Ground Penetrating Radar (GPR)

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• It uses Multi   frequency electromagnetic waves to Monitor the subsurface and solve the   engineering problems (ASTM D6432-99). Frequency is changeable   depending on the target depth. Energy is radiated downward into the ground   from a transmitter and is reflected back to a receiving   antenna.  The reflected signals are recorded and produce a   continuous cross-sectional “picture” or profile of shallow subsurface   conditions.   Reflections of the radar wave occur where there is a   change in the dielectric constant or electrical conductivity between two   materials.  As much as variation in soil is large there is a   variation in the dielectric constant and hence the radar method is successful   in monitoring the target, and solves problems (e.g. a block of rock in sand).   We hence used the GPR in solving lots of engineering problems. 

• As large contrasts in electrical properties between soil components increase, as GPR are images become more successful. Examples of large contrasts in electrical properties between two components are: rock inside soil and metallic objects inside soil. More subtle features can also be detected with GPR, such as dry soil vs. saturated soil, shallow stratigraphy, and shallow contamination, among others. Because this method uses travel time to record reflections from different layers it is highly precise and may be used to measure the thickness of surface materials (such as concrete) or the depth to a given interface. The GPR method is ideal for the vertical and lateral delineation of features that have widths and depths as small as a few inches (such as voids, or fractures) to as large as many feet (such as USTs, utilities, tunnels or stratigraphic layers), depending on soil conditions. One of the most famous and successful work of the GPR is the archeology. GPR can be used to show hidden rooms, buried temples and many other application. 

• Sinkhole        and cavity detection
• Utility        detection
• Micro-tunnels        problems
• Determining        the lake beds bottoms
• Locating        rebar in concrete
• Mine        Detection
• Mapping        buried wastes.
• Fault tracing
• Geological structures
• Archeology        

• The GPR is a device containing both a transmitter and a receiver. GPR is pulled along the ground surface to monitor the subsurface. The transmitter radiates high-frequency electromagnetic energy (center frequencies of 25 MHz up to 1 GHz) into the ground. The frequency of the used GPR is inversely proportional to the depth of penetration. This electromagnetic wave propagates into the subsurface at a velocity that is dependent on the relative dielectric constant of the medium through which the wave travels. When it encounters the interface of two materials having different electrical properties, a portion of the energy is reflected back to the surface. As conductivity of the soil increases, the attenuation GPR energy increase. When the target of a GPR survey is metallic, the characteristic response is readily identified because the electromagnetic wave is completely reflected upon reaching the metallic object (much the way a mirror works).
• Once the reflected signal is detected at the receiver antenna, it is transmitted to a control unit (notebook or Smart Cart) and displayed as a vertical GPR profile on the digital video logger in real time. GPR data are usually collected in “wheel mode”, where a digital odometer used to measure the distance of the collected profile. Before designing the survey, we should know the target depth and medium velocity to be able to use the suitable GPR antenna. Digital data are viewed by the operator in real time on a screen, and saved for later processing. Elfarouk uses 2D and 3D processing programs which is the latest technologies for monitoring the subsurface using the GPR technique.

•     Ground penetrating radar provides engineers a non-intrusive solution to the hidden geological hazards. A GPR survey carried out prior to the onset of construction operations can mitigate many geological hazards and optimize the engineering solutions. A GPR inspection can locate many geological hazards such as cavities, sinkholes or faults. For example buried cavities are a hazard, both to engineers and the general public. They can impede construction operations, undermine building foundations and be the cause of destructive ground subsidence. 

•  Many geological hazards can be identified with GPR method such as, cavities, sinkholes in karstic limestone terrain, unknown basements and culverts, abandoned wells and mineshafts, all of which present serious hazards. Geological hazards are always different geological structures compared to the surrounding soil. For example Cavities are air or water filled structures while surrounding soil is formed from fractured limestone rocks for example, this result in different electrical properties leads to easy identification by GPR method. Other geological hazards such as shallow faults or sinkholes  can be identified easily using the GPR method. 

