Abstract:
An electromagnetic sensitive fiber optic sensor, including a cylinder portion with a hole through the center, where a surface of the cylinder portion includes a magnetostrictive material, and a fiber cable threaded through the cylinder portion and wrapped around the cylinder portion multiple times. In another embodiment, a mandrel surrounds a magnetorestrictive or piezoelectric material and the fiber cable is wrapped around the mandrel.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application claims priority to U.S. Provisional Application Ser. No. 61,926,448 entitled “FIBER OPTIC SENSOR ARRAY FOR ELECTROMAGNETIC DATA COLLECTION” filed Jan. 13, 2014, which is incorporated by reference in its entirety herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to the field of Controlled Source Electromagnetic (CSEM) Surveying, Magnetoturelics (MT), and Seismic and Micro-Seismic surveying and various shortcomings in the technique related to resolution and sensitivity of the recording instrumentation and sensors. 
       BACKGROUND ART 
       [0003]    In the fields of Controlled Source Electromagnetic Surveying and Seismic surveying there is a desire to collect high resolution data from as many points and orientations in the area of interest as is possible. Currently, this leads to a plurality of signal receiving devices that are laid out in a matrix or other arrangement over the area of interest. In addition there are various techniques that are used to move the very high data volumes collected toward a central processing system. These methods include wireless transmission of data, data storage and subsequent collection. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0004]      FIG. 1A  shows an embodiment of the electromagnetic sensitive fiber optic sensor. 
           [0005]      FIG. 1B  shows an embodiment of the electromagnetic sensitive fiber optic sensor. 
           [0006]      FIG. 2  shows an embodiment of the magnetostrictive sensor. 
           [0007]      FIG. 3  show an embodiment of a sensor array layout. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0008]    The disclosed subject matter relates to the field of Controlled Source Electromagnetic (CSEM) Surveying, Magnetoturelics (MT), and Seismic and Micro-Seismic surveying and various shortcomings in the technique related to resolution and sensitivity of the recording instrumentation and sensors. A new sensor is provided that is a combination of an optical fiber that has been modified to have sections along its length coated or combined with a magnetostrictive and piezoelectric material, with the technique of spread spectrum virtual sensors using natural fiber span reflectometers or fiber brag gratings (FBG). The magnetostrictive material makes the fiber sensitive to magnetic field variations. The piezoelectric material makes the fiber sensitive to electric field variations. Fibers are naturally sensitive to acoustic or pressure waves and temperature changes. 
         [0009]    These types of sensors are particularly useful when recording data for interferometry and synthetic aperture soundings with electromagnetic signals. The practical implementation of the large sensor arrays required to make these measurements effective is greatly simplified with the proposed electromagnetic sensitive fiber optic sensor array. 
         [0010]    Recent advances in techniques for depositing materials onto fiber optic cables allow the creation of several types of sensors that improve the way in which data is collected for both CSEM and Seismic surveying. These techniques are described along with figures of the proposed sensors in ‘Fiber-optic magnetic field sensors’ by Erin Tate and ‘Optic fiber magnetic field sensors with TbDyFe magnetostrictive thin film as sensing materials’ by Yang et al. Examples of the Terfenol-D (TbDyFe) magnetostrictive material can be found at Etrema Products of Ames, Iowa, both of which are incorporated by reference herein. 
         [0011]      FIGS. 1A and 1B  show an embodiment of the electromagnetic sensitive fiber optic sensor  135 . The magnetostrictive material or piezoelectric material  100  is constructed in a cylinder form with a hole through the center to allow the fiber cable  105  to be threaded and wrapped multiple times through the magnetostrictive material  100 . Examples of the magnetostrictive material include materials such as  100  Terfenol-D (TbDyFe) which may be found at Etrema Products of Ames, Iowa. 
         [0012]    It can be shown that the number of turns of the fiber is proportional to the sensitivity of the sensor. For a magnetostrictive rod of length L, placed in a magnetic field of strength B: 
         [0000]    
       
         
           
             
               
                 
                   
                     Δφ 
                     
                       Δ 
                        
                       
                           
                       
                        
                       B 
                     
                   
                   = 
                   
                     
                       μ 
                       r 
                     
                      
                     
                       μ 
                       0 
                     
                      
                     
                       KLn 
                       0 
                     
                      
                     ɛ 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Where: n 0 =1.47, 
         [0000]    
       
         
           
             
               K 
               = 
               
                 
                   2 
                    
                   π 
                 
                 λ 
               
             
             , 
           
         
       
       
         
           
             ΔØ=change in phase of 1550 nm laser, 
             ΔB=change in magnetic field 
             ε=magnetostrictive sensitivity 
           
