To detect and confirm the presence of gas hydrates, we must start with the big picture, then progressively close in on them. Recall that gas hydrates only exist at a certain combination of temperature and pressure. These conditions only occur on land in regions of permafrost, and offshore on continental slope

Reflection seismic surveys performed in these regions help narrow the search. Gas hydrates may be present in the subsurface as thick layers, thin veins, nodules, or pore-filling cement; all of these forms tend to stiffen the sedimentary layer in which the hydrates are present. And, sound waves travel faster through stiff rocks than more pliable rocks. Therefore, if a relatively stiff layer of subsea sediments that contains gas hydrates overlays a more pliable layer that contains free gas, then the interface of those two layers will strongly reflect sound waves back towards the surface. Such a reflection is known, in this case, as a bottom simulating reflector (BSR).

Figure 1 - Subsea geographical map of the southeastern continental margin of North America. Gas hydrate locations, confirmed by seismic bottom simulating reflectors (BSR), are indicated with light red shading. The numbers refer to scientific offshore borehole drill sites.

Figure 2 - A seismic cross-section of the subsea continental margin in the vicinity of drill sites 994, 995, and 997. The presence of the BSR, imaged below sites 995 and 997, is an indication of the presence of gas hydrates. Yet, gas hydrates were also recovered from site 994.

Inside-out Magnetic Resonance By now, most people have heard about magnetic resonance imaging (MRI) in the medical industry. These magnetic resonance devices, the size of a truck and weighing several tons, make images of the bone, tissue, and blood within the human body (figure 4). What people are less likely to know, however, is that the oil industry uses magnetic resonance technology to make measurements of the subsurface formations. Schlumberger Scientist Bob Kleinberg is a pioneer in developing magnetic resonance tools for the petroleum industry. Visit NMR Six Miles Deep to find out more Figure 4 A typical magnetic resonance imager (MRI) device used for the medical examination of brain tissue. The patient is placed on the table and the patient's head is slowly moved into the bore of the imager. Around the bore is an antenna array and strong fixed magnets, which provide for the polarization of the hydrogen nuclei within the brain tissue. Such a device provides a permanent magnetic field of 3 Tesla (30,000 gauss), fills a room, and weighs over 4 tons. Picture of MAGNETOM Allegra courtesy of: www.Siemens.com.   .

Figure 3 - Recordings of well logs and core sample analysis from a an offshore borehole. This well was drilled to test for the presence of gas hydrates. A layer containing gas hydrates has been found between 200-430m below the top of the sediments. Higher values of resistivity and sound velocity are good indications that gas hydrates are present. The core analysis, on the right, confirmed that gas hydrates account for up to 15% of the total pore space.

Figure 5 - Nuclear magnetic resonance (NMR) can help provide accurate measurements of hydrate concentration in situ. Schlumberger designed and built a tool capable of making such determinations. This tool contains two permanent magnets arranged to project a strong magnetic field (550 gauss) out and into the surrounding geologic formation. The antenna and associated electronics operate at about 2 MHz. The measurement volume (yellow) is about 2 by 2 centimeters wide and 15 centimeters tall. Magnetic resonance well logging tools (figure 5) are about 15 feet long, about 5 inches in diameter, and weigh about 500 pounds. But when compared to their medical industry cousins, these MR logging tools work "inside-out". A medical MRI scan is made possible thanks to a very small entity: the proton. The human body is made up of billions of atoms, many of which are hydrogen atoms. Each hydrogen atom has a nucleus that contains a single proton. In its normal state, this hydrogen atom spins, or precesses, on an axis; think of it like a toy top spinning off its vertical axis (figure 6). When a body is placed lying down within the core of the main magnetic, the hydrogen nuclei within the body will align in one direction (towards the head) or the other (towards the feet). Fortunately, it is never exactly half and half, and there will be a large, residual net orientation of proton in one of the directions (figure 7). Now comes the really fun part of this. An antenna within the device broadcasts radio frequency (RF) waves at these oriented protons in a series of bursts: on again, off again. When “on”, these waves knock the protons over on their sides; when “off”, the protons radiate their own signals, recorded by the antenna, while they attempt to get back up. These signals contain information about the tissue within the body. Each set of bursts and received signals image only a tiny volume of the body. However, the MRI device repeats this sequence many times over, each time slightly altering the location on which it focuses its energy. When the process is done, a complete image of some part of the human body is available for viewing.

	Figure 6 - On the left, a toy top precesses about its vertical axis. The hydrogen, atom on the right, precesses about a magnetic field. Because it has only one proton, a single mass with a positive charge, it has a large magnetic moment (red arrow).			Figure 7 All of the hydrogen protons will align with the magnetic field of the permanent magnet. Since the body is in the middle of this magnetic field, the protons will align in one direction or the opposite direction, canceling out each other. But, there will always be a few extra protons aligned in one of the two directions.

The NMR logging tool must fit within a small diameter borehole, so it must be much smaller than current medical devices! The NMR logging tool seeks to investigate the pore space of a formation, which is generally occupied by hydrogen-bearing water, oil, or gas. The measurement principle is similar to that used by the medical NMR device, except the material being evaluated is outside of the permanent magnet, hence its works from the “inside-out”. The magnet aligns the hydrogen protons of the pore-filling fluids. An antenna alternately broadcasts RF waves, then records the signals emitted by the protons as they try to realign themselves with the field of the magnet. The signals come from a small volume of the formation (figure 5), but they represent an important characteristic of the rock: how much of the fluid within the pore space can move.

Now we can understand how “inside-out” magnetic resonance well logging can benefit the search for gas hydrates. Most formation fluids are mobile, and will move if given a place to go, such as into a borehole. But although gas hydrates fill the pore space (the porosity) of the formation, they act like a solid, and will create a very low signal for the well logging MR antenna to measure. This very low signal, indicative of non-mobile fluids, within a formation that is known to have porosity, confirms the presence and quantity of gas hydrates.