Land perforating radio detection and ranging is a geophysical technique which is peculiarly appropriate to image the dirt in two or three dimensions with a high spacial declaration upto a deepness of several metres. The word ‘RADAR ‘ is derived from an acronym for wireless sensing and ranging. Radar is an electronic and electromagnetic system that uses wireless moving ridges to observe and turn up objects. The footings ‘ground perforating radio detection and ranging ( GPR ) ‘ , ‘ground examining radio detection and ranging ‘ , ‘subsurface radio detection and ranging ‘ or ‘surface perforating radio detection and ranging ( SPR ) ‘ refer to a scope of electromagnetic techniques designed chiefly for the location of objects or interfaces buries beneath the Earth ‘s surface or located within a visually opaque construction. However, the description ‘GPR ‘ has become about universally recognized one. Normally GPR techniques are employed to observe backscattered radiation signifier a mark. Forward dispersing can besides give mark information, although for sub-surface work at least one aerial should necessitate to be buried and imaging transform would necessitate to be applied to the measured informations ( Davis and Annan 1989 ) . The GPR engineering is mostly applications-oriented and is available based on the mark stuff and its milieus ( Daniels 2007 ) .
The GPR was foremost appeared in history to find the subsurface features with wireless moving ridges in the late 1950 ‘s but more applications were started in 1960 ‘s in the field of geological studies ( Annan 2002 ) . In the 1970 ‘s, the first commercial GPR systems were available and introduced in civil and military technology every bit good as in archeology. In 1980 ‘s, the GPR was introduced in forensic probes, whereas, the first applications of GPR were initiated in agricultural and environmental technology in 1990 ‘s ( Lambot et al. 2009a ) . At that clip GPR studies were chiefly focused on qualitative imagination of subsurface. However, in the last decennary the GPR has extensively used in assorted subjects including agribusiness where GPR imaging to happen dirt belongingss and their spacial distribution. Much advancement in the engineering itself has been made in this period by bettering the dynamic scope of systems and efficiency of the aerial, velocity of acquisition, real-time image acquisition and visual image and basic processing of radio detection and ranging images ( Lambot et al. 2009a ) . The applications of GPR are abundant. From 1970 until the present twenty-four hours, several applications of radio detection and rangings have been reported. By and large, they have been used in assortment of media like dirt, stone, ice, lumber, groundwater, tunnels, fresh water, edifices, roads and rail paths and bed reviews.
There are two classs of GPR systems: the clip sphere systems ( besides called pulse radio detection and rangings ) and the frequence sphere systems. The clip sphere radio detection and rangings are by far the most normally used. They are based on the transmittal of a pulsation in the clip sphere while frequence domain systems transmit stepped-frequency uninterrupted moving ridges into the media. The frequence sphere radio detection and rangings are besides going popular presents because of low-cost electronic constituents and other advantages over pulse radio detection and rangings. The basic constituents of clip sphere GPR systems consist of a pulse generator, a transmittal aerial for conveying high frequence moving ridges into the media, a having aerial to have the direct and reflected urges, a switch for exchanging between transmittal and response if merely one aerial is to be used and a show unit which converts the received/recorded signals and expose them. These constituents may hold different agreements harmonizing to the radio detection and ranging theoretical account, but their functionality is by and large the same. The theoretical facets of radio detection and ranging constituents and its working rule can be found in item in ( Daniels 2007 ; Jol 2009 ) . Here, the basic rule of GPR and the factors act uponing radio detection and ranging signals are presented briefly.
