Toward the low frequencies: land and marine equipment

A large improvement in the vertical resolution of surface seismic (one order of magnitude from decametres to metres!) is viewed by oil companies as the most important step toward a wider use of seismic in reservoir description. Differential absorption of the higher frequencies during the propagation of the reflected signal and the inability to create and record a broadband signal have prevented the seismic industry from fulfilling this important need. Borehole seismic may improve the situation by shortening ray paths and by avoiding transmission through the highly absorbing weathering zone. Seismic data with a resolution of metres and dominant frequencies around 1 kHz have been recorded by cross-hole seismic (Sheline, 1998). However, the scarcity of wells with the appropriate geometry has led to the rather limited extent of cross-hole seismic images. Vertical resolution (i.e. the ability to discriminate an event) depends, among other parameters, on the S/N ratio and on the frequency content. If we consider a zero-phase seismic wavelet (Figure 1), its resolution depends on the width of the central peak and on its isolation with respect to the secondary lobes. The width is determined by the average signal frequency (Fav = (Fmax + Fmin) /2), and the isolation by the relative bandwidth measured in octaves n (2n = Fmax / Fmin). The typical frequency range of surface seismic (10-80 Hz) represents three octaves, that is three doublings of frequency (10-20 Hz/20-40 Hz/40-80 Hz - Figure 2). We assume that, to gain vertical resolution, it should be easier to add one octave by recording signal from 5 to 10 Hz than by recording it from 80 to 160 Hz. What has prevented the industry from using this opportunity? First, we have not wanted to record low frequencies because they have a reputation for being highly contaminated by noise and for causing subsurface structural damage. In addition, most low frequencies have been filtered out in the recording process to avoid the high amplitude surface noise and the corresponding loss of dynamic range. This explains why we have not focused on them in either source or recording technology, and for data processing. Today, we can take full advantage of new marine and land acquisition equipment that makes it possible to emit and record broadband signal, including low frequencies.