From early surface mining experiments to modern nuclear physics, formation logging has been the backbone of oil and gas development, letting geologists, geophysicists and engineers see more and more of the subsurface. The advent of well logging in the 1920s, and its subsequent development into a sophisticated technology, revolutionized the exploration and production industry. The ability to "look and measure" such things as formation type, formation dip, porosity, fluid type and numerous other important factors brought the drilling and completion of wells from an ill-defined art to a refined science. There have been three major areas of logging development: electric, sonic/acoustic and nuclear. To understand the development of each is to understand the industry's technical progress. Electric logging The genesis of well logging resides with Conrad Schlumberger who, while a physics professor at the School of Mines in Paris, conceived the idea of prospecting for metal ore deposits by using their electrical conductivity to distinguish them from less conductive surroundings. Working with his brother Marcel, Schlumberger began a series of test surface surveys in Europe, Africa and North America during a three-year period. Their discoveries included an oil-productive salt dome in Romania, a precursor of things to come. In 1926, the brothers formed the Society of Electrical Prospecting and began to develop the theory that adding resistivity information from deeper formations would increase the effectiveness of surface prospecting. By lowering an electric sonde down a 1,600-foot well in the Pechelbronn Field in France on September 5, 1927, the brothers created the first well log. The first log was painstakingly recorded point by point, meter by meter using makeshift equipment and was then plotted by stitching together the successive readings. The new technology worked simply. Three electrodes-A, M and N-are lowered to the bottom of the well bore on three insulated wires. The current from A passes through the drilling mud and spreads out into the formation. The potentials measured at M and N are transmitted to the surface where they are measured. By measuring the potential difference between M and N, and the strength of the current from A, the apparent formation resistivity then can be calculated. Following the initial success with the first electric resistivity logs, logging technology began to develop rapidly. In 1931, the accidental discovery of spontaneous potential (SP)-produced naturally by the borehole mud at the boundaries of the permeable beds-led to an innovative new logging technique: simultaneously recording SP and resistivity curves. This technique enabled producers to differentiate permeable oil-bearing beds from impermeable, nonproducing ones. By 1936, the industry could augment resistivity logs with formation-sample-takers, automatic film recorders and multispacing resistivity curves for deep wells. The 1940s were a period of rapid development in logging technology. In 1941, logging took another major step forward with the introduction of the spontaneous-potential dipmeter, which greatly improved the vertical resolution of openhole logs. The tool allowed the calculation of a layer's dip-the deviation of that layer from true horizontal-and the direction of the dip. This measurement was improved further with the resistivity dipmeter in 1947 and the continuous resistivity dipmeter in 1952. During the 1940s, development in other areas forced innovations in logging. One of the most important was the introduction of oil-based muds (OBM) in the Rangely, Colorado, oil fields in 1948. OBMs are nonconductive. Normally configured electrical surveys require a conductive mud (water-based) system, according to Lee M. Etnyre in Finding Oil and Gas from Well Logs. The solution to logging in OBMs was the induction log, developed in the late 1940s. In induction logging, "alternating current with a constant intensity is fed to the emitting coil. Currents are induced into the formation and circulate in coaxial loops around the sonde. These currents in turn induce a signal in the receiver. This signal is proportional to the conductivity of the formation," writes Robert Desbrandes in The Encyclopedia of Well Logging. In 1962, computerized processing of logs catapulted the sector ahead, as it allowed much faster log processing, thus the dramatic expansion of log-data-gathering capability. By 1970, the field of nuclear logging was advancing but perfection of the electric-logging method remained under way, especially in the amount and the speed of data collection. In 1971 combination-logging systems were introduced, allowing loggers to acquire different sets of data simultaneously rather than in sequential runs. Thus were born tools that provided gamma ray, spontaneous potential, resistivity, sonic and caliper measurements in a single run. Sonic logging In 1946, working in large part off technology developed during World War II, the logging sector produced the first sonic log, the casing collar locator (CCL). This technology allowed