Ground penetrating radar is a geophysical technique which is particularly appropriate to image the soil in two or three dimensions with a high spatial resolution upto a depth of several meters. The word 'RADAR' is derived from an acronym for radio detection and ranging. Radar is an electronic and electromagnetic system that uses radio waves to detect and locate objects. The terms 'ground penetrating radar (GPR)', 'ground probing radar', 'subsurface radar' or 'surface penetrating radar (SPR)' refer to a range of electromagnetic techniques designed primarily for the location of objects or interfaces buries beneath the earth's surface or located within a visually opaque structure. However, the description 'GPR' has become almost universally accepted one. Usually GPR techniques are employed to detect backscattered radiation form a target. Forward scattering can also yield target information, although for sub-surface work at least one antenna should need to be buried and imaging transform would need to be applied to the measured data (Davis and Annan 1989). The GPR technology is largely applications-oriented and is available based on the target material and its surroundings (Daniels 2007).
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The GPR was first appeared in history to determine the subsurface features with radio waves in the late 1950's but more applications were started in 1960's in the field of geological surveys (Annan 2002). In the 1970's, the first commercial GPR systems were available and introduced in civil and military engineering as well as in archaeology. In 1980's, the GPR was introduced in forensic investigations, whereas, the first applications of GPR were initiated in agricultural and environmental engineering in 1990's (Lambot et al. 2009a). At that time GPR surveys were mainly focused on qualitative imaging of subsurface. However, in the last decade the GPR has extensively used in various disciplines including agriculture where GPR imaging to find soil properties and their spatial distribution. Much progress in the technology itself has been made in this period by improving the dynamic range of systems and efficiency of the antennas, speed of acquisition, real-time image acquisition and visualization and basic processing of radar images (Lambot et al. 2009a). The applications of GPR are abundant. From 1970 until the present day, several applications of radars have been reported. Generally, they have been used in variety of media like soil, rock, ice, timber, groundwater, tunnels, freshwater, buildings, roads and rail tracks and bed inspections.
There are two categories of GPR systems: the time domain systems (also called pulse radars) and the frequency domain systems. The time domain radars are by far the most commonly used. They are based on the transmission of a pulse in the time domain while frequency domain systems transmit stepped-frequency continuous waves into the media. The frequency domain radars are also becoming popular nowadays because of affordable electronic components and other advantages over pulse radars. The basic components of time domain GPR systems consist of a pulse generator, a transmission antenna for transmitting high frequency waves into the media, a receiving antenna to receive the direct and reflected impulses, a switch for switching between transmission and reception if only one antenna is to be used and a display unit which converts the received/recorded signals and display them. These components may have different arrangements according to the radar model, but their functionality is generally the same. The theoretical aspects of radar components and its working principle can be found in detail in (Daniels 2007; Jol 2009). Here, the basic principle of GPR and the factors influencing radar signals are presented briefly.
Basic principle of GPR and theory consideration
The working principle of GPR is similar to reflection seismic and sonar techniques (Davis and Annan 1989). The GPR systems work in a frequency range of 10-5000 MHz (e.g. VHF-UHF). A radar system can use a single antenna for both transmitter and receiver (e.g. monostatic mode), both a transmitting and a receiving antenna (e.g. bistatic mode) and/or several transmitters and receivers (multichannel systems). Mostly bistatic antennas are used with GPR systems which mainly deploys a transmitter and a receiver antenna in a fixed geometry, which move on or over the surface to detect reflections from subsurface features (Annan 2009). When the antennas are coupled with ground, dipole- or bowtie-type antennas are commonly used while with. While for off-ground GPR systems, mostly horn antennas are used because they are more directive (Lambot et al. 2009a; Lambot et al. 2007). The GPR system produces electromagnetic (EM) pulses in microwave band of the radio spectrum. The transmitter antenna radiates short and high frequency pulses into the ground while the receiver antenna measures the amount of the variations in the reflected return signal as a function of time (Davis and Annan 2002). The variations in reflected signal caused by any subsurface contrast in electrical properties (Annan 1973). Wave propagation into the soil is mainly governed by soil dielectric permittivity (Îµ) (determining wave velocity), electrical conductivity (Ïƒ) (determining wave attenuation), magnetic permeability (Âµ) (determining wave velocity and affecting wave attenuation) and their spatial distribution (Lambot et al. 2009a; Lambot et al. 2007). The complex dielectric constant has two parts; a real one (Îµ') and an imaginary one (Îµ"). Radar signal velocities in low loss geological materials which are amenable to radar sounding are related to the real part of dielectric constant which is simply called relative permittivity (Îµr) (Davis and Annan 1989). The magnetic permeability (Âµ) of most of the environmental materials is equal to the magnetic permeability of free space (Âµ0) (Davis and Annan 1989; Lambot et al. 2009a; Lambot et al. 2007).
