<PAGE>

                                PROMISSORY NOTE
                                ---------------


$10,000,000.00                  Houston, Texas                 February 16, 2001


     FOR VALUE RECEIVED, the undersigned, CONCORD TECHNOLOGIES, LP, a Texas
limited partnership, GEOSPACE ENGINEERING RESOURCES INTERNATIONAL, LP, a Texas
limited partnership, GEO SPACE, LP, a Texas limited partnership, OYO
INSTRUMENTS, LP, a Texas limited partnership and OYOG OPERATIONS, LP, a Texas
limited partnership, jointly and severally ("Maker"), hereby promise to pay to
the order of SOUTHWEST BANK OF TEXAS, N.A., a national banking association
("Payee"), at its offices at Five Post Oak Park, 4400 Post Oak Parkway, Houston,
Harris County, Texas, or such other address as may be designated by Payee, in
lawful money of the United States of America, the principal sum of TEN MILLION
AND NO/100 DOLLARS ($10,000,000.00), or so much thereof as may be advanced and
outstanding hereunder, together with interest on the outstanding principal
balance from day to day remaining, at a varying rate per annum which shall from
day to day be equal to the lesser of (a) the Maximum Rate (hereinafter defined)
or (b) the Prime Rate (hereinafter defined) of Payee in effect from day to day,
and each change in the rate of interest charged hereunder shall become
effective, without notice to Maker, on the effective date of each change in the
Prime Rate or the Maximum Rate, as the case may be; provided, however, if at any
time the rate of interest specified in clause (b) preceding shall exceed the
Maximum Rate, thereby causing the interest rate hereon to be limited to the
Maximum Rate, then any subsequent reduction in the Prime Rate shall not reduce
the rate of interest hereon below the Maximum Rate until the total amount of
interest accrued hereon equals the amount of interest which would have accrued
hereon if the rate specified in clause (b) preceding had at all times been in
effect.

     Principal of and interest on this Note shall be due and payable as follows:

          (a) Accrued and unpaid interest on this Note shall be payable monthly,
     on the first (1st) day of each month commencing on April 1, 2001 and upon
     the maturity of this Note, however such maturity may be brought about; and

          (b) All outstanding principal of this Note and all accrued interest
     hereon shall be due and payable on February 15, 2002.
<PAGE>

     Principal of this Note shall be subject to mandatory prepayment at the
times described in the Agreement (hereinafter defined).  If an Event of Default
(hereinafter defined) has occurred and is existing, the principal hereof and any
past due interest hereon shall bear interest at the Default Rate (hereinafter
defined).

     Interest on the indebtedness evidenced by this Note shall be computed on
the basis of a year of 365 days, and the actual number of days elapsed
(including the first day but excluding the last day) unless such calculation
would result in a usurious rate in which case interest shall be calculated on
the basis of a year of 365 or 366 days, as the case may be.

     As used in this Note, the following terms shall have the respective
meanings indicated below:

          "Agreement" means that certain Loan Agreement dated as of February 16,
     2001 between Maker and Payee, as the same may be amended or modified from
     time to time.

          "Default Rate" means the lesser of (a) the sum of the Prime Rate plus
     two percent (2.0%), or (b) the Maximum Rate.

          "Event of Default" shall have the meaning given to such term in the
     Agreement.

          "Maximum Rate" means the maximum rate of nonusurious interest
     permitted from day to day by applicable law, including Chapter 303 of the
     Texas Finance Code (the "Code") (and as the same may be incorporated by
     reference in other Texas statutes).  To the extent that Chapter 303 of the
     Code is relevant to any holder of this Note for the purposes of determining
     the Maximum Rate, each such holder elects to determine such applicable
     legal rate pursuant to the "weekly ceiling," from time to time in effect,
     as referred to and defined in Chapter 303 of the Code; subject, however, to
     the limitations on such applicable ceiling referred to and defined in the
     Code, and further subject to any right such holder may have subsequently,
     under applicable law, to change the method of determining the Maximum Rate.

          "Prime Rate" shall mean that variable rate of interest per annum
     established by Payee from time to time as its prime rate which shall vary
     from time to time.  Such rate is set by Payee as a general reference rate
     of interest, taking into account such factors as Payee may deem
     appropriate, it being understood that many of Payee's commercial or other
     loans are

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<PAGE>

     priced in relation to such rate, that it is not necessarily the lowest or
     best rate charged to any customer and that Payee may make various
     commercial or other loans at rates of interest having no relationship to
     such rate.

     This Note (a) is the Note provided for in the Agreement and (b) is secured
as provided in the Agreement.  Maker may prepay the principal of this Note upon
the terms and conditions specified in the Agreement.  Maker may borrow, repay,
and reborrow hereunder upon the terms and conditions specified in the Agreement.

