code: def _compute_soil_depth_term(self, C, imt, z1pt0, vs30):
          
          a21 = self._compute_a21_factor(C, imt, z1pt0, vs30)
          a22 = self._compute_a22_factor(imt)
          median_z1pt0 = self._compute_median_z1pt0(vs30)
  
          soil_depth_term = a21 * np.log((z1pt0 + self.CONSTS['c2']) /
                                         (median_z1pt0 + self.CONSTS['c2']))
  
          idx = z1pt0 >= 200
          soil_depth_term[idx] += a22 * np.log(z1pt0[idx] / 200)
  
          return soil_depth_term
docs: Compute and return soil depth model term, that is the 9-th term in
          equation 1, page 74. The calculation of this term is explained in
          paragraph 'Soil Depth Model', page 79.