INTRODUCTION Controlled source audio-frequency magnetotellurics (CSAMT) is a frequency-domain electromagnetic sounding technique which uses a fixed grounded dipole or horizontal loop as an artificial signal source. CSAMT is similar to the natural-source magnetotellurics (MT) and audio-frequency magnetotellurics (AMT) techniques; the chief differences center around the use of the artificial CSAMT signal source at a finite distance. The source provides a stable, dependable signal, resulting in higher-precision and more economical measurements than are usually obtainable with natural-source measurements in the same spectral bands. However, the controlled source can also complicate interpretation by adding source effects, and by placing certain logistical restrictions on the survey. In most practical field situations these drawbacks are not serious, and the method has proven particularly effective in mapping the top 2 to 3 km of the earth's crust.