         
       
     
         [0016]    Careful selection of the fiber optic cable  105  will ensure minimal attenuation due to the wrapping through the mandrel. An example of bend insensitive fiber is Corning RC 1550 and is manufactured by Dow Corning located in Canton, N.Y. 
         [0017]    The equations above shows the relationship between the change in the magnetic field and the phase change that results from 1550 nm laser light being used to obtain a signal that is proportional the change in unit length of the magnetostrictive material due to the impinging magnetic field. 
         [0018]    Referring again to  FIG. 1 a    and  FIG. 1   b,  the fiber optic magnetostrictive sensor  135  is a single axis assembly. The fiber optic cable  105  is attached rigidly at each end of the magnetostrictive material  100  at  120  and  125 . The fiber optic cable between these two points of attachment,  120  and  125  is pre-stressed and is able to expand and contract along with the magnetostrictive material  100 . The fiber optic cable  105  is wrapped through the magnetostrictive material  100 , multiple times. 
         [0019]    Using a pair of FBG&#39;s  130  and  135  allows an electronic system, such as that specified in Reference  1 , to create a value that indicates the change in the overall dimension of the magnetostrictive material  100  that is proportional to magnetic field  140  impinging on the sensor. The FBG&#39;s  130  and  135  are place within the fiber optic cable  105  at a position before and after the sensor that allows a measurement to be made that will emphasis any dimensional changes in length of the magnetostrictive material  100 . 
         [0020]    The magnetic field  140  is also concentrated by a Permalloy flux concentrator  115  to improve sensitivity. 
         [0021]    The fiber optic magnetostrictive sensor  135  may be daisy chained with two other fiber optic magnetostrictive sensors to form a 3 axis sensor that is sensitive in three orthogonal directions 
         [0022]    Now referring to  FIG. 2 . An alternate embodiment of the magnetostrictive sensor type has a mandrel  200  that surrounds a magnetostrictive or piezoelectric material  205  that is then wrapped by a fiber optic cable  210 . The number of wraps is proportional to sensitivity. The sensor can be daisy chained to form a 3 axis measurement. The Y axis  215 , the Z axis  220  and the Y axis  225 . 
         [0023]    The piezoelectric sensor uses a relaxor electrostrictive polymers (REP) as the loaded member. The REP sensor is preloaded to ensure resonance over the Extremely low frequency (ELF) and Ultra low frequency (ULF) band over which the CSEM system operates. The preloaded REP sensor is then wrapped with the fiber optic cable as shown in  FIG. 2 . An example of the REP material is manufactured by Piezotech S.A.S. located in Hésingue—France and may be found at the following website: 
         [0024]    http://www.piezotech.fr/fr/2-products-piezoelectric-polymers/news/news-38-relaxorelectrostrictive-polymers-p-vdf-trfe-cfe-.html 
         [0025]    For the field of Seismic or Micro-Seismic data collection it is envisioned that the system described in U.S. Pat. No. 7,268,863 may be used for monitoring Seismic waves, which is incorporated herein by reference. 
         [0026]    Now referring to  FIG. 3 . The fiber optic magnetostrictive sensor  300 , fiber optic piezoelectric sensor  305  and fiber optic pressure wave sensor  310  are created along a length of continuous fiber optic cable and placed in a plurality of locations. The arrangement creates the fiber optic sensor array  315 . 
         [0027]    For example, the technique describe in U.S. Pat. No. 7,268,863 with the added feature of using Fiber Brag Gratings (FBG) before and after each sensor is used to create a sensor array that will generate data for the magnetic and electric fields returned from the sub surface formations, once they have been excited by an electromagnetic disturbance, naturally occurring or induced by a transmitter. 
         [0028]    The sensor array can also be left in place for long term data collection enabling passive monitoring or timed active monitoring. 
         [0029]    An important aspect of this system that the signals be temperature and pressure compensated. Since the effect of temperature on the fiber is linear in our region of interest and very slowly changing compared to the signal of interest, it is relatively easy to remove the effect. This can be verified by providing a similar fiber that is in the same environment as the sensing fiber and recording deviations due to temperature. Additionally, the system can also be sensitive to pressure waves. The changes are extremely weak in the operating mode and can be ignored. 
         [0030]    The sensor array layout depicted in  FIG. 3  is illustrative of one arrangement. And while the receivers  301  are evenly space in  FIG. 3 , they may also be randomly distributed or placed in some other geometric arrangement. These distributions affect the way in which an interferogram will display data and will result in more information or less information being developed for specific subsurface regions.