The delta waves are brain waves of high amplitude and oscillation frequency is in range of 0-4 Hz. Delta waves similar to other brain waves can be recorded using an electroencephalogram. Delta waves are recorded using EEG similar to other brain waves and is generally related with SWS or slow wave sleep and deepest sleeping stages such as 3 REM and 4 NREM sleep. The delta waves assist in identifying the depth of sleep. You can find the picture of delta waves taken during slow wave sleep of patient under stage 3. The red mark indicates the high amplitude of signals under EEG. This picture represents a thirty second epoch.
History and background information on Delta waves:
A famous scientist W. Grey Walter described the delta waves during early 1990s that enhanced the EEG (Electroencephalograph discovered by Dr. Hans Berger) for detecting delta and alpha waves.
Example of Delta Binaural beats: