Being so close to home, on each of the six days from Sat. March 18 through Thurs. March 23 we will leave at 7:30 AM from near the Motor Pool at the north end of campus and return there each night, probably about 7:00 PM. Vehicles will be provided. Students may bring their own vehicles and meet us in the field at 8:00 AM, but must do so entirely at their own risk. Each student will not need to bring any camping or cooking equipment, only daily food and drink. But many items on the personal equipment list are still necessary. Be prepared especially for wildly variable weather conditions. We may delay our work during fierce storms, but we will have to make up storm days (or for equipment failure) between Fri. March 24 and Sun. March 26. So keep your entire spring break open for now, including the two weekends.

Our task in Antelope Valley is to profile the sedimentary basin using a variety of geophysical methods:

• One or more cross-strike gravity profiles (30-50 stations spaced between 50 and 300 m) will detail overall basin geometry.
• GPS surveying will be needed to provide 0.3-meter-accurate elevations for gravity reduction.
• One or more refraction profiles >700 m long will calibrate and confirm depth and density results of gravity. We will also record microtremor noise on the refraction profiles for S-wave soundings. A high-resolution seismic reflection profile with g = 2 m, 200-300 m long, placed against a fault scarp, may detail slip history and the style of faulting, provided a suitably fine-grained location can be found.
• 4-10 time-domain electromagnetic soundings will locate the water table, and the clay content of the sedimentary fill.

In Virginia City we will embark on more speculative surveys:

• We will attempt a short but high-resolution microgravity (0.05 mGal accuracy) survey over topographic discontinuities. We may be able to test the validity of our detailed density model against the locations of known cavities (mine workings).
• Microgravity surveying will require GPS or theodolite elevations more accurate than 10 cm.
• A profile of 4-10 TEM soundings will evaluate the electrical properties of known ore bodies and faults.

• GPS
• Gravity
• TEM
• Seismic (3 team members)

Guidelines for proposing the survey plans are below. Certain questions need to be answered soon, and preparations begun early. These parts of the plans should be prepared by Mar. 7, and will be reviewed and discussed by the class that week. Each team should see J. Louie as soon as possible to begin preparations. The remainder of each plan must be finished by Mar. 14, so we can act on the plans during the days before we go in the field Mar. 18.

Finished plans should include complete and detailed checklists of every item that will go to the field, data sheets and/or software disks, instrument operation instructions, maps showing proposed survey locations, and schedules for work by each team. Each team should turn in one set of plans on March 14, which I will evaluate and use to affect your final report grades.

• **What instruments are needed for the survey?
• **What instruments are available to us?
• **Who oversees the instruments? Are they available?
• **Are the instruments in good order? Have they been recently tested in the lab and on the ground? Need they be calibrated?
• **What supplies are needed to operate the instrument and record data? Batteries? Special paper? How are batteries to be charged, or sensitive materials stored?
• **If the instruments need repair or supplies need to be obtained, can this be done before departure?
• What provision can be made for instrument failure in the field? Would any tools be useful? Spare parts?
• **How and when are the other participants to be trained in the use of the instruments? What manuals are available? Can brief instructions for field use be written? Data entry forms prepared?
• **What items and procedures need to be put on a checklist that can be completed during mobilization?
• **How can the instruments be shipped to the field area? Are they especially sensitive? Do special arrangements need to be made to borrow them from the Bureau of Mines, DRI, or other Universities?
• **Are there materials or supplies that can or must be obtained at the field area? Where and when will this be done?

• What location and elevation accuracy is needed for survey stations? Do they need to be surveyed in? Could pace-and-compass locations be adequate?
• What site or geologic factors will contribute to successful bedrock profiling with this technique? Where could these be present in the field area?
• What site or geologic factors will contribute to successful shallow profiling with this technique? Where could these be present in the field area?
• In what parts of the field area would this technique help to constrain the interpretations from another technique?
• In what parts of the field area would the use of this technique be difficult? Are there access or surveying problems?
• How will the instruments be moved to different sites in the field area? When will vehicles be required? Will some stations have to be reached on foot?
• How many people and how much time are needed for each station, or each experiment?
• How much area or how many kilometers of profile can we do while we are in the field?
• How can a schedule be set up so everyone uses each of the instruments in the field?
• Where will the survey stations be? What profiles or areas will they cover? In what order will they be measured?
• How will the data be labeled and stored when it is collected to avoid loss or confusion later?
• What data quality-control procedures can be used? Can data be immediately reduced or plotted in the field to check for accuracy?
• Can initial results be used to guide the other techniques? Or to adjust survey plans on the fly?