Basic rule of GPR and theory consideration
The working rule of GPR is similar to reflection seismal and sonar techniques ( Davis and Annan 1989 ) . The GPR systems work in a frequence scope of 10-5000 MHz ( e.g. VHF-UHF ) . A radio detection and ranging system can utilize a individual aerial for both sender and receiving system ( e.g. monostatic manner ) , both a transmission and a receiving aerial ( e.g. bistatic manner ) and/or several senders and receiving systems ( multichannel systems ) . Largely bistatic aerials are used with GPR systems which chiefly deploys a sender and a receiving system aerial in a fixed geometry, which move on or over the surface to observe contemplations from subsurface characteristics ( Annan 2009 ) . When the aerials are coupled with land, dipole- or bowtie-type aerials are normally used while with. While for off-ground GPR systems, largely horn aerials are used because they are more directing ( Lambot et al. 2009a ; Lambot et Al. 2007 ) . The GPR system produces electromagnetic ( EM ) pulses in microwave set of the wireless spectrum. The sender aerial radiates short and high frequence pulsations into the land while the receiving system aerial measures the sum of the fluctuations in the reflected return signal as a map of clip ( Davis and Annan 2002 ) . The fluctuations in reflected signal caused by any subsurface contrast in electrical belongingss ( Annan 1973 ) . Beckon extension into the dirt is chiefly governed by dirt dielectric permittivity ( Iµ ) ( finding wave speed ) , electrical conduction ( I? ) ( finding beckon fading ) , magnetic permeableness ( Aµ ) ( finding wave speed and impacting wave fading ) and their spacial distribution ( Lambot et al. 2009a ; Lambot et Al. 2007 ) . The complex insulator invariable has two parts ; a existent one ( Iµ ‘ ) and an fanciful one ( Iµ ” ) . Radar signal speeds in low loss geological stuffs which are conformable to radar sounding are related to the existent portion of insulator invariable which is merely called comparative permittivity ( Iµr ) ( Davis and Annan 1989 ) . The magnetic permeableness ( Aµ ) of most of the environmental stuffs is equal to the magnetic permeableness of free infinite ( Aµ0 ) ( Davis and Annan 1989 ; Lambot et Al. 2009a ; Lambot et Al. 2007 ) .
The chief features of a GPR system are its operating frequence ( centre frequence ) , declaration and deepness of incursion. GPR declaration, which is typically considered as one one-fourth the wavelength, is the ability of the system to separate two signals that are close to each other in clip. It is determined by the clip period of emitted pulsation which is controlled by frequence bandwidth of the GPR system. Because impulse radio detection and ranging systems are designed to accomplish bandwidths that are about equal to the operating frequence ( halfway frequence ) , the declaration of GPR additions with increasing operating frequence ( Davis and Annan 1989 ; Huisman et al. 2003 ) . The pick of an operating frequence is ever a tradeoff between declaration and incursion deepness, as higher frequences permit higher declaration but lower incursion deepness ( Davis and Annan 1989 ) . The depth scope of GPR is besides strongly controlled by the electrical conduction ( EC or I? ) of the land. Soil EC decreases the GPR signal by dispersing it into heat energy more rapidly with deepness. The GPR systems have been demonstrated to sound to deepnesss of 50 m in low conduction stuffs of less than 1 mS/m such as sand, crushed rock, stone and fresh H2O ( Davis and Annan 1989 ) . Therefore, the optimum deepness incursion can be achieved in dry sandy dirts which have really low land conduction value. While in moist and more conductive dirts like clays, silts and saline dirts, the incursion scope of GPR impulse restricted to merely a few centimetres. Higher frequence signals penetrate less as compared to take down frequence signals but they give better declaration. The basic rule of GPR is shown in Figure 1 ( Huisman et al. 2003 ; Lambot et Al. 2009a ; Lambot et Al. 2007 ) .
The speed of GPR urge is different between stuffs with different electrical belongingss ( Davis and Annan 1989 ) and the arrival clip for GPR pulses over the same distance through two stuffs with different electrical belongingss over the same distance will be different. The interval of clip that it takes for the moving ridge to go from the sender aerial to the receiving system aerial is merely called the travel clip. The travel clip is fundamentally measured in nanosecond ( ns ) ( 1 ns = 1×10-9 s ) . Since the speed of an electromagnetic moving ridge in air is 3×108 m/s ( 0.3 m/ns ) , hence, the travel clip for an electromagnetic moving ridge in air is about 3.3333 ns per metre distance. Since the permittivity of Earth stuffs is ever greater than the permittivity of the air, the travel clip of a moving ridge in a stuff other than air is ever greater than 3.3333 ns/m ( Daniels 2000 ) . The basic rule of GPR in inside informations 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 speed of EM pulsation remains basically changeless between 10 and 1000 MHz frequence and dirt EC of less than 100 mS/m. Above this frequence the speed additions due to relaxation of H2O molecule ( Davis and Annan 1989 ) . The dielectric belongingss of stuffs can be described by EC, permeableness and permittivity of the medium. To depict the electrical belongingss of dirt is beyond the range of this article. Here, a really basic debut about GPR construct is given. The relationship between the speed of EM pulsation and stuff insulator invariable or comparative permittivity harmonizing to ( Davis and Annan 1989 ) is given below.
— — – ( 1 )
Where, V is the EM pulsation speed, degree Celsius is the speed of an EM moving ridge ( 3×108 m/s ) which is a changeless and is comparative permittivity ( here it is equal to existent portion of dielectric invariable ( Iµ ‘ ) . The above dealingss shows that the speed of EM pulsation is reciprocally relative to the square root of the comparative permittivity of the stuff. Relative permittivity is a ratio of permittivity in a material relation to free infinite, i.e.