•   GPR data can be taken in the form of network or a grids in case of critical geological hazards. The acquisition can be made using a grid of 1m spacing in both X and Y directions .
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• By this way slice depths of GPR can be given to summarized pictures of the subsurface at certain depthes. what is found in the survey by this way can be woderful. It is like a contour of GPR radargrams taken in all directions and give what is found at any depth you select. it is like a plan view at certain depth collected from all GPR cross sections in X and Y direction. 
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• This is a very important technology applied by Elfarouk since long period of time and proved to be valid for solving many problems. We can trace subsurface faults, Bedding planes with deffects, Fractures, Lenses of clays and lenses of rocks and many other applications.

•  Techniques are based on the response of the earth to the flow of electrical current. Measurements are made by placing four electrodes in contact with the soil or rock (ASTM G57-06). A current is caused to flow in the earth between one pair of electrodes while the voltage across the other pair of electrodes is measured.  The depth of measurements is related to the electrode spacing. Several types of electrode configuration and survey geometry exist in resistivity measurements. Depending upon the survey geometry, the data are plotted as 1-D sounding or profiling curves, or in 2-D cross-section in order to look for anomalous regions and monitor the subsurface.  2D and 3D imaging use multi-electrode arrays (sometimes known as ERT) to monitor the subsurface soil problems for depths of several hundred meters. 

• The electrical resistivity of a material is a measure of the ability of that material to transmit an electrical current. In the electrical resistivity method a DC circuit is established in the ground via cables and electrodes, and the ground acts as the resistor to complete the circuit. There are several different arrays that can be used to collect the data; however, the most common are Wenner, Schlumberger and dipole-dipole. Electrical resistivity data are typically displayed in 2D sections or profiles where they supply lateral and vertical electrical resistivity information about materials either directly below a given transect (much like a road cut); or between two boreholes in the case of ERT.

• Delineation of large voids or caves 
• Ground      water exploration
• Find      source of underground water polluting lands
• Show      geoelectric model of the strata and compare with geologic conditions
• Detect      lateral changes and locate anomalous geologic conditions 
• Measure      soil resistivity for estimating metal corrosion rates and designing      grounding grids 
• Map      saltwater contamination plumes 
• Locate      buried wastes (e.g. locate landfill boundaries) 
• Delineation      of buried stream channels

• In practice, a linear array of electrodes is established in the ground and connected to the resistivitymeter. Once this is done, a known amount of current is introduced into the ground through a pair of electrodes (current electrodes). This current then travels through the ground and the electrical potential is commonly measured by two other electrodes (potential electrodes) some distance from the current electrodes, where the larger the separation between the current and potential electrodes the greater the depth of the measurement. During a resistivity imaging  survey, many apparent resistivity measurements are made for a suite of electrode pair separations, and these apparent resistivity values are plotted on a two-dimensional section, where the surface location of the measurement is plotted versus the depth of the measurement. The automated resistivity data acquisition provided allows for a tremendous amount of data to be acquired relatively quickly with very high lateral and vertical resolution, resulting in a 2D subsurface image representing the lateral and vertical variation of apparentresistivity along the line. Once the data have been acquired for a given transect, they can be downloaded to a field computer and subsequently viewed, color-contoured, and processed by 2D and 3D inversion programs to produce resistivity models. These model sections is correlated with the geological settings and drilling logs then interpreted for features of interest. 

•   Measuring resistivity data can be in 1D, 2D and 3D. Electrical resistivity data can be collected using different electrode arrays such as Schlumberger, Wenner, dipole-dipole, pole dipole and pole-pole arrays or any other arrays can be modeled using advanced Geoelectrical resistivity inversion programs. Inversion is defined as the process of determining the estimates of the resistivity model parameter on the basis of the data and the model. Inversion is a mapping from data space to model space, and it reconstructs the subsurface resistivity distribution from measured voltage and current data. Inversion is also known as inverse modeling, inverse simulation, and inverse problem. 

