05 Patent-US5552786A-Logging-Underground-Formations-Radar 1996
The patent discloses a borehole radar logging system that determines the electromagnetic wave speed in the surrounding formation by measuring the direct‑path signal between a transmitter and a spaced receiver within the borehole. The method involves emitting a radar pulse, recording the exact emission time, detecting the unreflected (direct) component at the receiver, and calculating the formation wave speed from the known transmitter‑receiver spacing and the measured travel time. The apparatus integrates a self‑triggered pulser, dipole antennas, a non‑metallic fiber‑optic trigger link, and digitizing electronics that capture both the direct arrival and reflected signals for formation imaging. The invention emphasizes that the direct signal propagates primarily through the formation adjacent to the borehole, making the measured speed a reliable in‑situ calibration for subsequent reflection‑based distance measurements. Experimental field tests in a gneiss formation demonstrated a consistent linear relationship between antenna spacing and direct‑arrival time, yielding a formation wave speed of ~0.31 ft/ns, independent of borehole fluid type. The patent claims cover the method steps, the timing‑determination means, and the specific tool architecture, including the use of pick‑up loops, LEDs, and fiber‑optic links to generate a precise emission marker.
Topics
Direct‑Path Wave‑Speed Measurement Concept
The core innovation of measuring the formation’s electromagnetic wave speed by detecting the unreflected signal that travels directly from the transmitter to the receiver through the surrounding rock.
Borehole Radar Tool Architecture and Fiber‑Optic Triggering
Detailed description of the logging tool: self‑triggered pulser, dipole antennas, non‑metallic spacer, pick‑up loop, LED, fiber‑optic cable, photodiode, power splitter, delay line, and digitizer.
Electromagnetic Propagation Modeling in a Borehole Environment
Analysis of TM₀₁ mode, cutoff frequency, wavelength in borehole fluid versus rock, and why the direct signal bypasses the borehole fluid and tool body.
Experimental Validation and Field Test Results
Field deployment in a 12.25" water‑filled borehole, measurements at four transmitter‑receiver spacings, observed linear time‑spacing relationship, derived wave speed, and comparison with water‑propagation hypothesis.
Patent Claims and Legal Scope of the Invention
Enumeration of the 20 claims covering method steps (emission, timing, direct‑signal detection, speed calculation), apparatus elements (transmitter, receiver, timing means, processing), and specific configurations (pick‑up loop, LED, fiber‑optic link).