fast-formation arrival

1. n. [Formation Evaluation]
An early signal in a cement-bond log. In some formations, particularly carbonates of low porosity, it is possible that the first acoustic signal to arrive at the receiver passes through the formation rather than through the casing, and hence its amplitude is unrelated to the cement bond. This manifests itself by a shortening of the transmitter-to-receiver traveltime and by anomalous patterns on the variable-density log. In such cases, it may be assumed that the cement bond is good, as the signal would be unlikely to be transmitted through the formation with sufficient amplitude to be detected if cement bond were poor.