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The main characteristics of a GPR system are its operating frequency (centre frequency), resolution and depth of penetration. GPR resolution, which is typically considered as one quarter the wavelength, is the ability of the system to distinguish two signals that are close to each other in time. It is determined by the time period of emitted pulse which is controlled by frequency bandwidth of the GPR system. Because impulse radar systems are designed to achieve bandwidths that are about equal to the operating frequency (center frequency), the resolution of GPR increases with increasing operating frequency (Davis and Annan 1989; Huisman et al. 2003). The choice of an operating frequency is always a trade-off between resolution and penetration depth, as higher frequencies permit higher resolution but lower penetration depth (Davis and Annan 1989). The depth range of GPR is also strongly controlled by the electrical conductivity (EC or Ïƒ) of the ground. Soil EC decreases the GPR signal by dissipating it into heat energy more quickly with depth. The GPR systems have been demonstrated to sound to depths of 50 m in low conductivity materials of less than 1 mS/m such as sand, gravel, rock and fresh water (Davis and Annan 1989). Therefore, the optimal depth penetration can be achieved in dry sandy soils which have very low ground conductivity value. While in moist and more conductive soils like clays, silts and saline soils, the penetration range of GPR impulse restricted to only a few centimeters. Higher frequency signals penetrate less as compared to lower frequency signals but they give better resolution. The basic principle of GPR is shown in Figure 1 (Huisman et al. 2003; Lambot et al. 2009a; Lambot et al. 2007).
The velocity of GPR impulse is different between materials with different electrical properties (Davis and Annan 1989) and the arrival time for GPR pulses over the same distance through two materials with different electrical properties over the same distance will be different. The interval of time that it takes for the wave to travel from the transmitter antenna to the receiver antenna is simply called the travel time. The travel time is basically measured in nanosecond (ns) (1 ns = 1x10-9 s). Since the velocity of an electromagnetic wave in air is 3x108 m/s (0.3 m/ns), therefore, the travel time for an electromagnetic wave in air is approximately 3.3333 ns per meter distance. Since the permittivity of earth materials is always greater than the permittivity of the air, the travel time of a wave in a material other than air is always greater than 3.3333 ns/m (Daniels 2000). The basic principle of GPR in details can be found in (Annan 2009; Ben-Dor et al. 2009; Blindow et al. 2007; Daniels 2007; Daniels 2000; Davis and Annan 1989; Huisman et al. 2003).
The velocity of EM pulse remains essentially constant between 10 and 1000 MHz frequency and soil EC of less than 100 mS/m. Above this frequency the velocity increases due to relaxation of water molecule (Davis and Annan 1989). The dielectric properties of materials can be described by EC, permeability and permittivity of the medium. To describe the electrical properties of soil is beyond the scope of this article. Here, a very basic introduction about GPR concept is given. The relationship between the velocity of EM pulse and material dielectric constant or relative permittivity according to (Davis and Annan 1989)is given below.
Where, v is the EM pulse velocity, c is the velocity of an EM wave (3x108 m/s) which is a constant and is relative permittivity (here it is equal to real part of dielectric constant (Îµ'). The above relations shows that the velocity of EM pulse is inversely proportional to the square root of the relative permittivity of the material. Relative permittivity is a ratio of permittivity in a material relative to free space, i.e.
Where, (Farad per meter or Fm-1) is the absolute permittivity of the material and is the permittivity of free space which is 8.854 x 10-12 Fm-1.
As the GPR operates at higher frequencies, mostly the materials behave as dielectric (insulator) at higher frequencies. The higher the frequency of the EM pulse, the shorter the wavelength of the pulse will be.
Where, is wavelength of EM wave (m), t is time period (s) and f is the wave frequency (Hz). Equation 3 shows that time period and frequency of pulse are inversely related with each other. The velocity of the EM pulse can be related with the depth of penetration and travel time. The reflection of signal only occurs when EM pulse of GPR encounters with a material with contrast in electrical properties like metals etc. Therefore, the depth of penetration of EM wave can be determined if we know the travel time (Ï„) and wave velocity.
