Shallow-water acoustic predictions are severely limited by uncertainty in sediment property characteristics. Inverse methods with controlled active sources and vertical arrays have been used to estimate seabed properties; however, some applications require a covert approach and horizontal bottomed arrays. This study addresses the accuracy of low-frequency (100–200 Hz) matched-field correlations using broadband signals from surface ships with unknown source levels at unknown ranges. Matched-field techniques are applied in a realistic shallow-water environment with a horizontal planar array and high signal-to-noise ratios. The simulations indicate significant potential for accurate estimates of thick-sediment characterizations of grain size out to ranges of tens of water depths in shallow water, despite moderate mismatch conditions in the environmental model. The results show that: (1) the horizontal aperture should contain at least three hydrophones per wavelength to ensure high quality inversions; (2) the horizontal aperture should be several times longer than a vertical aperture; (3) coherent (phase-only) matched-field processing outperforms standard intensity processing by about 2 dB in good input SNR conditions; (4) incorrect assumptions about the assumed sound-speed profile (e.g., incorrect mixed-layer-depth) do not significantly affect the inversion results. [Work sponsored by QinetiQ North America.]