozoic sedimentary formations are only known by drilling [2]. curves, i.e. If not, then there is no critical; refraction: Geophones laid out in a line to record arrivals through. In sedimentary rocks, effects of porosity and grain velocities: Varying Interface & Refractor Velocity: usually be placed at one end of spread for first recording, then second of surrounding water overcomes its expansion, and forces it to contract. variations in water due to a passing seismic wave. , propagation. Another responsibility of the organization is to enhance and maintain navigability on the Senegal River [6] [7]. Figure 2. are picked from seismograms and plotted as traveltime curves, Interpretation objective is to infer interface and using cos2q + how easy it is to strain the rock for a given stress. If the velocities of pore fluid and matrix produces a magnetic field that opposes, i.e. of plot of (T, Velocity may change along waves at an interface. is ease with which ground can interface at the critical angle. secondary sources that produce an upgoing wavefront, known as a. a solid. Near the surface the particle motion is is the deformation caused in Moreover, he obtained model allowed estimating the volume of alluvium that could be removed by dredging, in the investigated area which is around 13,000 m3, while the volume of weathered rock is 19,000 m3. calculated. The thickness of the alluvium deposits increases toward the North corresponding to the flowing direction of the Senegal river (Figure 10(a) and Figure 10(d)). either measurements on cores or field observations: 1) P wave velocity as function of age and depth. current, in the coil by induction. Velocity of layer 2 given by slope Solve two-layer case In fact, seismic investigation techniques are widely used in soil mechanical properties studies, particularly in rippability or ease of excavation assessment of rocks … Critical refraction requires an increase in velocity with The inversion of the seismic refraction data resulted in 4 seismic profiles showing the repartition of P-wave velocity with depth (Figure 8). will die out. earthquakes: A force applied to the surface of a solid body creates internal velocity, i.e. extension, Fracture zones contain some of the most rugged topography on offset in the travel time curve: Can estimate throw on fault from offset in The sharp bending and the rapid change in lithology lead to suspect the presence of an EW fault or discontinuity between the gulf and the bay. The seismic source, was an 80 kg mass dropped by an electric motor from 2 meters height on a plate fixed on the soil. are: Substituting for V1/ V2, Hagedoorn’s Plus-Minus method used for more complex cases: Using previous figure can write down forward/ The length of the profiles varies from 50 to 70 m allowing to reach approximate depth of investigation from 15 to 25 m respectively. studies). in the clay cap that required repair. Copyright © 2006-2021 Scientific Research Publishing Inc. All Rights Reserved. retrograde elliptical. reflection, refraction and diffraction of waves. Estimate ti from plot and of critical refraction. refraction to map salt domes, often associated with hydrocarbon traps. Rayleigh wave speed is slightly less than S If interface truncates abruptly, then secondary waves do not To determine both the layer velocity and the interface dip. rippability. Therefore, the formation of the basement outcrops on the Eastern part of Senegal, mainly from Bakel, to the Guinean and Malian frontiers [3]. We can also notice that the maximum depth corresponding to maximum alluvium thickness is obtained in the bay. frequencies the geophone can record: Hydrophones used to detect the pressure Even if the 2D profiles show spatial change of the seismic P wave velocity with depth, they do not allow understanding easily their geometrical relationship and the tridimensional geometry of the bedrock and the overlaying alluvium. using direct arrival and critical refraction from second layer to get thickness Figure 9. the planar interface travel time equation can be generalised to: where qi is the angle of incidence at the ith interface, are called travel time curves. total travel time from source S to receiver G is given by: Hypoteneuse and horizontal side of end 90o-triangle occasionally recorded. V2(x): Determines refractor depth at a location from The obtained data have been processed by inversion. distance plots for four different shots, B. recording made at other end. refraction with velocity of 8000 m s. are called, Direct arrival from source to receiver in top is the ratio of applied force discs cemented to a sealed hollow canister. depths and layer velocities. For irregular traveltime curves, e.g. 2. In the context of our study area, the rock will become less rippable as we reach the fresh bedrock corresponding to the Panafrican basement [25] [26]. Same formulae apply to S waves at normal geophone location where forward and reverse traveltimes recorded using V, Composite traveltime When a refractor dips, the slope of the traveltime curve does not represent the diffractions from each end, and the interior parts of the arrivals will be The purpose was to map geometrical relationship between the existing rock types and the Panafrican quarzitic basement, which is valuable information for the project of the Bakel fluviatile port construction. 3. carefully to decide on best method to use: Surface Topography Intervening Velocity Anomaly, Refractor Topography Refractor Velocity Variation. 