— — – ( 2 )
Where, ( Farad per metre or Fm-1 ) is the absolute permittivity of the stuff and is the permittivity of free infinite which is 8.854 ten 10-12 Fm-1.
As the GPR operates at higher frequences, largely the stuffs behave as insulator ( dielectric ) at higher frequences. The higher the frequence of the EM pulsation, the shorter the wavelength of the pulsation will be.
— — – ( 3 )
Where, is wavelength of EM moving ridge ( m ) , t is clip period ( s ) and f is the moving ridge frequence ( Hz ) . Equation 3 shows that clip period and frequence of pulsation are reciprocally related with each other. The speed of the EM pulsation can be related with the deepness of incursion and travel clip. The contemplation of signal merely occurs when EM pulsation of GPR brushs with a stuff with contrast in electrical belongingss like metals etc. Therefore, the deepness of incursion of EM moving ridge can be determined if we know the travel clip ( I„ ) and wave speed.
— — – ( 4 )
Where, vitamin D is the deepness of incursion ( m ) and I„ is travel clip ( s ) .
Figure 1. Land perforating radio detection and ranging basic rule. Left ( Lambot et al. 2009a ) demoing contemplation of radio detection and ranging moving ridges ; Right ( Huisman et al. 2003 ) demoing assorted wave extension waies of a radio detection and ranging in a two superimposed dirt.
The GPR wave propagates from sender aerial to receiver antenna in different waies ( Figure 1 ) which include air wave, land moving ridge, critically reflected moving ridge, reflected moving ridge and refracted moving ridge ( Huisman et al. 2003 ) . When EM wave base on ballss in a two superimposed dirt, as shown in Figure 1, holding different dielectric invariables and severally, the signal is either reflected from the contrasting bed or refracted ( transmitted ) into the 2nd bed. The radio detection and ranging signal amplitude is reduced at the reflecting boundary depending on the contrast of the electrical belongingss and the thickness of the bed ( Davis and Annan 1989 ) . The contemplation coefficient at a half-space for a normal incident signal is given as:
— — – ( 5 )
Where, R is the contemplation coefficient, and and are the dielectric invariables of both the beds severally. The contemplation coefficient is called as the amplitude of the incident boundary. From above relation it is apparent that the amplitude of radio detection and ranging moving ridge is the ratio of the dielectric invariables of the two stuffs. The incident moving ridge on a boundary holding amplitude 1 is reflected back towards having aerial. The scope of contemplation coefficient ( amplitude ) can be given as: -1 a‰¤ R a‰¤ 1. If the lower stuff is metal, the contemplation coefficient is -1 and so maximal amplitude can be obtained.
Factors impacting GPR readings
A elaborate description of the factors impacting GPR signals can be found in ( Doolittle and Collins 1995 ) while ( Ben-Dor et al. 2009 ) summarized these factors more briefly. Harmonizing to ( Doolittle and Collins 1995 ) the chief factors act uponing the conduction
of dirts are: ( 1 ) porousness and grade of H2O impregnation ; ( 2 ) sum and type of salts in solution ; ( 3 ) the sum and type of clay ; and ( 4 ) sprinkling. In broader position, as mentioned antecedently, the moving ridge extension into the dirt is governed by three factors e.g. dielectric permittivity, EC and magnetic permeableness ( Davis and Annan 1989 ; Lambot et Al. 2009a ) . Soil EC is the basic physical belongings that affects the radio detection and ranging moving ridges. At changeless radio detection and ranging frequence, the signal fading is straight related with EC of the medium as shown in Equation No. 6 ( Davis and Annan 1989 ) . Metallic objects buried in the land can alter the electrical conduction drastically. The metal objects backscatter radio detection and ranging moving ridges and forestall their farther incursion. These metals on one manus increase the land conduction, while on the other manus, they strongly backscatter the radio detection and ranging waves themselves towards receiver antenna. Soils, rocks or deposits which are usually dielectric ( dielectrics ) will allow the incursion of radio detection and ranging moving ridges without fading. When the EC of dirts or stones increases so the moving ridges energy dissipates at shallower deepnesss. The more electrically conductive a stuff is, the more fading is in the EM moving ridge. In extremely conductive medium, the electrical constituent of the propagating EM moving ridge is conducted off in the land and accordingly the moving ridge as a whole dissipates ( Ben-Dor et al. 2009 ) . Soil salt is one of the two most factors that increases the dirt conduction enormously. The other most factor act uponing GPR signal is the dirt wet content ( Daniels et al. 1995 ) because of high insulator invariable ( ~80 ) as compared to other environmental stuffs ( 5-30 ) ( Davis and Annan 1989 ) . Besides dirt salt and wet content, there are other factors which can increase