Use of Long Gauge Length Distributed Acoustic Sensing for Improved Spatial Resolution of First Break Detection in Vertic

The gauge length used in the distributed acoustic sensing (DAS) interrogator affects the spatial resolution of the recorded vertical seismic profiling (VSP) data. Conventional wisdom is to use a small enough gauge length to minimize its detrimental bandwidth effects but a large enough gauge length to maximize the signal-to-noise ratio. However, this paper challenges this view by proposing to use unusually long gauge lengths and demonstrating that high spatial resolution of the first breaks is possible. When the gauge length is longer than the seismic wavelength of the source wavelet, the passage of the wavelet is recorded as two events: one marking the entry of the wavelet into the gauge region, and the other marking its exit from the gauge. Because the first wavelet is isolated from the second wavelet, the first arrivals are detected without ambiguity and with a spatial resolution that no longer depends on gauge length as long as it is sufficiently long to help prevent overlap of the entry and exit wavelet signals. Examples of this counter-intuitive result are demonstrated by simulation.