Structures within the deep geothermal reservoir in the Kakkonda field have been identified by a newly-developed reflection method using acoustic emission (AE) as a wave source. Reflected waves in three-component AE signals were detected independently of the wave amplitude by examining the linearity of a three-dimensional hodogram. There are many possible reflectors in this field, such as geological boundaries and fractured regions. In this paper are described some deep reflectors beneath the conventionally developed shallow geothermal reservoirs that were revealed by three-dimensional inversion of natural AE waveforms. These identified reflectors agree fairly well with geological constraints based on core samples from the field.