6.5 利用无人飞行器进行航空地球物理探测
无人飞行器又称为无人航空器系统(Unmanned Airborne Vehicle,UAV;Unmanned Aircraft Systems,UAS),飞机发明后不久,就有UAV。20世纪50~60年代美国将UAV用于军事侦察,到20世纪末,美国等国家广泛应用UAV执行各种军事任务,随后UAV开始作民用。现在有32个国家开发和制造大约250种类型的UAV。20世纪80年代起,美国一些研究机构就研发利用UAV进行地球物理探测的技术(磁测和甚低频VLF测量)。
6.5.1 实际运用
21世纪起始,有几家公司利用UAV做航空磁测商业飞行[36~37]。加拿大Fugro Airborne Surveys公司2003年与设计制造UAV的美国Insitu公司(小而有名,现已并入波音公司)合作,制成航磁UAV系统——GeoRanger。2005年在试验地飞行几百测线公里,与以往的航磁结果进行对比,验证取得成功之后,在加拿大魁北克加佩斯附近的圣劳伦斯湾海面上进行了首次商业飞行,共完成13 000测线公里。Fugro公司计划于2006年使用GeoRanger在加拿大和澳大利亚飞行100 000测线公里,见图67及图68。该公司的技术人员还申请了专利(PCT/CA2005/001557)。
图6-7 加拿大Fugro公司的GEORANGER无人航空磁测飞机[37]
图6-8 加拿大Fugro公司GEORANGER的地面控制站[37]
万能飞翔地球物理公司(Universal Wing Geophysics Corporation)2007年在加拿大三个地区使用UAV共完成28 100测线公里的磁测工作。英国Magsurvey Ltd.公司经过多年研发,制成Prion(UAV),装载铯光泵磁力仪做磁测。加拿大山德尔地球物理公司(Sanders Geophysics Ltd.,SGL)与渥太华卡列顿大学(Carleton University)合作制成新型的搭载磁力仪的UAV,命名为GeoSurvⅡ。GeoSurvⅡ样机长3.8m,翼展4.8m,最大起飞质量70kg,磁力仪安装在翼尖。要求飞行高度10m,时速110km,留空4h。还要求可爬升到2500m高度,以便测量UAV自身的磁场特征而不受地下的影响。按计划GeoSurvⅡ在2008年进行试验飞行,同时还要研究解决障碍物探测和避撞等问题。
6.5.2 应用前景
从飞行高度来看,有的UAV可以飞得很低,可以在离地面20m以下的高度做起伏飞行;有的UAV可以飞得很高,在20~30km的高空飞行。在高、低两端,有人驾驶的飞机不能执行任务,全靠UAV。对于地球物理探测来说,在其他的高度上,与有人驾驶的飞机相比,UAV也有它的优越性。在几十、几百甚至几千米高度上采用UAV进行地球物理探测是我们应该积极、认真考虑的问题。英国巴纳德微系统公司(Barnard Microsystems Limited)的巴纳德博士,在与FUGRO、SGL等实力雄厚的航空物探公司的专家们探讨之后,对于运用UAV进行地球物理探测问题,提出了全面、详尽的见解[38~39]。
小型的UAV可配备导航系统(如GPS)、激光扫描高度计并用计算机控制,可以在20m以下贴地飞行,能够获得精确的高分辨率数据,可以代替费力费钱的地面磁测,还有希望代替地面重力测量。小型的UAV只需要一小块平地即可操作,不需要飞机场;油耗少、留空时间长,可在夜间飞行,因此单位测线公里的成本低。由于成本低,可以在同一地区多次观测,提高数据的精度和可靠性,并且可以在不同时间进行观测,探测物理场随时间的变化,提供4维(4D)资料。UAV对环境影响小(排放少、噪音低)。根据目前的实际情况,利用小型的UAV做航空磁测是切实可行的。MagSurvey Ltd.公司的Prion(UAV)、FUGRO AIRBORNE SURVEYS公司的航磁GeoRanger以及Sanders Geophysics Ltd.公司正在研发的GeoSurv II,起飞质量只有几十千克,载荷几千克。配备质量轻的铯、钾光泵磁力仪、磁通门磁力仪和其他配套仪器。我国也有这一类小型无人飞机,还有氦光泵磁力仪,应该可以做得很轻,制作磁通门磁力仪的技术也很成熟,与国产的小型UAV结合,可组成中国的航磁UAV。
目前利用小型UAV作航空重力测量还不太可能。现有的航空重力仪,质量100~150kg。航空重力梯度仪更重,为300~400kg,小型UAV搭载不动。但是新型重力梯度仪的出现,值得注意。意大利比萨大学物理系的Bertolini教授等正在研制一种在常温下工作的轻便单轴重力梯度仪,位置(位移)转换器是一个蓝宝石微波介电谐振器,已做成实验室原型,质量2kg,准备进行静态试验。他们设想将这种重力梯度仪用于地面和航空重力测量[40~41]。
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