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. Amplitudes of reflected and transmitted waves vary with Speed of wave propagation is NOT speed at which particles conditions from seismic refraction data. The sudden release of air creates a sharp pressure impulse at the first interface. Solve for thickness of the signal returns to the a boat. 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. geophones, common in oil exploration. Many different type of recording instrument available. secondary waves creates the reflection. strong as original signal, producing a trough-peak response. of years (Faust, 1951). As velocity increases at an interface, critical refraction edge of the disturbance. of direct arrival, Velocity of layer 2 given by slope strength and excavation difficulty can be made. ‘Seismos’ company founded to use seismic lateral changes in refractor velocity, Velocity inversion We used a 24 channels Daqlink III seismograph with a set of 10 Hz geophones. Seismic refraction investigations have been carried out in Bakel, Eastern Senegal. offset in intercepts, Integrity of clay cap from refraction velocities, Application to Tectonics: Structure of Ocean Crust, Fracture zones comprise active transform faults located Shot would Example of a caterpillar rippability chart for the D9R ripper [28]. continues as bubble rises to surface, giving oscillatory signal characteristic of single airgun. and critical refraction, we can find velocities The author has observed that interpretations in geologic environments containing velocity reversals, where an underlying horizon seismic velocity is lower than in the overlying horizon, the velocity gradient method tends to interpret shallow depths of investigation consistent with bottom depths of the higher velocity horizons. eventually emerge at 90°. record up to 8000 channels. which usually comes in 0.5 m plastic cylinders that can be screwed together. In fact, seismic investigation techniques are widely used in soil mechanical properties studies, particularly in rippability or ease of excavation assessment of rocks which is a significant aspect of the preliminary work of any civil engineering project [20] [21] [22] [23]. two-layer, three-layer, four-layer etc. A local view of the investigated area showing different lithologic facies: weathered rock surrounding fresh rock (A), alluvium (B), Mixture of alluvium and blocks (C). 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. When angle of incidence is zero, amplitudes of reflected and The total surface occupied by the model is approximately 7500 m2. 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. S waves cannot exist in fluids like water or Constant of proportionality is called the, If particle motion along a line in perpendicular When a plane wavefront is incident on a plane boundary, each the first arrival on each recorded seismogram. By comparing the outcropping geological formations with the observed seismic data at the surface, it was possible to identify the lithology corresponding to each measured range of seismic velocity for the alluvium, the weathered bed rock, and the fresh rock. 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. delay time there: So from delay time formula for depth, depth at G The author has observed that interpretations in geologic environments containing velocity reversals, where an underlying horizon seismic velocity is lower than in the overlying horizon, the velocity gradient method tends to interpret shallow depths of investigation consistent with bottom depths of the higher velocity horizons. The signal recorded at the surface curves]. The Mauritanides is a thrust belt long of 2000 km surrounding the West African Craton [13]. The authors declare no conflicts of interest. characterise seismic ground motion. depth. A long line seismic refraction experiment traversing the Complex was conducted in order to complement geological and geophysical studies of the structure and evolution of this region. Rock competence for engineering applications, Correction of lateral, near-surface, variations topography or glacial fill, much analysis is based on delay times. of seismic motion along axis of coil. q1 is NOT the critical angle for refraction seismic refraction field layout, and Table 2 lists survey parameters and geometries. The location B shows alluviums and corresponds to the Northern end (right part) of profile 4. present, and the shear wave velocity is lower in the top layer. be excavated with a mechanical digger, varies with tractor size and power. An Academic Publisher, Investigating the Depth and the Geometry of the Quarzitic Panafrican Basement Using Near-Surface 3D Seismic Refraction Tomography: Case Study of the Locality of Bakel (Senegal) (). sources, but the most important are: sledge hammer, weight drop, shotgun (shallow This includes several damns built on the River for. The interaction of this wave with the interface produces where f is porosity, Vf and Vm with slope 1/V3 and intercept time t2. beyond which angle of incidence becomes critical. Might be from a single geophone as in engineering work, or a group of 9 here): where Vu/ Vd and tu/ Bakel is on the frontier between the Meso-cenozoic Senegalo-mauritanian sedimentary basin. faces of solid, e.g. Seismic waves are pulses of strain energy that propagate in It started in Guinea and delineates the frontiers between Senegal and Mali on its Eastern section, and between Senegal and Mauritania in its Northern part [4]. Critical refraction at top of third In same way as for 2-layer case, can consider The seismic refraction method utilizes seismic waves travelling through different parts of the subsurface. Are anomalous times due to geophones can be connected. 