• Electrical resistivity tomography method (ERT) can be used to detect weak points in the soil such as cavities, sinkholes or dangerous gas pockets. The parameters of the survey are chosen depends on a number of factors. For example in cavity detection, we should take care of as the size and depth of anticipated cavity, reason for delineating Cavity, desired resolution of cavity, nature of background materials or bedrock surrounding the cavity, type of materials that may fill the cavity (such as clay or water), depth to groundwater, size of the investigation area and sources of cultural interference in the investigation area. 

•  Once the data have been acquired for a given transect, they can be downloaded to a field computer and subsequently viewed, color-contoured, and processed by 2D and 3D inversion programs to produce resistivity models. These model sections is correlated with the geological settings and drilling logs then interpreted for features of interest.   

• The electrical resistivity of a material is a measure of the ease with which an electrical current can flow through that material. Since most minerals are insulators, electrical current flow through sedimentary soils and rocks is primarily electrolytic and takes place through pore spaces, along grain boundaries and through fractures. As a result, permeable materials (such as coarse sands or sandstones) are less resistive (or more conductive) when saturated than when dry. In addition, because ionic conduction is enhanced by the presence of dissolved salts in the pore fluid, soils and rocks saturated with salineor high-TDS groundwaterwill have significantly lower levels of resistivity than soils and rocks bearing fresh water.

•  In Crosshole seismic survey three boreholes are drilled (ASTM D4428 M-84) One is used for the source of seismic waves, while the other two boreholes are used as receivers to record the transmitted seismic waves. The source and receivers are lowered simultaneously (e.g. each one meter).data are recorded. By knowing the distance between the source borehole and the receiver boreholes and Time difference between recorded signals (travel times for the waves), the corresponding seismic velocity can be calculated (P-wave or S-wave).In downhole seismic test the seismic source is a sledge hammer placed at the top of the well, while receivers are lowered in just one recording well. 

• Downhole seismic is a surface method that uses the properties of acoustic and shear waves to generate a detailed vertical profile of the variation of seismic velocity (SH) with depth. These vertical profiles are used to calculate the elastic constants for foundation studies and to measure Vs30 for earthquake design ground motion.

• Borehole P-wave and s-wave determination for engineering site investigation.
• Determination of the shear rigidity      constants of the soil
• Liquefaction assessment.
• Seismological studies
• Resolves hidden layer velocity
• Depth to the bedrock
• Stratigraphic delineation 
• Calculation of the design shear wave velocity Vs30  

• Like seismic refraction, the downhole seismic method is based on seismic theory, which tells us that a seismic wave travels at the velocity of the material that it is currently traveling through, and when a wave reaches a boundary between two materials having different seismic velocities, that seismic wave will be refracted (or bent) either toward the normal to the interface or away from the normal to the interface (depending on whether the velocity increases or decreases at the boundary). Once the wave enters the new material, it will travel at the velocity of the new material. However, unlike seismic refraction, downhole seismic is also based on the assumption that the first arrival on the seismograph from a geophone at a given depth is from the direct wave, since the waves travel nearly vertically. Because the downhole seismic method makes use of the direct wave it does not have difficulty resolving hidden layers and does not require that layer velocities increase with depth. As a result, this method is highly reliable for identification of all seismic layers of interest, and is particularly useful for resolving thin layers or velocity reversals that the seismic refraction method will not detect.

•  Typically two different types of shots are made for a given depth – one shear wave shot and one p-wave shot. In order to create a shear wave shot a source is needed that generates shear waves. This shear wave source usually consists of a long plank weighted down by a vehicle or a shear wave “brick” secured to the ground. The plank or brick is generally established close to the borehole such that its length is equally distributed on either side of the borehole, and is oriented in a predetermined direction. Once the shear wave source has been established, hammer blows to the side of it are made – first on one side and then on the other side. This procedure is necessary because the shear wave is polarized: a blow to one side of the source will cause an excursion to the right on the seismograph and a blow to the other side of the source will cause an excursion to the left. Comparison of seismic records resulting from blows to either side of the plank or brick ensures that the shear wave is properly identified. In order to create a p-wave shot vertical hammer blows to an aluminum plate are made.

