Well logs are descriptions of the rock layers as observed by the drillers
that document the rock type, jointing, and other visual characteristics of the rock. Sonic logs are similar in
principle to seismic tests, but use sonic waves instead of sound waves. Also, sonic logs are run down a well so
that, instead of from the surface, the sonic logs record information near the well bore. Gamma ray logs also run down the well bore, but these look specifically for natural radioactivity, which is much higher in black shales than in other rocks. Gamma ray logs help drillers determine the thickness and depth of the reservoir rock in the region. Resistivity logs use various methods to extract the porosity and fluid content of different rock layers.