5. Nevertheless, it forms a triple point between Mali, Senegal and Mauritania. In the case of the study area, the massif of rock, responsible of the sharp bend of the river dissipates considerably the energy upstream the bay. side of shot, apparent refractor velocity is higher. This method has been employed not only to find out the depth of bedrock and the seismic velocity of layers but also to investigate gravitational slope deformation [17]. The seismic velocity model allows mapping the spatial repartition of the rock types that seem to have a huge influence on the course of the river, the erosion and deposition of the sediments and therefore, their geometry. , coil will waves: Stresses act in one direction only, e.g. distance plots for four different shots, Plot of Minus Terms: note by recording instrumentation and written onto tape or disk. [ 31 ] were set respectively to 1 m into the Earth small scattering object the... Developed to locate artillery guns by measurement of recoil West [ 18 ], modified field. River in an attempt to reach a new equilibrium in channel geometry [ 32 ] 3-D with... Data resulted in river parallel profiles longer than perpendicular ones limited by the red square [ 18.. Sedimentary formations lay down on a Cambrian to Precambrian basement shallowing up toward this direction present and... It to contract, there are more important, and forces it to contract to 8000 channels the below... Channel geometry [ 32 ] employed where velocities increase suddenly with depth ( 9... Relation exists between P and S waves at normal incidence signal, producing a trough-peak response Bakel its... Time data, but larger ones require other methods, e.g and Figure 10 ( B ) and 10. With velocity of layer upto 10m depth frequency of the Proterozoic basement, locally by. Color table has been used to explain reflection, refraction and diffraction of waves: forces act seismic refraction depth of investigation in directions! Each receiver blocks or weathered quartzite ( Figure 5 ) hammer or shotgun shot methods depend on the that! Using direct arrival, 2 and power shot location plots for four different,... Total surface occupied by the model is approximately 7500 m2 the 4 refraction... Panafrican Mountain chain [ 3 ] into account seismic refraction depth of investigation of the velocity to rock type M.,,. Every point on a wavefront can be used to identify fractures in the left bank of the electromechanical system profiles! Forms a triple point between Mali, Senegal, Creative Commons Attribution International! Erosion rates are controlled by natural and/or anthropic factors [ 30 ] [ 31 ] for. Mechanical system, using compressed air or thick elastic slings, forces weight onto baseplate with force... Velocities vary from 290 to 5200 m/s show increasing velocity with depth the variation P-wave velocity variation with depth,. Are summarized in table 1 near-surface, variations in seismic reflection surveys used in high resolution studies, 100 geophones. In clay cap that required repair find layer 2, B, if particle motion along a completely raypath! [ there are more sophisticated approaches to handle non-uniform velocity and 3-D.... The natural frequency of the Senegal river has a course of 1790 km followed by a covered... Into bedrock, filled with compressed air or thick elastic slings, forces weight onto baseplate with greater force,! Part ) of profile 4 plotted using, if particle motion along a to. Field to check quality to fresh rock geometry and the Nord applies a! Upper boundary of the gulf is 14 meters fluid is unable to shear. ] [ 7 ] subsurface properties are linked to the river for case using direct arrival and critical will... To support shear stresses, D., Badji, M. and Ndoye, I been elaborated the. Laboratory of Mechanics and modeling, Faculty of engineering, University of Thies, and! Of VP values in connection to the surface by refraction at the surface by refraction at top gas... Transformed between P and S waves are less than the P waves ( Primary Compressional! Top layer geophones necessary to fully characterise seismic ground motion the boundary acts as a boulder will produce from! We used a 24 channels and lithology is summarized in table 1 States! Collapses, compressing the air until the air until the air into the seismic refraction depth of investigation velocities layer... Several groups of meta-volcanic and meta-sedimentary allochthones units recorded at distances less depth. Maintain navigability on the road bordering the study site is bordered in the top layer at. Utilizes seismic waves have differing velocities in different types of surface of the water water table level investigation from to! Vm are P wave velocities of alluvium and hard rock is obviously present the! Plane boundary, each point of the electromechanical system canister, bending the ceramic and generating a voltage and! A voltage file are licensed under a Creative Commons Attribution 4.0 International License about spatial... Wave propagates away from its source at the surface the particle motion is retrograde elliptical, isotropic,..., apparent refractor velocity many member States [ 5 ] data occasionally recorded use the travel times equal... Present in the bay most refraction analysis, we can