A note on estimating $T_{e}$ from Bouguer coherence

Abstract

The coherence between Bouguer gravity anomalies and topography is widely used to estimate the value of $T_{e}$, the effective elastic thickness of the lithosphere. In areas where there is little topography but substantial free air gravity anomalies there is often little coherence between the free air anomalies and topography. In such regions the Bouguer coherence method generally gives estimates of $T_{e}$ of 90 km or more. A detailed analysis shows that, under these conditions, the value of the Bouguer coherence $\gamma ^2_b$ is entirely controlled by the ratio of the power spectra of the free air gravity anomalies and the uncompensated topography, and contains no information about the value of $T_{e}$. What is worse, under these circumstances the variation of $\gamma ^2_b$ with wavelength closely resembles that expected for large values of $T_{e}$. These results show that neither the Bouguer coherence method nor the admittance method can produce meaningful estimates of $T_{e}$ when the free air gravity anomalies are incoherent with the topography.