The Oil Drum

Generally you have to run the instrumentation package that plots readings from the depth measurements that are recorded as the cable is reeled back into the instrument truck.

Each of the instruments in the pod works in a different way.  One reason, for example, that the tool is so long is to give distance between the transmitter and the receiver.  Thus you can induce a current in the ground using a coil at one end of the pod, and read its size from a reciever coil at the other, as a way of measuring resistivity.  Alternately you can generate a sound at one end of the pod and time how long it takes to reach a sensor on the rock at the other.  Then based on the reading you can determine from graphs, tables and past knowledge what the porosity and water content are likely to be.   The signals all come up the wire to the instrument truck where they are recorded in real time, and can be interpreted later.

Generally when you talk about large field data and how properties vary, you are using the logs from a number of wells to give you the picture of what the rock formations look like at different points around the field, and then using this and other geophysical surveys (that I will try and cover next week) to map out the undergound shape of the rock layers.