     Notwithstanding anything to the contrary contained herein, no provisions of
this Note shall require the payment or permit the collection of interest in
excess of the Maximum Rate.  If any excess of interest in such respect is herein
provided for, or shall be adjudicated to be so provided, in this Note or
otherwise in connection with this loan transaction, the provisions of this
paragraph shall govern and prevail, and neither Maker nor the sureties,
guarantors, successors or assigns of Maker shall be obligated to pay the excess
amount of such interest, or any other excess sum paid for the use, forbearance
or detention of sums loaned pursuant hereto.  If for any reason interest in
excess of the Maximum Rate shall be deemed charged, required or permitted by any
court of competent jurisdiction, any such excess shall be applied as a payment
and reduction of the principal of indebtedness evidenced by this Note; and, if
the principal amount hereof has been paid in full, any remaining excess shall
forthwith be paid to Maker.  In determining whether or not the interest paid or
payable exceeds the Maximum Rate, Maker and Payee shall, to the extent permitted
by applicable law, (a) characterize any non-principal payment as an expense,
fee, or premium rather than as interest, (b) exclude voluntary prepayments and
the effects thereof, and (c) amortize, prorate, allocate, and spread in equal or
unequal parts the total amount of interest throughout the entire contemplated
term of the indebtedness evidenced by this Note so that the interest for the
entire term does not exceed the Maximum Rate.

     If default occurs in the payment of principal or interest under this Note,
or upon the occurrence of any other Event of Default, as such term is defined in
the Agreement, the holder hereof may, at its option, (a) declare the entire
unpaid principal of and accrued interest on this Note immediately due and
payable without notice, demand or presentment, all of which are hereby waived,
and upon such declaration, the same shall become and shall be immediately due
and payable, (b) foreclose or otherwise enforce all liens or security interests
securing payment hereof, or any part hereof, (c) offset against this Note any
sum or sums owed by the holder hereof to Maker and (d) take any and all other
actions available to Payee under this Note, the Agreement, the Loan Documents
(as such term is defined in the Agreement) at law, in equity or otherwise.
Failure of the holder hereof to exercise any of the foregoing options shall not
constitute a waiver

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<PAGE>

of the right to exercise the same upon the occurrence of a subsequent Event of
Default.

     If the holder hereof expends any effort in any attempt to enforce payment
of all or any part or installment of any sum due the holder hereunder, or if
this Note is placed in the hands of an attorney for collection, or if it is
collected through any legal proceedings, Maker agrees to pay all costs,
expenses, and fees incurred by the holder, including all reasonable attorneys'
fees.

     THIS NOTE SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF
THE STATE OF TEXAS AND THE APPLICABLE LAWS OF THE UNITED STATES OF AMERICA. THIS
NOTE IS PERFORMABLE IN HARRIS COUNTY, TEXAS.

     Maker and each surety, guarantor, endorser, and other party ever liable for
payment of any sums of money payable on this Note jointly and severally waive
notice, presentment, demand for payment, protest, notice of protest and non-
payment or dishonor, notice of acceleration, notice of intent to accelerate,
notice of intent to demand, diligence in collecting, grace, and all other
formalities of any kind, and consent to all extensions without notice for any
period or periods of time and partial payments, before or after maturity, and
any impairment of any collateral securing this Note, all without prejudice to
the holder. The holder shall similarly have the right to deal in any way, at any
time, with one or more of the foregoing parties without notice to any other
party, and to grant any such party any extensions of time for payment of any of
said indebtedness, or to release or substitute part or all of the collateral
securing this Note, or to grant any other indulgences or forbearances
whatsoever, without notice to any other party and without in any way affecting
the personal liability of any party hereunder.


                              CONCORD TECHNOLOGIES, LP

                              By:  OYOG, LLC,
                                   its general partner


                                   By:  /s/ Thomas T. McEntire
                                        ----------------------
                                        Thomas T. McEntire
                                        Chief Financial Officer

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<PAGE>

                              GEOSPACE ENGINEERING RESOURCES
                              INTERNATIONAL, LP

                              By:   OYOG, LLC,
                                    its general partner


                                    By:  /s/ Thomas T. McEntire
                                         ----------------------
                                         Thomas T. McEntire
                                         Chief Financial Officer



                              GEO SPACE, LP

                              By:   OYOG, LLC,
                                    its general partner


                                    By:  /s/ Thomas T. McEntire
                                         ----------------------
                                         Thomas T. McEntire
                                         Chief Financial Officer


                              OYO INSTRUMENTS, LP

                              By:   OYOG, LLC,
                                    its general partner


                                    By:  /s/ Thomas T. McEntire
                                         ----------------------
                                         Thomas T. McEntire
                                         Chief Financial Officer


                              OYOG OPERATIONS, LP

                              By:   OYOG, LLC,
                                    its general partner


                                    By:  /s/ Thomas T. McEntire
                                         ----------------------
                                         Thomas T. McEntire
                                         Chief Financial Officer

                                       5