the EC of the land such as porousness, clay types, clay mineralogy, cation exchange capacity and dissolved ions in the dirt H2O nowadays in macro pores ( McNeill 1980 ) . All these factors attenuate the radio detection and ranging waves incursion and backscattering. Free ions, which allow for greater EC, act as major factors for diminishing GPR backscattering. Sulfates, carbonate minerals, Fe, salts of all kinds and charged clay atoms create a extremely conductive land and readily rarefy radio detection and ranging energy at shallow deepness ( Ben-Dor et al. 2009 ) . In really favorable conditions of low conduction dirts ( e.g. littorals, crushed rocks, stone and fresh H2O holding EC & lt ; 1mS/m ) , GPR waves can perforate to few 10s of a metre. While in extremely unfavorable conditions ( wet dirt with high sum of soluble salts or in heavy clay dirts ) , the incursion deepness of radio detection and rangings moving ridges is less than a metre, no affair what frequence of aerial is used ( Davis and Annan 1989 ) . In topographic points where soluble salts and exchangeable Na accumulate in the surface soil, high fading occurs and incursion is restricted to few centimeters. In dirts with high EC ( ECe a‰? 4 dS/m ) or sodium surface assimilation ratio above 13, GPR techniques are hard to use ( Ben-Dor et al. 2009 ) . The attenuated radio detection and ranging moving ridges dissipate into heat energy more rapidly with deepness. It is easy to decode a individual dirt characteristic contributing to dirty electrical conduction with GPR moving ridges. However, when many factors interact and contribute to dirty electrical conduction so it ever remained hard to gauge these features with radio detection and ranging moving ridges.
Magnetic permeableness is another factor which affects the GPR ability to perforate in the dirt. Magnetic permeableness is an ability of a medium to be magnetized when an EM field is imposed upon it. The higher the magnetic permeableness, the more EM energy will be attenuated during its transmittal, doing a devastation in of the magnetic part of the EM moving ridge, merely as the electrical constituent is lost with increased EC ( Ben-Dor et al. 2009 ) . The dirts and stones incorporating magnetic minerals such as Fe oxide, have high magnetic permeableness and hence, rarefy radio detection and ranging moving ridges in transmittal. However, most of the natural environmental stuffs such as dirts, stones and deposits have really low magnetic permeableness equal to the magnetic permeableness of free infinite ( Davis and Annan 1989 ) . Therefore, magnetic permeableness seldom creates jobs in radio detection and ranging moving ridges transmittal as compared to EC.
The speed of the propagating radio detection and ranging moving ridges is reciprocally relative to the square root of the permittivity of the medium as shown in Equation 1. The fading of radio detection and ranging signal is besides affected by medium permittivity. ( Davis and Annan 1989 ) gave a relationship between radio detection and ranging signal fading, EC and dielectric invariable in low-loss media as shown below.
— — – ( 6 )
The relation shows that radio detection and ranging signal fading is reciprocally relative to the square root of the permittivity at a given frequence. The stuffs holding high EC and low permittivity can rarefy radar signal greatly. Another factor that affect signal fading is radar frequence. The maximal deepness of probe lessenings quickly with increasing antenna frequence. High frequence ( & gt ; 100 MHz ) can rarefy the radio detection and ranging waves greatly ( Davis and Annan 1989 ) . Antennas places and types can besides impact radio detection and ranging moving ridges incursion into the dirt.
Available GPR detectors in planetary market
( Lambot et al. 2009a ) tabulated the several commercially available GPR theoretical accounts being used and present in the market today. Harmonizing to ( Lambot et al. 2009a ) , the normally used GPR makers are GSSI, Sensors & A ; Software Inc. , MALA GeoScience, IDS, 3D-Radar AS, Utsi Electronics, RASCAN Systems LLC and Pipe Hawk. They manufacture a scope of GPR theoretical accounts holding a individual channel, two channel and multichannel radio detection and ranging systems in a scope of frequences from a few MHz to some GHz. The GSSI, Sensors & A ; Softwares, and Mala equipments are the most used in research by universities and research establishments and for agricultural/environmental technology applications ( Lambot et al. 2009a ) . Other companies are more dedicated to civil technology applications ( e.g. , concrete review, buried pipe sensing, etc. ) . Table 1 nowadayss a list of the chief GPR makers, including the name of their radio detection and ranging merchandises and their cardinal characteristics. All available GPR systems belong to clip sphere radio detection and rangings ( pulse radio detection and rangings ) household, except for 3d-Radar which is based on two stepped-frequency continuous-wave systems.
