TESTING CAUSE AND EFFECT HYPOTHESES

    

A.  INTRODUCTION 

      The last lecture introduced correlation as a means for inferring
      cause and effect relationships.  The purpose of today's lecture
      is to show how correlation can also be used as a means to test
      cause and effect hypotheses.
   
 

B.  BACKGROUND

      The geologic example used to illustrate how correlation can be
      used as a means to test cause and effect hypotheses is known
      as
land subsidence, the slow, downward movement of the land
      surface.  There are various causes of this phenomenon, but we
      are considering only one specific type (called deep subsidence
      by the authors of your text), which can occur when pressurized
      fluids (water, gas, and/or oil) are removed from the subsurface:
        

      1.  CONFINED GROUNDWATER

           Sometimes the geologic conditions allow groundwater to occur
           within highly permeable materials that are sandwiched between
           by lowly permeable materials.  This is called confined (artesian) 
           groundwater.  Notice that confined groundwater occurs under
           under pressure, which can result in flowing wells.


      2.  OIL AND GAS RESERVOIRS

           Oil and natural gas are fluids produced deep within the earth as
           buried organic matter is subjected to heat and pressure. Oil and
           gas rise towards the land surface until they are trapped by lowly
           permeable formations.   These fluids occur under pressure and
           are thus analogous to confined groundwater.

 

C.  OBSERVATIONS

       1.  WHERE SUBSIDENCE OCCURS

            Deep subsidence occurs in various locations across the United
            States.  What do these locations have in common?
 
            a.  These are locations where fluids are pumped from layers of
                 sandy sediment within the subsurface. 

            b.  The fluids are under pressure:  both confined groundwater
                 and pressurized oil and gas reservoirs.
 

 

       2.  EXAMPLES

            a. Central Valley, California:
 
                This is the world's largest area of subsidence and is an area
                 where groundwater is pumped from confined aquifers.
 

            b. Long Beach, California:

                This is an area where as much as 30 feet of subsidence has
                occurred over an oil and gas field.

 

 

D.  HYPOTHESIS

       When fluids under pressure within the subsurface are removed, the
       fluid pressure drops.  If the formation from which fluids are taken is
       composed of compressible sediments, a reduction in fluid pressure
       can cause the overlying formations to slowly subside


        

 E.  HYPOTHESIS TESTING      

        1.  CORRELATION

             To test the hypothesis that removal of pressurized fluids causes
             land subsidence, pumping and subsidence data can be plotted
             versus time.

             Example:   Santa Clara Valley, California
 

      
        2.  PREDICTION

             This hypothesis can also be tested by applying the model in an
             effort to solve subsidence problems.

             Example:  subsidence in the Long Beach, California area has 
                              been arrested by using injection wells to pump sea
                              water back into the oil and gas reservoir.

 

 

 

 

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