SNMR is performed in the Earth's magnetic field
Fig. 1: Surface Nuclear Magnetic Resonance (SNMR) in Nigeria. Image from:
Generation of a report (log) for a well by lowering a wire with an NMR instrument
NMR well logging was initially developed in the Earth's magnetic field before permanent magnets and electromagnets were used
Well logging or borehole logging consists of generating a detailed report (log) of the geological formations along the length of a well of borehole. This can be accomplished by visual inspection or by measurement of physical properties with instruments that are lowered with a wire in the well. The oil and gas industry perform well logging to obtain a record of rock properties. There exist different categories of well logging such as electrical, acoustic etc. NMR logging uses the magnetic resonance of hydrogen nuclei or protons to detect fluid type e.g. water or oil and additionally to determine rock porosity (pore size) and permeability. For instance, based on the duration of the T2 relaxation time, it is possible to determine clay bound water and capillary bound water i.e. water held by capillary forces onto the rock (ref. https://iopscience.iop.org/article/10.1088/1757-899X/579/1/012034/pdf).
An illustrative video is found at this link https://vimeo.com/grahamdavis/petrophysics-for-dummies-6-nmr.
Figure 2: Video on NMR and petrophysics. The featured screen capture illustrates the T2 distribution for clay-water and capillary-bound water.
Some related information from Wikipedia is provided below.
(https://en.wikipedia.org/wiki/Well_logging) “The Schlumberger brothers from Alsace, France, who founded Schlumberger Ltd in France in 1926 are considered the inventors of electric well logging."
"On September 5, 1927, a crew working for Schlumberger lowered an electric sonde or tool down a well in Pechelbronn, Alsace, France creating the first well log."
"During World War II, the US Government gave a near wartime monopoly on open-hole logging to Schlumberger, and a monopoly on cased-hole logging to Lane-Wells ."
(https://en.wikipedia.org/wiki/Schlumberger) “Schlumberger Limited is the world's largest oilfield services company. Schlumberger has four principal executive offices located in Paris, Houston, London, and the Hague .”
"After the discovery of nuclear magnetic resonance by Bloch and Purcell in 1946, the nuclear magnetic resonance log using the Earth's field was developed in the early 1950's by Chevron and Schlumberger.”
Information from the PetroWiki is provided below.
“In 1978, the Los Alamos Natl. Laboratory developed a logging tool that employed permanent magnets and used a pulsed radio frequency (RF) (pulse-echo) NMR method.
Ultimately, two wireline tools using different magnet and coil configurations emerged from these efforts:
Numar’s mandrel device (MRIL)
Schlumberger’s skid (sometimes called "pad") design [combinable magnetic resonance (CMR) tool]
Commercial logging began with these tools in 1991 and 1995, respectively. These wireline-tool designs continue to evolve (see section on Tool Design in this chapter)."
https://petrowiki.org/PEH:Nuclear_Magnetic_Resonance_Applications_in_Petrophysics_and_Formation_Evaluation#Logging_Tools (Logging tools - click on Expand)
It is noted that “a variety of multiple-echo pulse sequences have been developed for different purposes . In well logging and petrophysical studies, the most widely used is the Carr-Meiboom-Purcell-Gill (CMPG) sequence .
At the Schlumberger site (http://www.slb.com/) it is possible to consult a video that is representative of logging and the related principle of "Pulsar Multifunction Spectroscopy Service" at the link http://www.slb.com/services/characterization/petrophysics/wireline/pulsar-spectroscopy.aspx.