Listen Without a Source
Every survey in the rest of the library begins by making a bang. This one never does. Microseismic ears hear a frac job grow crack by crack, fiber-optic cable turns thousands of meters of well into a continuous receiver, ambient noise carries velocity structure for anyone patient enough to correlate it, and the McGarr bound sets the loudness budget for everything we inject. The earth as its own source, and the quiet revolution in how we hear it.
You can explain what passive listening buys that active surveys cannot, locate microseismic events and read a frac's growing geometry from them, state the DAS bargain, gauge length against sensitivity, and place fiber where it beats geophones, extract signal from ambient noise by correlation, bound the magnitude an injected volume can drive and carry that number into an operating permit, and design the monitoring that keeps an injection honest.
Why listen
Active surveys sample the earth on the survey's schedule; passive listening samples it on the earth's. The taxonomy, 2D to 4D to multicomponent to passive, shows where the quiet methods fit and what only they can hear.
Every induced crack is a tiny earthquake with a location and a magnitude; recorded in a neighboring well, the cloud of events is the only direct image of where a frac actually went.
The case file runs the listening end to end: geophones in the offset well, events located stage by stage, and the frac geometry read from the cloud, height growth, asymmetry, and the stage that screened out.
The fiber revolution
A telecom fiber cemented behind casing is thousands of meters of continuous sensor with no electronics downhole; DAS rewrites the economics of putting ears in wells.
Rayleigh backscatter turns strain into signal, and the gauge length is the price: sensitivity bought with spatial averaging. Knowing the bargain is knowing what fiber can and cannot resolve.
The same fiber hears differently downhole and along a trench: geometry, coupling, and directionality decide where DAS replaces geophones and where it only supplements them.
The vertical seismic profile is listening with a known source and known depths, the bridge between surface seismic and the borehole, and the calibration passive methods borrow.
Monitoring that must last decades cannot depend on crews: permanently installed receivers make listening a property of the field itself, which is what storage obligations and life-of-field surveillance require.
Correlate enough noise between two receivers and the earth's response between them emerges, velocity structure extracted from traffic, weather, and ocean swell, a survey with no source at all.
The loudness budget
McGarr bounds the seismic moment an injected volume can drive, two thirds of a magnitude per decade of volume; the listening program and the injection permit are both written against that arithmetic.
The events cluster where slip tendency is highest: the stress tensor resolved on each fault orientation predicts which structures the microseismic will light up before the pumps ever start.
Strain, temperature, and next-generation sensing on the same glass: where distributed sensing is heading, and what a fully instrumented subsurface will hear that we currently guess at.