Airborne Gradiometer Survey For The Proposed San Andreas Fault Drill Site

[01321337 .塔尔瓦尼]航空重力梯度测量是一项最近才发展起来的技术，但它具有巨大的潜力。它能够在几乎任何地形上以相当低的成本，非常迅速地进行高精度的绝对重力梯度测量。它很可能会彻底改变各种类型的重力测量，这些测量主要集中在浅层地壳上，而且由于该仪器可以确定绝对数值，因此也可以用于时间推移研究。前者可以包括，例如，涉及探测盲断层的研究，确定沉积填充物或玄武岩流动的厚度和在植被密集地区的岩性接触，以及与环境和矿物勘探有关的问题。后者可以帮助在人类的时间尺度上追踪浅地壳中的流体流动或火山中的岩浆运动，以及浅地壳内其他物质的运动。目前只有两个机载梯度仪在运行。两者都是基于洛克希德·马丁公司的GGI梯度仪，并已被洛克希德·马丁公司和BHP公司联合用于机载工作。显然，这项技术还处于初级阶段，需要进行一些初步实验，以检验它是否适合地壳工作。该项目包括对圣安德烈亚斯断层深度观测站（SAFOD）拟议钻探地点的航空梯度测量，该观测站是EAR Earthscope计划的一个组成部分。SAFOD将涉及钻一个4公里深的洞，该洞将穿过加利福尼亚州帕克菲尔德附近的圣安德烈亚斯断层。除了帮助确定拟议钻探地点的深层结构外，梯度仪进行绝对测量的事实还可以用于其他用途。在时间推移实验中，即在同一空间上进行两次测量，很有可能监测到地下的变化。这些变化可能是流体的运动或与地震有关的变化。对于后一种试验，在震前建立基线测量显然很重要。如果进行大量的测量，就有可能看到浅层地下的流体运动是否是地震的可靠前兆

0132137TalwaniAirborne gravity gradiometry is a technology that has only recently been developed, but which has enormous potential. It is capable of making absolute gravity gradient measurements of great precision very rapidly over almost any kind of terrain, and at a relatively modest cost. It could well revolutionize measurements involving gravity for various types of studies focused on the shallow crust, and because the instrument determines absolute values, for time lapse studies as well. The former could include, for example, studies involving the detection of blind faults, determination of the thickness of sedimentary fill or of basalt flows and lithologic contacts in densely vegetated areas as well as environmental and mineral exploration related problems. The latter could help trace fluid flow in the shallow crust or magma movement in volcanoes, as well as other movements of material within the shallow crust on the human time scale.Only two airborne gradiometers are actually operational today. Both are based on Lockheed Martin's GGI gradiometer and have been adapted for airborne work jointly by Lockheed Martin and BHP. It is clear that this technology is in its infancy and some preliminary experimentation is necessary in order to test its suitability for crustal work. This project involves an aerial gradiometer survey of the proposed drill site for the San Andreas Fault Observatory at Depth (SAFOD), a component of EAR's Earthscope Initiative. SAFOD will involve drilling a 4 km deep hole which will intersect the San Andreas Fault near Parkfield, CA. In addition to helping determine the deep structure at the proposed drill site, the fact that the gradiometer makes absolute measurements can be put to another use. It is quite possible that in a time lapse experiment, that is by making two measuremenbts over the identical space separated in time, changes in the subsurface can be monitored. These changes could be the motion of fluids or changes associated with an earthquake. For the latter experiment, it is obviously important to establish a base line measurement before the earthquake. If a number of measurements are made it may be also possible to see if fluid motions in the shallow subsurface are reliable precursors to earthquakes.***