• What procedures will be used to reduce and interpret the data? Would any results of the other surveys be needed? Would this survey's results be useful to another's?
• How and when will the data be disseminated to the other students? How will needed accessory information, such as station locations and instrument settings, be provided?

• Are there any items or concerns that need to be added to this list?

References

Telford et al., our textbook.

Compton, 1962, Manual of Field Geology, chapters 2, 3, 4, 11.

Dobrin and Savit, 1988, Introduction to Geophysical Prospecting, pages 3-8 and as noted below:

• **What instrument would be easiest to use while providing enough accuracy? Handheld GPS? Geodetic GPS? Kinematic or stationary? Absolute or differential? Alidade and plane table? Laser transit?
• **What maps, airphotos, and remote-sensing images are available? In what forms will copies be needed before, during, and after field work?
• What horizontal and vertical control has already been established in the field area? Are control points accessible? How will they be tied in?

• **What special precautions need to be taken to assure the stability of the gravimeter? Are extra batteries needed?
• Are any absolute gravity control stations available near the field area? When should they be measured?
• What gravity measurements have been made previously in the field area? How will our survey improve upon that work?
• Where should control stations be established, and how often should they be measured?
• Are any data reduction or modeling packages available? Can they work in the field?
• What sources of local rock density measurements are available?
• What accuracy is needed to make a useful interpretation of basement topography? What procedures will enhance accuracy?

• **Are the instruments working and ready for the field? What can be rented or borrowed?
• **What power sources are required?
• What is the maximum electrode spread, loop area, or time gate? What is needed to sound to our target depths?
• What are the expected depths of penetration for each of the available instruments?
• Are special procedures needed to properly ground electrodes in materials such as dry gravels? To lay out loops of known area? What tools and supplies will be needed?
• What will be the effect of the water table?
• What interpretation or analysis packages are available? Can analysis be done in the field? How will data be downloaded?

• **What instruments are available and working?
• **Are recording base stations available? Gradient instruments?
• What magnetic surveys have previously been performed in the field area? How will our survey improve on them?
• Where should drift control stations be located, and how often should they be measured?
• What accuracy is needed to detect the target basement topography? What if the bedrock changes from volcanic to metasedimentary?
• What will be the effect of volcanic materials within the sediments?
• What interpretation or analysis packages are available? Can analysis be done in the field? How will data be downloaded?

• **What recorders, cables, and geophones are available? Need any be rented or borrowed?
• What seismic surveys have been performed in the region? What were their results?
• What seismic velocity measurements are available for our area? If none, what are the likely velocities, and their contrasts at the basement interface?
• What line lengths are needed to locate basement refractions for different target depths? What are the possible dips?
• Which geophones should be used? Can an S-wave experiment be conducted?
• Will hammer blows provide enough seismic energy? Should another source be considered? At which sites will hammer surveys have the most chance of succeeding?
• **How will proper operation of the roll-along switch be assured?
• What arrivals will likely be observed? How will they be interpreted?
• What 3-d or pseudo-3-d shallow surveys are possible with the available time and equipment?
• What fan shots (off-line) will be possible and helpful in interpretation?
• What areas are most conducive to getting good high-resolution data? What condition of the water table is helpful?
• **Where are the buried colluvial wedges of possible earthquakes most accessible to reflection surveying?
• **Where are fault offets of buried neogene channels most accessible to reflection surveying?
• How small a hammer source can be used? What tests will need to be done in the field on different sources?
• What interpretation or analysis packages are available? Can analysis be done in the field? How will data be downloaded?