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Where, d is the depth of penetration (m) and Ï„ is travel time (s).
Figure 1. Ground penetrating radar basic principle. Left (Lambot et al. 2009a) showing reflection of radar waves; Right (Huisman et al. 2003) showing various wave propagation paths of a radar in a two layered soil.
The GPR wave propagates from transmitter antenna to receiver antenna in different paths (Figure 1) which include air wave, ground wave, critically reflected wave, reflected wave and refracted wave (Huisman et al. 2003). When EM wave passes in a two layered soil, as shown in Figure 1, having different dielectric constants and respectively, the signal is either reflected from the contrasting layer or refracted (transmitted) into the second layer. The radar signal amplitude is reduced at the reflecting boundary depending on the contrast of the electrical properties and the thickness of the layer (Davis and Annan 1989). The reflection coefficient at a half-space for a normal incident signal is given as:
Where, R is the reflection coefficient, and and are the dielectric constants of both the layers respectively. The reflection coefficient is called as the amplitude of the incident boundary. From above relation it is evident that the amplitude of radar wave is the ratio of the dielectric constants of the two materials. The incident wave on a boundary having amplitude 1 is reflected back towards receiving antenna. The range of reflection coefficient (amplitude) can be given as: -1 â‰¤ R â‰¤ 1. If the lower material is metal, the reflection coefficient is -1 and then maximum amplitude can be obtained.
Factors affecting GPR readings
A detailed description of the factors affecting GPR signals can be found in (Doolittle and Collins 1995) while (Ben-Dor et al. 2009) summarized these factors more briefly. According to (Doolittle and Collins 1995) the principal factors influencing the conductivity
of soils are: ( 1 ) porosity and degree of water saturation; (2) amount and type of salts in solution; (3) the amount and type of clay; and (4) scattering. In broader perspective, as mentioned previously, the wave propagation into the soil is governed by three factors e.g. dielectric permittivity, EC and magnetic permeability (Davis and Annan 1989; Lambot et al. 2009a). Soil EC is the basic physical property that affects the radar waves. At constant radar frequency, the signal attenuation is directly related with EC of the medium as shown in Equation No. 6 (Davis and Annan 1989). Metallic objects buried in the ground can change the electrical conductivity drastically. The metal objects backscatter radar waves and prevent their further penetration. These metals on one hand increase the ground conductivity, while on the other hand, they strongly backscatter the radar waves themselves towards receiver antenna. Soils, rocks or sediments which are normally dielectric (insulators) will permit the penetration of radar waves without attenuation. When the EC of soils or rocks increases then the waves energy dissipates at shallower depths. The more electrically conductive a material is, the more attenuation is in the EM wave. In highly conductive medium, the electrical component of the propagating EM wave is conducted away in the ground and consequently the wave as a whole dissipates (Ben-Dor et al. 2009). Soil salinity is one of the two most factors that increases the soil conductivity tremendously. The other most factor influencing GPR signal is the soil moisture content (Daniels et al. 1995) because of high dielectric constant (~80) as compared to other environmental materials (5-30) (Davis and Annan 1989). Besides soil salinity and moisture content, there are other factors which can increase the EC of the ground such as porosity, clay types, clay mineralogy, cation exchange capacity and dissolved ions in the soil water present in macro pores (McNeill 1980). All these factors attenuate the radar waves penetration and backscattering. Free ions, which allow for greater EC, act as major factors for decreasing GPR backscattering. Sulphates, carbonate minerals, iron, salts of all sorts and charged clay particles create a highly conductive ground and readily attenuate radar energy at shallow depth (Ben-Dor et al. 2009). In very favourable conditions of low conductivity soils (e.g. sands, gravels, rock and fresh water having EC<1mS/m), GPR waves can penetrate to few tens of a meter. While in highly unfavourable conditions (wet soil with high amount of soluble salts or in heavy clay soils), the penetration depth of radars waves is less than a meter, no matter what frequency of antenna is used(Davis and Annan 1989). In places where soluble salts and exchangeable sodium accumulate in the topsoil, high attenuation occurs and penetration is restricted to few centimetres. In soils with high EC (ECe â‰¥ 4 dS/m) or sodium adsorption ratio above 13, GPR techniques are difficult to apply (Ben-Dor et al. 2009). The attenuated radar waves dissipate into heat energy more quickly with depth. It is easy to decipher a single soil characteristic contributing to soil electrical conductivity with GPR waves. However, when many factors interact and contribute to soil electrical conductivity then it always remained difficult to estimate these characteristics with radar waves.