• Once a particular shot is made, the seismograph then records the time at which the first arrival from the seismic shot is received at the downhole seismometer. With a multi-channel seismograph shear wave and p-wave arrivals for a given depth can be routed to separate channels via a control box. The geophone is then lowered a small distance down into the borehole, secured, another shot is made and the process is repeated until the desired depth is reached. Once all the data have been collected for a given borehole, a time-depth plot of seismic first arrivals is made, which can be used to calculate the velocity of the materials through which the wave has traveled.
• We are using advances software for data analysis for getting shear wave reversals ad VP/VS variation with depth. Downhole and crosshole methods are the traditional methods used to obtain shear wave and p-wave profiles of the subsurface for geotechnical design, and as such are commonly used for calculations of Vs30, liquefaction assessment, and calculation of geotechnical design parameters such as Modulus of elasticity (shear modulus), Poisson’s ratio, bulk modulus and Young’s modulus 

• Magnetic   geophysical surveys measure small, localized variations in the Earth's magnetic   field. The magnetic properties of naturally occurring materials, such as   magnetic ore bodies and basic igneous rocks allow them to be identified and   mapped by magnetic surveys. Strong local magnetic fields or anomalies are   also produced by buried steel objects. Magnetometer surveys find basement   igneous and metamorphic rocks, underground storage tanks, drums, piles and   reinforced concrete foundations by detecting the magnetic anomalies they   produce.  

• magnetic anomalies depend on many factors, such as the amount of magnetite or pyrrhotite in the rock, the parent magnetic field strength and direction, the depth of the rock beneath the ground surface, and structural lineaments such as fractures and faults.
• The magnitude of the induced field (and hence the anomaly) is proportional to the intensity of the earth’s magnetic field and the magnetic susceptibility of the underlying material. 

• The case of exposed igneous rocks, the most likely type of remanent magnetism is thermoremanent magnetism, which occurs when igneous magma is cooled below the Curie point in the presence of an external field (the earth’s magnetic field). 

• The magnetics method is a surface method that uses the response of either magnetic materials or atomic particles to an external magnetic field in order to measure the lateral variation in the intensity of the earth’s magnetic field. The magnetics method began its history in the oil industry to delineate petroleum basins and in the mining industry for mineral exploration; however, it is now used for many environmental and engineering types of applications. Because of its sensitivity to ferromagnetic (steel) objects and its great depth of detection, by far the most common shallow use for this method is to detect buried steel objects, utilities, abandoned steel-cased wells and steel piping.
• The shape and amplitude of an induced magnetic anomaly over a ferromagnetic object is dependent upon the geometry, size, and magnetic susceptibility of the object; depth to the object; and the inclination of the earth’s magnetic field in the survey area. In simple cases, such as abandoned steel-cased wells, the depth to the buried object may be calculated based on well-known half-width or slope methods. Magnetic anomalies over buried objects such as drums, pipes, and buried metallic debris are generally dipolar and exhibit an asymmetric, south up, north down signature (maximum on the south side and minimum on the north side).

