The Woofer, the Squarker and the Tweeter — The ‘Varisponse’ — An experiment in Geophone Response

The audio range of high fidelity equipment is extremely wide, from 2 Hz to 15 kHz approximately. A single audio speaker cannot maintain efficient operation across the spectrum, so in order to compensate for the losses in efficiency, hi-fi enthusiasts use three speakers with over-lapping spectral operation. Thus, a broad-band signal is produced using a ‘woofer’ for low frequencies, a ‘squarker’ for medium range frequencies, and a ‘tweeter’ for the higher frequencies.

The useful seismic spectrum is limited to a range of 2 Hz to 400 Hz. The range of 2 Hz to 120 Hz is of particular interest to the oil explorer with present technology. In order to image reservoirs successfully, methods of signal enhancement and noise reduction have been designed to improve low signal-to-noise ratios.

In order to reduce the harmful signal-masking effects of surface waves, geophone strings spread over the surface waves’ length are used in an attempt to attenuate coherent ground roll. Where waves are dispersive, a higher valued geophone coil resonant frequency may provide assistance in attenuating such waves. Alternatively, in vibrator work, a change of sweep frequency character may assist not only surface wave reduction (by simply not sweeping over the surface wave frequencies) or a concentration of sweeps in the target zone spectrum may enhance reflected signal strength. One innovation — that of Varisweep* — allows a change of sweep frequency with vibrator position, thereby using the source array sweep frequency design to enhance the desired signal, and yet use the source array length to attenuate long wavelength coherent noise.

This paper proposes the combination of the Varisweep approach of source design to receiver design, such that the receiver array improves the spectral response in terms of both signal and noise.