Multibeam echosounders (MBES) are sonar systems used to gather geophysical information from the seafloor and water column. At high frequencies (>100 kHz), MBES can be calibrated for their ensonification patterns. However, deep-water MBES feature long transmit arrays and varying geometries that make calibration difficult. In addition, modern MBES systems have high source levels, broad bandwidths and long pulses that can potentially impact the local soundscape. The goals of this study were to experimentally characterize the radiated sound field of deep-water MBES. A MBES survey was conducted using a 12 kHz Kongsberg EM122 MBES on the SCORE range, a US Navy broadband hydrophone array off the coast of San Clemente Island. The hydrophone data were analyzed and the radiation pattern was determined and compared to theoretical models of the transmit patterns. The analysis identified a limitation with the hydrophones that prevented the full characterization of the MBES radiation pattern. Despite the limitation, the full transmit radiation pattern revealed the presence of unexpected lobe-like structures in the alongship direction that are atypical of model transmit patterns. The discovery of these lobes may have an impact on backscatter measurements and is directly applicable to impact assessment