Magnetic permeability is another factor which affects the GPR ability to penetrate in the soil. Magnetic permeability is an ability of a medium to be magnetized when an EM field is imposed upon it. The higher the magnetic permeability, the more EM energy will be attenuated during its transmission, causing a destruction in of the magnetic portion of the EM wave, just as the electrical component is lost with increased EC (Ben-Dor et al. 2009). The soils and rocks containing magnetic minerals such as iron oxide, have high magnetic permeability and therefore, attenuate radar waves in transmission. However, most of the natural environmental materials such as soils, rocks and sediments have very low magnetic permeability equal to the magnetic permeability of free space (Davis and Annan 1989). Therefore, magnetic permeability seldom creates problems in radar waves transmission as compared to EC.
The velocity of the propagating radar waves is inversely proportional to the square root of the permittivity of the medium as shown in Equation 1. The attenuation of radar signal is also affected by medium permittivity. (Davis and Annan 1989) gave a relationship between radar signal attenuation, EC and dielectric constant in low-loss media as shown below.
The relation shows that radar signal attenuation is inversely proportional to the square root of the permittivity at a given frequency. The materials having high EC and low permittivity can attenuate radar signal greatly. Another factor that affect signal attenuation is radar frequency. The maximum depth of investigation decreases rapidly with increasing antenna frequency. High frequency (> 100 MHz) can attenuate the radar waves greatly (Davis and Annan 1989). Antennas positions and types can also affect radar waves penetration into the soil.
Available GPR sensors in global market
(Lambot et al. 2009a) tabulated the several commercially available GPR models being used and present in the market nowadays. According to (Lambot et al. 2009a), the commonly used GPR manufacturers are GSSI, Sensors & Software Inc., MALA GeoScience, IDS, 3D-Radar AS, Utsi Electronics, RASCAN Systems LLC and Pipe Hawk. They manufacture a range of GPR models having a single channel, two channel and multichannel radar systems in a range of frequencies from a few MHz to some GHz. The GSSI, Sensors & Softwares, and Mala equipments are the most used in research by universities and research institutions and for agricultural/environmental engineering applications (Lambot et al. 2009a). Other companies are more dedicated to civil engineering applications (e.g., concrete inspection, buried pipe detection, etc.). Table 1 presents a list of the main GPR manufacturers, including the name of their radar products and their key features. All available GPR systems belong to time domain radars (pulse radars) family, except for 3d-Radar which is based on two stepped-frequency continuous-wave systems.
Table. List of GPR manufacturers and commercial products. Adapted from (Lambot et al. 2009a).
AntennasÂ in the range 15-2600 MHzÂ
Sensors & Software Inc
Antennas: 12.5-1000 MHz
Antennas: 250-1000 MHz
RAMAC (X3M, ProEx, CX)
Antennas in the range 25-1000 MHz
Antenna array: 200, 400, 1300 MHz
Up to 16 channels
Antennas: 25-2000 MHz
RIS MF Hi-Mod
3d-Radar Antenna arrays
Frequency domain radar
Antenna arrayÂ : 100-2000 MHz
Antennas: 30-2000 MHz
AntennasÂ : 30-4000 MHz
RASCAN Systems LLC
Investigation depthsÂ : 15-35 cm
GPR is a very promising tool for imaging primarily the subsurface features (Annan 2002) and one of a very few methods available which allows the inspection of objects or geological features which lie beneath an optically opaque surface (Davis and Annan 1989). A review of GPR history is presented by (Annan 2002). (Davis and Annan 2002) and (Huisman et al. 2003)reviewed GPR applications in agriculture for measuring soil water content. Principally, all wave propagation paths of GPR (Figure 1) can be used to measure soil water content, but ground waves and reflected waves are most commonly used for determination of near surface moisture content and other soil properties (Huisman et al. 2003).
