Method for imaging subterranean formations

A method for using a tool in a wellbore in a formation to predict an anomaly in the formation ahead of the device, the tool comprising a transmitter for transmitting electromagnetic signals through the formation and a receiver for detecting response signals, the method comprising the steps of:

• a) bringing the tool to a first position inside the wellbore;
• b) energizing the transmitter to propagate a transient electromagnetic signal into the formation;
• c) detecting a response signal that has propagated through the formation as a result of the transient signal in step (b);
• d) calculating a derived quantity for the formation as a function of time based on the detected response signal for the formation;
• e) plotting the derived quantity for the formation against time;
• f) moving the tool to at least one other position within the wellbore and repeating steps (b) to (e);
• and
• g) predicting an anomaly in the formation ahead of the device by plotting the derived quantity for the formation calculated in respect of the tool at the at least two positions in a plot having logging depth and time as its axes and fitting at least one curve to the plots of the derived quantity for said at least two positions.