• Delineation of Geological structures such as faults
• Utility         and pipe detection 
• Delineation         of steel objects
• Landmine         detection
• Delineation         of landfill boundaries
• Location         of oil pipelines in the marine
• Lateral delineation of geologic contacts
• Ground water exploration and         fault delineation
• Mapping basic igneous intrusives         & faults
• Archeological         studies
• Advantages 

• We         can cover large distances in small time (e.g. 6 km/day) by one         instrument.
• Easy         to work and easy to locate objects
• Processing         is easy and take little time
• Delineation of Geological structures such as faulte

• Magnetometers are highly accurate instruments that measure local magnetic fields to a high degree of precision. Magnetometer systems used for commercial applications include proton precession, caesium vapour and gradiometer magnetometers. The systems operate on broadly similar principles utilising proton rich fluids surrounded by an electric coil. A current is applied through the coil, which generates a magnetic field that temporarily polarises the protons. When the current is removed, the protons realign or process along the line of the Earth's magnetic field. The proton precession produces a small but measurable electric current in the coil, at a frequency proportional to the magnetic field intensity.
• Gradiometers measure magnetic field gradient rather than total field strength. Magnetic gradient anomalies generally give a better definition of shallow buried features such as buried tanks and drums but are less useful for geological tasks. The depth penetration of magnetic surveys is unaffected by high electrical ground conductivities, which makes them useful on sites with saline groundwater, clay or high levels of contamination where the GPR and Electromagnetic methods struggle.

•    Data acquisition for magnetic surveys involves taking a series of point readings at regular intervals on a survey grid. The spacing between grid lines and reading stations is dependant upon the application. Generally smaller targets require higher resolution surveys and denser survey grids. Data is stored digitally on site, and later downloaded on to a PC for post-survey processing and interpretation. Various interpretation techniques are applied to the data using specialist interactive software to identify the targeted anomalies. 

• A combination of contouring and color shading is used to highlight anomaly patterns. Survey results are presented as plans tied in to site co-ordinates, in an engineering compatible format readily understandable by the client.

• The objective of magnetic data interpretation is usually to locate anomalous material, its depth, dimensions, and properties. Consequently, the interpretation of magnetic data could be classified into two major categories; qualitative and quantitative interpretation.

• Quantitative interpretation is the process of finding the depth, and geometrical and petrophysical characteristics of geological bodies, locally revealed in plane and initially identified as a result of qualitative interpretation. This stage also includes estimation of the accuracy and reliability of the results. Success in the interpretation depends on the quality of the initial model whilst “any misguided efforts will merely multiply mistakes”. Magnetic data is usually plotted on 2-D map and contoured. 
• In general, magnetic data can be interpreted in several ways: 
• Direct detection of structural trends or geological provinces
• Basement depth estimation
• Forward modelling; detection and analysis of specific anomaly sources by trial and error.
• Inverse modelling: determining specific anomaly sources by inversion of data.

• Seismic Refraction Test  This test use a suitable energy source is triggered (e.g. accelerated weight drop or sledge hammer) which is placed near a line of sensors (geophones) on the surface and hence the subsurface velocity variation with depth can be determined. P-wave or S-wave velocity of the subsurface layers can then be used to find the Elastic properties of the soil and to give picture about its rigidity, for example we can determine the shear rigidity modulus of the soil (ASTM D5777-96) .  

• Seismic refraction can be used to identify the subsurface properties of the soil while shallow seismic reflection studies can be used to show subsurface faults and depth of the bedrock.

• Seismic Reflection Test measures the two-way travel time of seismic waves from the ground surface downward to a geologic contact where part of the seismic energy is reflected back to geophones at the surface (ASTM 7128-05).  Reflections occur when there is a contrast in the density and velocity between two layers.  The reflection method provides a high resolution cross section of soil/rock strata along a profile line.  For geotechnical and environmental work, reflection measurements are typically made from about 10 to 100 m deep on land.  

