ds |
= | dt/l. |
(2.17) |

In this case the error diminishes as

[ ds^{2} ] |
= | [ dt^{2} ]/ [ N l^{2} ] , |
(2.18) |

where the brackets indicate an averaging over the different outcomes (or random variables) of the experiment. This equation says that slowness variance is inversely proportional to the squared segment length of the ray that passes through the cell times the number of rays that pass through the cell. Thus the model error can be assessed as a function of data errors and raypath segment lengths. For arbitrary source-receiver distributions and model parameterizations, the covariance matrix [