A number of applications are reported in literature such as: mineral and groundwater exploration; geotechnical and archaeological investigations; remote sensing and planetary exploration; geophysical and glacier investigations; and surface and subsurface agricultural soil characterization. For instance, in different agriculture-related areas, the GPR has been used to determine water table (Bano 2006; Benson and Mustoe 1999; Bian et al. 2009; Blindow and Balke 2005; Doolittle et al. 2006; Doolittle et al. 2000; Nakashima et al. 2001; Pyke et al. 2008; Roth et al. 2004; Shih et al. 1986a; Smith et al. 1992; Takeshita et al. 2004; Teixeira et al. 2002; Truman et al. 1988), to identify soil stratigraphy (Arcone et al. 2002; Bristow 2004; Cagnoli and Russell 2000; Comas et al. 2004; Davis and Annan 1989; Dominic et al. 1995; Jol et al. 2004; Pipan et al. 2004; Radzevicius et al. 2000; Van Overmeeren 1998), to monitor subsurface contaminants (Atekwana et al. 2000; Benson and Mustoe 1999; Daniels et al. 1995; Hamzah et al. 2008; 2009; Kim et al. 2000; Sénéchal et al. 2002), to find the depth of soil horizons and thickness (Collins and Doolittle 1987; Doolittle 1987; Simeoni et al. 2009), to delineate hard pans (Olson and Doolittle 1985; Raper et al. 1990), to infer soil colour or organic carbon content (Collins and Doolittle 1987; Doolittle 1982), to identify subsurface hydraulic parameters (Beres Jr and Haeni 1991; Jadoon et al. 2008; Lambot et al. 2009b; Lambot et al. 2006a) and to characterize the depths of organic soil materials (Collins et al. 1986; Doolittle et al.; Doolittle; Shih and Doolittle 1984). In addition, GPR has been used to study changes in soil properties which affect forest productivity (Farrish et al. 1990)and stress in citrus trees (Shih et al. 1986b).
To review GPR applications in all disciplines is not the mandate of this article. Therefore, we will restrict our review to only those soil properties which directly or indirectly influence crop/plant productivity. Variation is soil physical, chemical and mechanical properties can influence the crop yield. GPR can also be used to determine shallow or near-surface soil properties. (Doolittle and Asmussen 1992)stated that the GPR techniques have been used to assess soil compaction, plough pan development, variations in soil texture, organic matter content, soil water content and thickness of top soil or horizons. The use of GPR for measuring these soil properties is given in Table 2.
Table 2. Review of soil properties measured with GPR techniques.
(Chanzy et al. 1996), (Freeland et al. 1997; 1998a), (Van Overmeeren et al. 1997), (Weiler et al. 1998), (Al Hagrey and MÃÂ¼ller 2000), (Charlton 2000), (Parkin et al. 2000), (Redman et al. 2000), (Charlton 2001), (Bano and Girard 2001), (Lensen et al. 2001), (Davis and Annan 2002), Redman, 2002 #658}, (Schmalz and Lennartz 2002), (Huisman and Bouten 2002; 2003; Huisman et al. 2003; Huisman et al. 2002; Huisman et al. 2001), (Garambois et al. 2002), (Grote et al. 2002; 2003), (Hubbard et al. 2002), (Rubin 2002), (Stoffregen et al. 2002), (Takeshita et al. 2002), (Stoffregen et al. 2002), (Galagedara et al. 2003a; 2005a; Galagedara et al. 2002; 2003b; Galagedara et al. 2005b), (West et al. 2003), (Sénéchal et al. 2005; Sénéchal et al. 2004), (Chen et al. 2004), (Moysey and Knight 2004), (Conyers 2004), (Schmalholz et al. 2004), (Serbin and Or 2004a; b; 2005), (Lunt et al. 2005), (Fiori et al. 2005), (Pettinelli et al. 2005), (Wollschläger and Roth 2005), (Causse and Sénéchal 2006), (Hanafy and al Hagrey 2006), (Paixão et al. 2006), (Turesson 2006), (Strobbia and Cassiani 2007), (Weihermüller et al. 2007), (Lambot et al. 2008; Lambot et al. 2006b), (Bradford 2008), (Deiana et al. 2008), (Gerhards et al. 2008), (Minet et al. 2009), (Müller et al. 2009), (Wang et al. 2009),
Texture (Sand, silt, clay), topsoil depth or depth to claypan or bed rock
(Truman et al. 1988), (Boll et al. 1996), (Meadows et al. 2006), (West et al. 2003), (Petersen et al. 2005), (Ziekur and Schuricht 2002), (Gerber et al.),
(Tsoflias and Becker 2008), (Koh 2008), (Tsoflias and Becker 2007), (Hu et al. 2006), (Al Hagrey and MÃÂ¼ller 2000), (Shih and Myhre 1994), (Shih et al. 1985),
(Freeland et al. 1998b), (Freeland et al. 2008), (Petersen et al. 2005),
Organic matter or organic carbon content