•  The seismic refraction method has a long history with the engineering problems and oil industry. Because this method uses travel time to measure the seismic properties of materials it is highly precise in the measurement of seismic velocity and quite accurate in the measurement of material thicknesses in many instances. Because seismic velocity is diagnostic for different types of material, and generally increases with degree of induration or hardness, the seismic refraction method not only can measure depth to a hard layer but can be used to non-invasively classify the type of material (e.g. soft sedimentary vs. igneous) and rippability of the layer encountered. In addition, seismic layers often correlate closely with geologic contacts. Elfarouk utilizes different equipments and different techniques to solve various engineering
•  ismic-reflection methods are active-source geophysical methods that were developed for oil and gas exploration applications. More recently, seismic-reflection methods have seen greater use as near-surface investigatory tools due to technological advances that increase their ability to produce high-resolution, two-dimensional images of the subsurface. Seismic-reflection methods are best suited to image subsurface environments composed of approximately horizontal layers. Because the shallow subsurface is commonly stratified, seismic-reflection methods can benefit numerous near-surface environmental and hydrogeological studies.   

• Refraction       
• Determining    soil properties Elastic moduli of soil and rock and hence give soil properties and site      characterization.
• Estimate      overburden thickness or depth to bedrock 
• Mapping      geologic strata and anomalous conditions 
• Solve      many engineering problems based on lateral or vertical variation and      contrasts.
• Advantages 
• Can      cover big areas compared with drilling, however dose not cancel it.
• Provides      data to depths of 100 m or more 
• Provides      a 2D cross-section of P-wave or S-wave velocity for the subsurface
• The      source of seismic energy can be small as simple as 7-kg sledgehammer 

• Reflection
• Primary      application is for determination of depth and thickness of geologic strata      
• Mapping      structural and geologic conditions such as subsurface faults 
• Recent      applications have attempted to use higher frequencies to identify smaller      targets such as caves or tunnels.

Advantages 

• Provides      a high resolution cross-section of soil/rock along profile line 
• Depth      range as shallow as 10 m to greater than 300 meter .

• Both      P-waves and S-waves can be measured with the appropriate equipment 

• The seismic refraction method is based on the fact that when a wave reaches a boundary between two materials having different seismic velocities, that seismic wave will be refracted (or bent) either toward the normal to the interface or away from the normal to the interface, depending on whether the velocity increases or decreases at the boundary. In practice, a linear array (or spread) of geophones is established along the ground surface and connected to a multi-channel seismograph. The seismic source is then established at a certain location along the line, and a seismic “shot” is then made. The first arrivals at each geophone location on the seismic record are recorded and plotted Vs distance and hence Geoseismic models are introduced based on different mathematical method
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• Seismic  Refraction seismic tomography uses the time of first arrivals or first preak to calculate the profile in the same way as refraction survey processing using methods such as GRM, time delay, or intercept. The recording of the signal must be long enough to detect the arrivals on all geophones. Usually for 50-100 meters long seisimic line it is enough to record for a time interval of 500ms. The needed recording time may vary depending on the subsoil conditions we are investigating and the type of wave we are measuring. P waves will be almost twice as fast as S waves in many stratigraphics contexts.
• seismic reflection survey needs lots of geophones to monitor the subsurface (e.g. 48 or 96 geophones) the time recorded is a function of the desired depth of penetration.

• Data processing includes the selection of first break arrival times, the generation of time-distance plots for each line, the assignment of selected portions of the travel time data to individual refractors, and the phantoming of travel time data for the target (lower) refractor.
  

•  Seismic Refraction modeling is made by A computer programto make   two‐dimensional, layered earth model. These programs requires input data consisting of shot‐point and geophone locations, refraction traveltimes, and identification of the refraction layer associated with each traveltime. 
• The first approximation model is generated by a computer adaptation of the delay‐time method, followed by a series of improved approximations that are made by use of a ray‐tracing procedure. The final result of the program is a model designed to minimize the discrepancy between field‐measured traveltimes and computed traveltimes of rays traced through the model.
• Seismic Reflection Modeling  Once this preprocessing work has been done, mathematical method begins. Once this is done, layer thicknesses and velocities are calculated and a geophysical interpretation of the geological parameters may be made. The end product is a seismic refraction profile that indicates the seismic layers detected, the depths to the interfaces between layers as they vary along the line and the seismic velocities encountered.