Estimating the low-frequency seismic impedance trend is critical for reservoir characterization based on seismic inversions. Conventional seismic reflection data have an information gap between ∼2 and 10 Hz and do not contain information about the background impedance trend. On the other hand, controlled source electromagnetic (CSEM) data are typically low resolution, and contain information about the Earth's resistivity structure. Thus, cross-property rock physics relations between elastic and electrical properties can extract information from the CSEM data to provide constraints on the low-frequency seismic impedance trend. This concept is tested with well-log data and applied to a resistivity section inverted from CSEM data. Results show that appropriate cross-property rock physics relations can capture the background low-frequency impedance trend and provide useful constraints on models for inversion.