The modified image method is used to invert active electromagnetic (AEM) data from a 1984 U. S. Navy survey of Cape Cod Bay. The high-frequency data (7200 Hz) give a robust value for the altitude of the helicopter-towed AEM bird and for the first-layer skin depth and, hence, for the first-layer conductivity. The inversion of low-frequency (385 Hz) bottom-probing signals produces more noise-sensitive estimates for the water depth and for the conductivity contrast K, the ratio of the bottom to water conductivities. The results show good agreement with 'ground-truth' radar altimeter, sea conductivity, and sonar depth data. To demonstrate the accuracy of the modified image method of inversion, we incorporate ground-truth measurements along a flight line and the experimental frequencies in a forward Sommerfeld calculation to generate synthetic data which then are inverted using this technique.