Demonstration of LDEO/WLS web site with Leg 183/Kerguelen LIP geophysical logging data and exercises developed for the TEXTEAMS GMO Leadership Development institute. (Mary Reagan/LDEO and Kathy Ellins/UTIG) Log data represents a continuous recording of in-situ geologic material with spatially correct positioning of measured variations. The spatial positioning of core material may be greatly aided with the use of downhole logging as coring often is incomplete or yields more than 100% recovery. Log depths are based on precise cable measurements using a calibrated wheel and an optical encoder. Core depths are based on pipe depths are reported by the driller or core-technician. Mary Reagan, Program Manager of the LDEO Wireline Logging Service (WLS) has set up a web page (http://www.ldeo.columbia.edu/BRG/EDUCATION/index.html) with specially designed logging exercises based on data collected on ODP Leg 183. Links for additional resources will be provided for people who want more in depth information. Time Frame - 1˝ hours Materials:
Advanced Preparation Information that addresses the following questions has been specially prepared for the TEXTEAMS workshop by Mary Reagan at LDEO. This can be previewed at the LDEO web site.
The presenter/teacher will need to explain terms such as “resistivity” and “porosity”, and discuss how the composition of the rock or sediments can be inferred from knowledge about these physical properties. Glossary of Tools FMS Formation MicroScanner (FMS). The Formation MicroScanner sonde (FMS) consists of four orthogonal imaging pads each containing 16 microelectrodes, which are in direct contact with the borehole wall during the recording. The current intensity variations are measured by the array of sensors on each of the pads. Resistivity measurements can be correlated with lithology, bedding planes, fractures, faults, foliations, and the angle of inclination (dip) of layers. Dual Laterolog. The Dual Laterolog (DLL) provides two resistivity measurements with different depths of investigation into the formation: deep (LLd) and shallow (LLs). In both devices, a current beam 2 ft-thick (Ao) is forced horizontally into the formation by using focusing (also called bucking) currents (A1-A2, A'1-A'2); two monitoring electrodes (M1, M2, M'1, M'2) are part of a loop that adjusts the focusing currents so that no current flows in the borehole between the two electrodes. For the deep measurement both measure and focusing currents return to a remote electrode on the surface; thus the depth of investigation is greatly improved, and the effect of borehole conductivity and of adjacent formations is reduced. In the shallow laterolog, instead, the return electrodes which measure the bucking currents are located on the sonde, and therefore the current sheet retains focus over a shorter distance than the deep laterolog. The Dual Laterolog response ranges from 0.2 to 40,000 ohm.m, thus permitting a good characterization of highly resistive rocks such as oceanic basalts and gabbros. HNGS Hostile Environment Gamma Ray Spectrometry Tool. Natural gamma radiation in sedimentary formations results primarily from the unstable isotopes of potassium, thorium, and uranium and its daughter products. By counting gamma rays at different energy levels, that is, by measuring the gamma-ray spectrum, one can estimate the concentration of the three different elements by assuming that the proportion of stable and unstable isotopes is in natural equilibrium. The Hostile Environment Natural Gamma Sonde (HNGS) uses two bismuth germanate (BGO) scintillation detectors to measure the natural gamma ray radiation of the formation. The large detector volume, coupled with the higher gamma ray stopping power of BGO, makes these detectors highly suitable for gamma ray spectrometry applications.
Procedure
Part A In part A of this exercise, you will download log data from Leg 183 (Kerguelen Plateau). You will then plot those data to determine the depth of the sediment - basement* transition. You will also compare how this transition is reflected in several different types of log data. *“Basement” refers to the oceanic volcanic crust. The rock material of which the Kerguelen LIP is composed. Downloading Procedure. Go to the ODP Logging Services web site (http://www.ldeo.columbia.edu /BRG/ODP) and select Database and then Data Search. On the Data Search page select 183 under the Leg field and click on the Submit button. The database will then return a listing of all of the Leg 183 holes. Start with Hole 1137A. Click on the hole name and a new page will appear showing all of the log data collected in that hole. (Note: You will need a user name and password to access this page since the data is still protected under the one-year moratorium. Your instructor will provide them.) At this point, you have the option of downloading a variety of data types. For this part of the exercise, you will need the resistivity and porosity data. Just click on the links for Dual Laterolog and HNGS Hostile Environment Gamma Ray Spectrometry Tool. When the data for each appears on your screen, simply select Save As from the file menu to save the data to your hard disk. Analyzing the Data. Once you have the data files, you will need to plot the data in order to determine where the boundary between the sediments and the basement occurs. For the resistivity data (dual laterolog) you have two different measurements, one for deep penetration (LLd) and one for shallow (LLs). For a more complete discussion of the Dual Laterolog (DLL), see the DLL page in the Logger’s Manual. You can plot these either together or separately against depth. For the porosity data (APS) you have several measurements, in this exercise we will use the. For a more complete discussion of the Accelerator Porosity Sonde (APS), see the APS page in the Logger’s Manual. You should plot the AFEC (APS Far Detector Count Rate) column against depth. For both of these data types, you are looking for the depth at which the sediment/ basement transition takes place. That transition will be reflected as a change in the value of the measurement. You will see that the depth changes somewhat with different measurement techniques. If you plot other data types, you will find that the transition is more apparent in some than in others. Why do you think this happens? Part B In part B of this exercise, you will download Formation MicroScanner (FMS) image data from Leg 183 (Kerguelen Plateau) from the area around the sediment/basement transition to observe how this transition is reflected in image-format data. Downloading Procedure.The procedure is the same as for the first exercise, except in this case you will want to download the 100-meter FMS images from the area around the sediment/basement transition. (If you want to see more detail, look at one of the 20-meter images from the same interval.) Analyzing the Data. The FMS files are provided in GIF format and so can easily be viewed using your browser software. FMS images are created from measurements made by four orthogonal pads. The tool measures the microresistivity variations of the formation. Pale colors on an FMS image indicate more resistive parts of the formation, while dark colors indicate less resistive (more conductive) parts. If you were to print out an FMS image and roll the paper into a cylinder, you would see a representation of the inside of the hole. Notice the S-shaped curves that appear in several parts of the images. These are usually fractures in the hole, either natural or produced by the drilling process. They have this sinusoidal shape in the FMS image because it is an unrolled representation and the fractures usually bisect the hole at an angle. If you view these same fractures in the rolled image, they would appear as slices through the hole.
Formative AssessmentThe presenter/teacher may check that participants/students understand (1) the application of geophysical logging, and (2) how to access logging data from the ODP/LDEO-WLS web page. Participants/teachers will receive a preliminary summary of the logging program for Leg 183. |