Fracture mapping is essential in hydraulic fracturing and finds applications in the oil and gas industry. Mapping of fractures using traditional radioactive or microseismic methods involves contamination and is prone to being inaccurate, thereby reducing the yield in fracturing applications. Therefore, wireless sensor networks (WSNs) with each sensing node having a small form factor and low power consumption are currently being investigated for use in such applications. This paper presents a fully battery-less system of coherent sensing nodes using wireless energy harvesting. These nodes are capable of mapping fractures reliably at temperatures up to 250 °C and pressures up to 24 MPa. Each node comprises a microchip having dimensions of 1.1 mm X 0.56 mm, two coils of 8 mm diameter each, and resonating capacitors. The microchip was fabricated in the Taiwan Semiconductor Manufacturing Company (TSMC) 0.18 μm process. The node receives a 40.68 MHz radio frequency (RF) signal in the industrial, scientific, and medical applications (ISM) band and transmits back a locked sub-harmonic 13.56 MHz ISM band RF signal. The sub-harmonic signal is generated on-chip using a digital divide-by-3 circuit, drastically reducing the microchip power consumption compared to injection-locked oscillators or phase-locked loops (PLLs). The sensor nodes used in the system have a form factor of 17 mm X 12 mm X 0.2 mm and a minimum average power consumption of 1.5 μW.