Table. List of GPR makers and commercial merchandises. Adapted from ( Lambot et al. 2009a ) .
AntennasA in the scope 15-2600 MHzA
Sensors & A ; Software Inc
Antennas: 12.5-1000 MHz
Antennas: 250-1000 MHz
RAMAC ( X3M, ProEx, CX )
Antennas in the scope 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 sphere radio detection and ranging
Antenna arrayA : 100-2000 MHz
Antennas: 30-2000 MHz
AntennasA : 30-4000 MHz
RASCAN Systems LLC
Probe depthsA : 15-35 centimeter
GPR is a really promising tool for imaging chiefly the subsurface characteristics ( Annan 2002 ) and one of a really few methods available which allows the review of objects or geological characteristics which lie beneath an optically opaque surface ( Davis and Annan 1989 ) . A reappraisal of GPR history is presented by ( Annan 2002 ) . ( Davis and Annan 2002 ) and ( Huisman et al. 2003 ) reviewed GPR applications in agribusiness for mensurating dirt H2O content. Chiefly, all wave extension waies of GPR ( Figure 1 ) can be used to mensurate dirt H2O content, but land moving ridges and reflected moving ridges are most normally used for finding of close surface wet content and other dirt belongingss ( Huisman et al. 2003 ) .
A figure of applications are reported in literature such as: mineral and groundwater geographic expedition ; geotechnical and archeological probes ; remote detection and planetal geographic expedition ; geophysical and glacier probes ; and surface and subsurface agricultural dirt word picture. For case, in different agriculture-related countries, the GPR has been used to find H2O tabular array ( 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 place dirt 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 supervise subsurface contaminations ( Atekwana et al. 2000 ; Benson and Mustoe 1999 ; Daniels et Al. 1995 ; Hamzah et Al. 2008 ; 2009 ; Kim et Al. 2000 ; Senechal et Al. 2002 ) , to happen the deepness of dirt skylines and thickness ( Collins and Doolittle 1987 ; Doolittle 1987 ; Simeoni et Al. 2009 ) , to define difficult pans ( Olson and Doolittle 1985 ; Raper et Al. 1990 ) , to deduce dirty coloring material or organic C content ( Collins and Doolittle 1987 ; Doolittle 1982 ) , to place subsurface hydraulic parametric quantities ( Beres Jr and Haeni 1991 ; Jadoon et Al. 2008 ; Lambot et Al. 2009b ; Lambot et Al. 2006a ) and to qualify the deepnesss of organic dirt stuffs ( Collins et Al. 1986 ; Doolittle et Al. ; Doolittle ; Shih and Doolittle 1984 ) . In add-on, GPR has been used to analyze alterations in dirt belongingss which affect forest productiveness ( Farrish et Al. 1990 ) and emphasis in citrous fruit trees ( Shih et al. 1986b ) .
To reexamine GPR applications in all subjects is non the authorization of this article. Therefore, we will curtail our reappraisal to merely those dirt belongingss which straight or indirectly influence crop/plant productiveness. Variation is soil physical, chemical and mechanical belongingss can act upon the harvest output. GPR can besides be used to find shallow or near-surface dirt belongingss. ( Doolittle and Asmussen 1992 ) stated that the GPR techniques have been used to measure dirt compression, plough pan development, fluctuations in dirt texture, organic affair content, dirt H2O content and thickness of top dirt or skylines. The usage of GPR for mensurating these dirt belongingss is given in Table 2.
Table 2. Review of dirt belongingss 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 MAA?ller 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 ) , ( Senechal et al. 2005 ; Senechal 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 ) , ( Wollschlager and Roth 2005 ) , ( Causse and Senechal 2006 ) , ( Hanafy and al Hagrey 2006 ) , ( Paixao et al. 2006 ) , ( Turesson 2006 ) , ( Strobbia and Cassiani 2007 ) , ( Weihermuller 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 ) , ( Muller et al. 2009 ) , ( Wang et al. 2009 ) ,
Texture ( Sand, silt, clay ) , topsoil deepness or deepness to claypan or bed stone
( 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 MAA?ller 2000 ) , ( Shih and Myhre 1994 ) , ( Shih et al. 1985 ) ,
( Freeland et al. 1998b ) , ( Freeland et al. 2008 ) , ( Petersen et al. 2005 ) ,
Organic affair or organic C content