much more accurate depth measurement inside casing and much more exact placement of perforations and completion equipment. Sonic logs generally work by generating signals in the 20- to 30-kilohertz (kHz) range, although some tools operate at higher ranges. There are two basic types of acoustic logs: transmission and reflection. "In the transmission method, one or more transmitters emit acoustic energy, which is transmitted by the formation and/or casing and is detected by one or more receivers. In the reflection method, one or more transducers emit acoustic energy, part of which is reflected by the borehole wall and/or casing and is detected by the same transducer," writes A. Timur in Petroleum Engineering Handbook. During the next three decades, sonic logging moved into a number of measurement fields, including porosity measurement, fracture detection, mechanical rock properties measurement, borewall and casing inspection and gas-bearing formations identification, according to Etnyre. Although nuclear logging has supplanted some of the functions of acoustic logging, it is still a vital part of the logging suite and is regularly run in some form in combination logging tools. Nuclear logging Meanwhile, logging using radiation of nuclear origin got its start in 1940. The initial nuclear logging tools recorded the radiation emitted by formations as they were crossed by boreholes. Gamma radiation is used in well logging as it is powerful enough to penetrate the formation and steel casing, according to Desbrandes. From passive radioactive monitoring in the gamma ray tool, the logging industry moved rapidly to active nuclear bombardment and measurement. In a formation density log, first introduced in 1962, the borehole wall is irradiated with a gamma ray source. A gamma-ray-counter then records the reflected rays. The number returned, versus diffused, relates to the formation density. Nuclear logging took a step further with the introduction of neutron logs in the late 1960s. These measure returned gamma rays generated by fast- or slow-moving neutrons. After traveling a while, a neutron is captured by an atom and prompts the emission of a gamma ray. The amount of hydrogen present in the formation affects how long a neutron can travel without being captured. Considering this, porosity and content can be determined. The biggest breakthrough in recent logging history has been the advent of nuclear magnetic resonance (NMR) logging. The technology has proven more potentially beneficial-and more confounding-than its early developers could have imagined. NMR tools function by creating a magnetic field in the borehole and then sending out pulses that polarize the hydrogen in water, oil and gas in the formation. As these hydrogen nuclei realign themselves to the original magnetic field, they induce signals in the tool's receiver, which are recorded by electronics. The amplitude of the signal relates directly to porosity, and the signal relaxation time relates to the size of the pore spaces containing fluids, providing an indication of permeability. NMR is a fluids-only measurement; however, due to the interactions of the pore fluids with rock surfaces, the rock matrix can significantly influence the fluid response. The technology has existed since the early 1960s, but it has taken the industry several decades to refine the process, with Numar-now a Halliburton subsidiary-the first to bring a continuous NMR logging tool to the market. The result has been an offering of tools and associated products that provide better depth of investigation and more information while traveling at the same pace as a traditional triple combo. Pipe-conveyed logging For 20 years or more, highly deviated holes have required that loggers be able to run their suites of tools on pipe. Initially those efforts took the form of traditional logging tools run on coiled tubing with electric line run inside the tubing. Almost instantaneous information received continually at the surface-now known as logging-while-drilling (LWD) and measurement-while-drilling (MWD)-while the well was being drilled had always been a goal. That goal was within range of the sophisticated logging tools by the advent of the combination tools in the early 1960s. The barrier to earlier implementation of MWD and LWD was not the logging tools but the method by which to send the information to the surface while drilling with jointed pipe. As it turns out, one of the key advances in logging tools was not another logging technology but rather mud-pulse technology that allows near continual transmittal of logging information from tools on the bottom of the drill string to processors at the surface through measurement of short, variances in mud pressure created by a component of the logging suite downhole. It is now possible to employ almost any logging suite combination on the bottom of drillpipe and actually log the hole as it is drilled. While many operators are reluctant to allow decisions on a well to be made solely on MWD/LWD logs, reliability and correlation have improved dramatically.