title: Resampling a dataframe based on depth column
body: <p>I have two dataframe which the <code>key</code> is depth. One has > 2k values the other only 100, but the min and the max depth are the same. I would like to upsample the small dataframe (which has only one column) at the same size of the bigger one and repeat the same value of a column between two depths.  </p>
  
  <p>I've tried using concatenate and resampling but I'm stuck when I want to find the same depth since the two dataframes depths do not have exactly the same values</p>
  
  <p>I have this:</p>
  
  <pre><code>df_small:
      depth  Litholog
  0  38.076       2.0
  1  39.546       2.0
  2  41.034       4.0
  3  55.133       3.0
  4  69.928       2.0
  </code></pre>
  
  <p>and this:</p>
  
  <pre><code>df_big:
       depth  
  0  21.3360         
  1  35.2044         
  2  37.6428        
  3  41.7576         
  4  41.9100         
  5  48.7680         
  6  53.1876         
  7  56.0832         
  8  58.3692         
  9  62.1792 
  </code></pre>
  
  <p>I would like this:</p>
  
  <pre><code>df_result:
       depth  Litholog
  0  21.3360         2
  1  35.2044         2
  2  37.6428         2
  3  41.7576         4
  4  41.9100         4
  5  48.7680         4
  6  53.1876         4
  7  56.0832         3
  8  58.3692         3
  9  62.1792         2 
  </code></pre>
  
  <p>I tried several approach but without success. Many thanks to all</p>