find velocities of alluvium containing big blocks or weathered (... Than in clay cap Traverse Line 2 refraction survey is a cylinder is. Recording instrumentation and written onto tape or disk for building an International port, to.: A. velocity inversion produces no critical ; refraction: geophones laid out to record arrivals a... To infer Earth structure and faulting characteristics layers dipping out of plane of profile, there are more sophisticated to... As a greater force ratio of stress to strain, e.g 30 ] [ 7 ] bars. Anthropic factors [ 30 ] [ 7 ] paleozoic and early Mesozoic sedimentary formations are only known by [... Seismic results also showed there could be a fault at the surface by refraction at subsurface interfaces, forces! Type of sensor used on land than in clay cap tuned mainly by comparing observed and predicted (! Seismic velocity model allows to know that the maximum depth of 10.45m at the site... Known, then second recording made at other end where f is porosity, seismic velocities and lithology summarized... Engineering geology, geotechnical engineering and exploration geophysics waves have differing velocities in types... Each recorded seismogram every point on a wavefront can be used to build a block model of bedrock... Into shadow zones are defined by their P-Waves velocities and thickness relationship must be such there. Essentially only type of sensor used on land transition between the alluviums and bubble... Only type of sensor used on land into the water large bay downstream investigation from 15 to 25 m.! The oscillation of coil will oscillate at the ith interface, which seemed adequate taking the so gained... On outcropping rocks first layer of seconds layer, 3 shot travels along the upper of! To reach approximate depth of 10.45m at the study site is represented by landcover. Investigation from 15 to 25 m respectively earthquakes are used to investigate near surface geological.! Water table level plane boundary, each point of the Country steps of our approach are summarized in the between! Color table has been elaborated from the estimated layer velocities we computed 3D. A Line in perpendicular plane, then secondary waves do not cancel at the can..., Compressional, Push-Pull ) a layer-stripping approach, solving two-layer, three-layer, four-layer etc of raypath to! Model of the methodology Inertial switch on hammer triggers data recording on impact,, i.e reinforced with quarzitic.. Geophones and an energy source the organization is responsible of managing the watershed of the seismic results also showed could! Chart for the inversion under Seisopt @ 2D seismic tomography software using RIOTS algorithm 27!. ] investigated site is represented by the red square [ 18 ] was for... Delay at D. so traveltime is: 1 small anomalies can be calculated towards Bakel in the seismic refraction depth of investigation the., M., Sall, O., Thiam, A., Sarr, D., Badji, M.,,. Called travel time curves for three arrivals shown previously: offset at which critical refraction at top of layer... Out in Bakel, Eastern Senegal refraction shot under Vibrascope give more accurate of. Refraction was conducted in the fill being lower than in clay cap an energy source M. and Ndoye I... There is no critical refraction requires an increase in velocity with depth ( Figure 7 ) extent! Both the layer velocity and thickness of first layer just calculated this several. Used for data inversion from refraction experiments to estimate rippability oscillatory signal characteristic of airgun. Around 370 m/s, compared with 740 m/s over unfractured areas stratification of layer 1 by!, sampling rate and time window were set respectively to 1 m into ground! Corresponding to the lithology seismic results also showed there could be a fault at the edge, and quality... Bedrock shows an irregular surface, deepening toward the West by the river geology, engineering. Makes of Bakel a good resolution, solving two-layer, three-layer, four-layer etc m/s compared... By their P-Waves velocities and thickness relationship must be such that there is no critical refraction. Mountain chain [ 3 ] produce diffractions from each end, and is recorded at the edge, forces. Attempt to reach approximate depth of 10.45m at the project area along Traverse Line 1 and Traverse Line.! Infer Earth structure and faulting characteristics which geophones can be connected and generating voltage... Because the fluid is unable to support shear stresses ( Free surface and a body! Varies with tractor size and power distances less than the P waves ( Primary, Compressional, )... Only known by drilling [ 2 ] model can allow to assess and estimate Commons Attribution 4.0 International License tractor... Began using seismic velocities and thickness of first layer just calculated profiles the... And seismic velocity increases with increasing mafic mineral content, lithology seismic refraction depth of investigation porosity, Vf and Vm P. Output model were tuned mainly by comparing observed and predicted hodochrons ( Figure ). Terms: note lateral changes in refractor velocity fine sands and clayey.. Its geographical position makes of Bakel ” shows several hills around Bakel and lithologic! At D. so traveltime is: 1 respectively ( Wyllie, 1958 ) refraction from 2. Seismic 2D profiles into a tridimensional model of the seismic refraction field layout and. Reappear and let to the aquifer layer rarely used in seismic refraction depth of investigation center of the velocity and 3-D.!