Distributed Acoustic Sensing in Geophysics. Группа авторов
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of conventional fiber can be engineered to deliver better DAS performance (Figure 1.24). We will show how an SNR improvement can be achieved, along with a wider dynamic range, using engineered fiber with precisely uniform scattering centers. This approach differs from a simple increase in irregular backscattering intensity (Westbrook et al., 2017). We also consider the trade‐offs between spatial resolution, signal‐to‐noise performance and frequency response, and present data acquired from several different seismic and microseismic surveys.
Figure 1.21 3D VSP: Two intersecting images processed from DAS seismic data acquired with the dynamite shot positions indicated from Miller et al. (2016).
Figure 1.22 DAS hydraulic fracture monitoring in the treatment well with a fine spatial resolution and wide dynamic range for simultaneous cluster fluid allocation and microseismic monitoring.
DAS performance is largely governed by how much light can be usefully collected from the optical fiber. In general, we require low‐loss fiber for long range sensing, but higher scattering fiber to generate more light. These two apparently contradictory requirements can be balanced by engineering bright scatter centers in the fiber, without introducing significant excess loss for the forward propagating light. This can be achieved, for example, by using fiber Bragg grating technology.
For long fiber lengths, 100 times more light than Rayleigh level can be safely used (Farhadiroushan et al., 2021). That gives 20 dB reduction of acoustic noise caused by quantum shot noise at frequencies of around 1 kHz. This improvement can be even more at low frequencies as pink noise is suppressed by the regular structure of scattering. So, noise reduction can be more than 30 dB at around 1 Hz. This prediction was successfully confirmed in field surveys and are presented at the end of the chapter.
Figure 1.23 DAS hydraulic fracture monitoring in the offset (a) with a fine spatial resolution for microseismic monitoring (c), and localizing of microseismic events in time and space (b).
Figure 1.24 DAS with standard fiber and engineered fiber with precision bright scatter center zones.
1.3.1. Precision engineered fiber concept
We will start our consideration from Equation 1.6 in Section 1.1 (titled ‘Distributed Acoustic Sensor (DAS) Principles and Measurements’), which represents the scattered E(z, t) field as a convolution of input optical field with scattering coefficient r(z), for a gauge length L0.
where e(z) is a coherent optical pulse and Ω(z) ∝ v(z) is the Doppler shifted angular frequency, which is proportional to the local acoustic speed—see Figure 1.25.
The scattering coefficient for engineered fiber can be represented by a spatially periodic function (Farhadiroushan et al., 2021), meaning a reflection coefficient r(z) can be represented by a set of defined scatter center zones separated by sampling distance LS.
where comb(z ) is the Dirac comb function, or sampling operator. If the gauge length is s times larger than sampling distance, L0 = s LS, s = 1, 2…, then r(z) = r(z − L0), and the reflectivity function r(z)can be taken out of the brackets:
Figure 1.25 Optical fiber with defined scatter center zones and the corresponding Doppler shifted angular frequency sampled between the zones. The length occupied by optical pulse is less than the distance between the zones. The gray line corresponds to spatially integrated DAS output, following a linear spline approximation.
(1.36)
To prevent cross‐interference and fading, the spatial length of the optical pulse should be smaller or equal to the distance between scatter center zones, so the spatial sampling of the optical field (Equation 1.35) can be represented by a train of pulses:
(1.37)
The optical pulses from each zone are separated (see Figure 1.25), so the maximum signal intensity and maximum SNR can be delivered if the pulsewidth is equal to the sampling distance, or τ(z) = θ(z + LS) − θ(z), where θ(z) is the Heaviside step function whose value is 0 for negative argument and 1 for positive argument. In this case, intensity can be calculated from the interference between pulses with the same index j, and, for each pulse, an acoustic signal A(z) = F · ∂Φ/∂t, where Φ = ΔΩ(z)t