A method of heating molten aluminum flowing in a heated trough member comprising the steps of providing a source of molten aluminum and providing a rough member comprised of a first side and a second side, the first and the second sides having outside surfaces, the sides formed from a ceramic material resistant to attack by molten aluminum. The seismic source waveform’s amplitude spectrum is approximated by a frequency-weighted exponential function of frequency, having two parameters to adjust to fit the frequency shift data, thereby providing a better fit to various asymmetric source amplitude spectra. In the second mode, the second transducer generates the second signal and the first transducer senses a reflection of the second signal from a reference target. The differential in etch rates maintains the opening dimension within the mask and allows the formation of a wedge-shaped trench within the shield layer that separates the layer into two shields.