The writer points out the difficulties in mapping the oil structure of the Balcones fault zone in Texas in the usual way, by a study of surface geology. Core drilling has been used in many places, but has not proved very successful on account of the uncertainty involved in identifying the beds and in correlating them between holes.

He discusses the application, in this area, of the Sundberg electromagnetic method of mapping structure, describing the basic principles and showing it to be much superior to core drilling in speed and efficiency.

The survey made by the Sundberg method in the Salt Flat, or Bruner, oil field was begun in October, 1928, when only the discovery well was productive, and before the trend of the producing fault structure was known. The fault was mapped electrically by one surveying crew in about a month, and the later development of the field has proved the results correct, as shown by the map. A comparison of these electrical results with the data obtained from later drilling gave excellent agreement, as shown by five different profiles across the field.