**1. Introduction**

As discussed by Sicking and Malin (2019) [1], new methods of passive seismic data acquisition and processing allow ambient subsurface fluid connectivity to be mapped. Complementary to the information on fluid pathways from microearthquakes (MEQ) generated by massive high-pressure (active) fluid injections (e.g., [2]), the new passive technique of Fracture Seismics (FS) can directly reveal both the locations and relative permeabilities of fluid-filled fractures. Further, and perhaps more importantly, passive FS mapping shows the degree of flow connectivity at distance scales greater than a few tens of meters from the point of active fluid injection.

A variety of independent evidence indicates that locations where MEQ and FS activity is most intense are where fluid flow is most likely [1]. FS maps typically reveal long connectivity structures that conduct fluid flow, as illustrated in Figure 1. The top panel shows the intensity of FS signals, and the bottom panel indicates the fracture-connectivity skeletal pathways through the cloud where FS signals are most intense and fluid flow is most likely.

The FS pattern in Figure 1 is from a fracture stimulation in the Marcellus gas shales, and the connectivity of its permeability-related signals may be considered a surprise. However, in this case the FS flow prediction was confirmed both by gas pressure communication and by a tracer experiment. These measurements showed a high degree of connectivity over ~600 m between vertical Well B and horizontal Well A, as indicated by the FS backbone in the lower panel. This backbone was observed in the FS data from Stage 3, and then later also at Stage 4, in advance of the arrival of tracer signals in Well B, which appeared at the time Stage 5 was stimulated.

**Figure 1.** (**Top**) A horizontal map slice of the above background Fracture Seismic (FS) intensity (red high, blue low), activated during the third stage of a fracture stimulation treatment. (**Bottom**) The above-background FS intensity (red high, blue low) backbone of the same horizontal map slice in the FS intensity cloud for hydrofracturing Stages 3–5. Point A is the wellhead of a horizontal well with a trajectory indicted by the black line. Point B shows the location of a vertical well with pressure and chemical tracer monitors. Both pressure and tracer communicated between Well B and Well A, where the FS skeleton connects the wells over a distance of 600 m. Figure is from Lacazette et al (2013) [3].

There are good observation-based theoretical reasons to expect the kind of spatially extended crustal flow connectivity on the hundred-meter scale seen in Figure 1. The purpose of this paper is to discuss these reasons. We show that the distribution of features observed in such FS maps is expected. The expectation that crustal fluid flow over length-scales >10–20 m will be through high-connectivity flow structures is discussed below, from both (a) direct observations and physical logic and (b) the power law scale invariance in the observed self-similarity of fracture-connectivity patterns. The first approach was developed over the last three decades in a series of papers (e.g., [4–10]). The second approach has a vast and densely spaced literature (e.g., [11] and the many references therein). Our purpose here is to bring these insights together and convey an understanding of how upper-crustal fluid flow is likely to be structured, based on observations and the critical state physics of brittle rock.
